X-Linked Hypophosphatemia

Mendelian MONDO:0010619 Pathograph 21 Show in embeddings browser Metabolic Bone Disorders

X-linked hypophosphatemia (XLH) is a PHEX-related, X-linked renal phosphate-wasting disorder. Germline loss-of-function PHEX variants cause excess bone-derived FGF23 through an incompletely resolved molecular link; FGF23 reduces renal phosphate reabsorption and dysregulates calcitriol, producing lifelong hypophosphatemia, childhood rickets, adult osteomalacia, skeletal deformity, dental disease, and progressive musculoskeletal burden.

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1
Inheritance
7
Pathophys.
16
Phenotypes
3
Gaps
21
Pathograph
1
Genes
8
Medical Actions
6
Differentials
2
References
2
Deep Research
🏷

Classifications

ISDS Skeletal Nosology
abnormal mineralization
👪

Inheritance

1
X-linked dominant inheritance HP:0001423
Hemizygous males and heterozygous females can both be clinically affected, and intrafamilial severity is variable rather than reliably predicted by sex. An affected male transmits the PHEX variant to all daughters and no sons; an affected female has a 50% chance of transmission to each child.
X-linked dominant inheritance Expressivity: VARIABLE
Show evidence (4 references)
PMID:22319799 SUPPORT Other
"XLH is inherited in an X-linked manner; hemizygous males and heterozygous females are similarly affected."
GeneReviews explicitly describes the X-linked inheritance pattern and clinical involvement of both sexes.
PMID:22319799 SUPPORT Other
"Affected males transmit the PHEX pathogenic variant to all of their daughters (who will be heterozygotes and will be affected) and none of their sons."
This directly supports father-to-daughter and absent father-to-son transmission.
PMID:22319799 SUPPORT Other
"Affected females have a 50% chance of transmitting the pathogenic variant to each child: male and female offspring who inherit the pathogenic variant will be affected."
This directly supports the one-in-two transmission risk from an affected heterozygous female to each child.
+ 1 more reference
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Discussions and Knowledge Gaps

3
What molecular intermediates connect PHEX loss in mineralizing cells to increased FGF23 synthesis and secretion?
KNOWLEDGE GAP OPEN gap_xlh_phex_to_fgf23_coupling
Attached to
pathophysiology#PHEX Loss of Function in Mineralizing Cells pathophysiology#Excess Bone-Derived FGF23 Signaling
The causal PHEX gene and downstream FGF23 excess are well established, but treating PHEX as though it directly degrades FGF23 would overstate current evidence. Resolving the intervening osteocyte pathways could expose upstream targets that complement systemic FGF23 neutralization.
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"The pathogenesis of XLH is complex and incompletely understood. Many features of the disease can be explained by increased secretion of the phosphaturic hormone fibroblast growth factor 23 (FGF23) from bone."
The guideline explicitly separates established FGF23 consequences from the incompletely understood upstream pathogenesis.
Does long-term correction of FGF23-mediated phosphate wasting prevent or reverse enthesopathy, osteoarthritis, adult dental disease or neurosurgical complications?
KNOWLEDGE GAP OPEN gap_xlh_long_term_structural_response
Attached to
treatments#Burosumab phenotypes#Enthesopathy phenotypes#Osteoarthritis
Short-term trials establish biochemical correction and healing of active fractures, but irreversible or slowly evolving structural outcomes have not been shown to respond. This distinction matters when counseling adults about which symptoms are plausibly treatment-responsive.
Show evidence (3 references)
PMID:39814982 SUPPORT Other
"Evaluate the long-term efficacy and safety of treatment with burosumab in patients with XLH with respect to critical outcomes such as growth, body weight and composition, bone shape, physical function, hyperparathyroidism, tooth mineralization, hearing function, neurosurgical complications and..."
The guideline's future-research agenda directly identifies these long-term structural and functional outcomes as unresolved.
PMID:39814982 SUPPORT Other
"Thus, the trial showed no evidence of an effect of burosumab on enthesopathy burden"
The pivotal adult trial did not demonstrate a short-term enthesopathy effect.
PMID:39814982 SUPPORT Other
"No data are available on the impact of treatment with burosumab or oral phosphate and active vitamin D on osteoarthritis structural damage on X-rays or the course of osteoarthritis."
The guideline explicitly identifies effects on osteoarthritis structure and course as unproven.
What are the comparative efficacy, optimal dosing and long-term developmental outcomes of starting burosumab before 12 months of age?
KNOWLEDGE GAP OPEN gap_xlh_infant_burosumab_comparative_outcomes
Attached to
treatments#Burosumab
The first infant study included only 16 participants without randomization or a control group. It expands safety experience but cannot establish comparative clinical benefit, durable prevention of deformity or a universal regulatory age threshold.
Show evidence (1 reference)
PMID:42044650 SUPPORT Human Clinical
"This open-label, non-randomised, phase 1/2 study included infants from clinical sites in Austria, France, Italy, Spain and the UK."
The design supports only preliminary non-comparative inference in infants.
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Pathophysiology

7
PHEX Loss of Function in Mineralizing Cells
Germline inactivating PHEX variants are the proximal defect. PHEX is expressed mainly in osteoblasts, osteocytes and odontoblasts, but the exact molecular route from PHEX deficiency to increased FGF23 production remains unresolved.
osteoblast CL:0000062 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves osteoblast (CL:0000062). CL:0000062 is a cell type from the Cell Ontology. osteocyte CL:0000137 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves osteocyte (CL:0000137). CL:0000137 is a cell type from the Cell Ontology.
PHEX hgnc:8918 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves PHEX (hgnc:8918). hgnc:8918 is a gene from the HUGO Gene Nomenclature Committee.
Genetic context PHEX hgnc:8918 HUGO Gene Nomenclature Committee (hgnc) Relation: this genetic context concerns this gene This genetic context concerns PHEX (hgnc:8918). hgnc:8918 is a gene from the HUGO Gene Nomenclature Committee. functional_impact_category: LOSS_OF_FUNCTION
Germline loss of PHEX function underlies the X-linked disorder.
Downstream
Excess Bone-Derived FGF23 Signaling PHEX loss is upstream of increased FGF23 synthesis, but the intervening molecular coupling is incompletely understood.
Show evidence (2 references)
PMID:11062477 SUPPORT Human Clinical
"Inactivating mutations of the gene PHEX, encoding a member of the neutral endopeptidase family of proteins, are responsible for XLH"
This establishes inactivating PHEX variants as the disease cause without using the ADHR finding as evidence for the unresolved PHEX-to-FGF23 coupling step.
PMID:39814982 SUPPORT Other
"X-linked hypophosphataemia (XLH) is a rare metabolic bone disorder caused by pathogenic variants in the PHEX gene, which is predominantly expressed in osteoblasts, osteocytes and odontoblasts."
The guideline supports the causal gene and its principal mineralizing-cell expression sites.
Excess Bone-Derived FGF23 Signaling
PHEX loss produces excess synthesis and circulating activity of the bone-derived phosphaturic hormone FGF23. This endocrine signal acts on the kidney to reduce phosphate reclamation and alter vitamin D metabolism.
phosphate ion homeostasis GO:0055062 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal phosphate ion homeostasis (GO:0055062). GO:0055062 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Reduced Proximal Tubular Phosphate Reabsorption Excess FGF23 downregulates proximal tubular sodium-phosphate transport and causes phosphaturia.
FGF23-Driven Calcitriol Dysregulation Excess FGF23 decreases calcitriol synthesis and increases its degradation in proximal renal tubules.
Show evidence (2 references)
PMID:39814982 SUPPORT Other
"XLH is characterized by increased synthesis of the bone-derived phosphaturic hormone fibroblast growth factor 23 (FGF23), which results in renal phosphate wasting with consecutive hypophosphataemia"
This directly supports the endocrine FGF23 excess and its renal phosphate-wasting consequence.
PMID:29947083 SUPPORT Human Clinical
"inherited loss-of-function mutations in the PHEX gene cause excess circulating levels of fibroblast growth factor 23 (FGF23), leading to lifelong renal phosphate wasting and hypophosphatemia"
The adult trial background independently states the PHEX-FGF23-phosphate axis in XLH.
Reduced Proximal Tubular Phosphate Reabsorption
FGF23 signaling in renal proximal tubules downregulates the NPT2a and NPT2c sodium-phosphate transporters, lowering tubular phosphate reabsorption and causing persistent urinary phosphate loss despite hypophosphatemia.
epithelial cell of proximal tubule CL:0002306 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves epithelial cell of proximal tubule (CL:0002306). CL:0002306 is a cell type from the Cell Ontology.
phosphate ion transport GO:0006817 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased phosphate ion transport (GO:0006817). GO:0006817 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Reduced Circulating Phosphate Ongoing renal phosphate loss lowers the circulating phosphate concentration.
Show evidence (2 references)
PMID:39814982 SUPPORT Other
"downregulation of the sodium-dependent phosphate transporters NPT2a (encoded by SLC34A1) and NPT2c (encoded by SLC34A3)"
The updated recommendations specify the proximal tubular transporter mechanism.
PMID:7550339 SUPPORT Human Clinical
"impaired phosphate uptake in the kidney, which is likely to be caused by abnormal regulation of sodium phosphate cotransport in the proximal tubules"
The original PHEX report links XLH to abnormal proximal tubular sodium-phosphate cotransport.
FGF23-Driven Calcitriol Dysregulation
Excess FGF23 decreases renal production and increases degradation of 1,25-dihydroxyvitamin D. Calcitriol is therefore inappropriately low or normal for the degree of hypophosphatemia, limiting compensatory intestinal mineral absorption.
vitamin D metabolic process GO:0042359 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves abnormal vitamin D metabolic process (GO:0042359). GO:0042359 is a biological process from the Gene Ontology. ⚠ ABNORMAL
Downstream
Skeletal Mineralization Failure An inadequate calcitriol response compounds the substrate deficit for mineralization.
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"a decrease in 1,25-dihydroxyvitamin D (1,25(OH)2D) synthesis and an increase in 1,25(OH)2D degradation in the proximal renal tubules"
This directly supports both directions of FGF23-mediated calcitriol dysregulation.
Reduced Circulating Phosphate
Lifelong renal phosphate wasting produces serum phosphate below the age-adjusted reference range. This is a causally active systemic substrate deficit as well as the central biochemical finding used in diagnosis and treatment monitoring.
Downstream
Skeletal Mineralization Failure Insufficient extracellular phosphate impairs growth-plate and osteoid mineralization.
Dentoalveolar Mineralization Defect Chronic hypophosphatemia contributes to dental mineralization failure, with additional local PHEX-dependent mechanisms not fully resolved.
Hypophosphatemia The reduced circulating-phosphate state is expressed clinically as hypophosphatemia below the age-adjusted reference range.
Show evidence (1 reference)
PMID:29947083 SUPPORT Human Clinical
"leading to lifelong renal phosphate wasting and hypophosphatemia"
The adult trial explicitly identifies the lifelong systemic phosphate deficit.
Skeletal Mineralization Failure
Chronic phosphate deficiency impairs mineralization at open growth plates and in bone osteoid. The age-dependent tissue expressions are rickets in growing children and osteomalacia, insufficiency fractures and pseudofractures after growth-plate closure.
osteoblast CL:0000062 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves osteoblast (CL:0000062). CL:0000062 is a cell type from the Cell Ontology. chondrocyte CL:0000138 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves chondrocyte (CL:0000138). CL:0000138 is a cell type from the Cell Ontology.
bone mineralization GO:0030282 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased bone mineralization (GO:0030282). GO:0030282 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Rickets Failure to mineralize the growing skeleton produces rickets.
Osteomalacia Failure to mineralize osteoid produces adult osteomalacia.
Lower-Limb Deformity Weight bearing on undermineralized growing bone promotes varus or valgus deformity.
Short Stature Persistent growth-plate disease impairs linear growth and disproportionately shortens the legs.
Pseudofractures and Insufficiency Fractures Osteomalacic bone develops non-traumatic pseudofractures and insufficiency fractures.
Bone and Musculoskeletal Pain Active osteomalacia and insufficiency injury contribute to bone and musculoskeletal pain, although structural joint disease can add other pain generators.
Joint Stiffness Mineralization disease can contribute to adult stiffness, but coexisting osteoarthritis and enthesopathy prevent assignment of a single settled intermediate mechanism.
Gait Disturbance and Impaired Mobility The combined skeletal consequences restrict walking and mobility.
Show evidence (2 references)
PMID:29947083 SUPPORT Human Clinical
"fractures, and pseudofractures due to osteomalacia"
The adult trial directly attributes fractures and pseudofractures to osteomalacia.
PMID:39814982 SUPPORT Other
"XLH is characterized by increased synthesis of the bone-derived phosphaturic hormone fibroblast growth factor 23 (FGF23), which results in renal phosphate wasting with consecutive hypophosphataemia, rickets, osteomalacia, disproportionate short stature"
The guideline supports the systemic mechanism and principal skeletal consequences.
Dentoalveolar Mineralization Defect
Impaired dentin and alveolar-bone mineralization creates structurally weak teeth with enlarged or exposed pulp pathways and susceptibility to endodontic infection, including abscesses in teeth that can appear sound.
Downstream
Dental Abscesses and Periodontal Disease Intrinsic dental structural defects predispose to pulpal and periapical infection and early tooth loss.
Show evidence (2 references)
PMID:39814982 SUPPORT Other
"Patients also show hypomineralization of teeth and are prone to tooth abscesses and periodontitis"
The guideline directly links tooth hypomineralization with abscess and periodontal susceptibility.
PMID:42409089 SUPPORT Other
"X-linked hypophosphatemia (XLH) is a PHEX-related disorder causing elevated FGF23, chronic hypophosphatemia, and impaired skeletal and dental mineralization, leading to frequent endodontic abscess in clinically sound teeth due to intrinsic structural failure, periapical lesions and early tooth loss."
The dental scoping review supports the structural basis and characteristic infection pattern.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for X-Linked Hypophosphatemia Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

16
Ear 1
Hearing Impairment HP:0000365 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hearing impairment (HP:0000365). HP:0000365 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22319799 SUPPORT Other
"sensorineural hearing loss has also been reported"
GeneReviews supports hearing loss as an associated manifestation without implying a settled mechanism.
Genitourinary 1
Nephrocalcinosis HP:0000121 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Nephrocalcinosis (HP:0000121). HP:0000121 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:29460029 SUPPORT Human Clinical
"Dental disease (63%), nephrocalcinosis (42%), and hearing impairment (14%) were also common"
The adult cohort supports prevalence of nephrocalcinosis but is not used alone to infer treatment causality.
PMID:39814982 SUPPORT Other
"Oral phosphate supplements and active vitamin D can also promote hyperparathyroidism and cause nephrocalcinosis in a dose-dependent manner"
The guideline specifically identifies nephrocalcinosis as an adverse effect associated with conventional therapy.
Head and Neck 2
Dental Abscesses and Periodontal Disease Tooth abscess HP:0030757 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Tooth abscess (HP:0030757). HP:0030757 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:42409089 SUPPORT Other
"frequent endodontic abscess in clinically sound teeth due to intrinsic structural failure, periapical lesions and early tooth loss"
The dental review describes the characteristic infection pattern and consequences.
Craniosynostosis HP:0001363 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Craniosynostosis (HP:0001363). HP:0001363 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:40220947 SUPPORT Human Clinical
"The pooled prevalence of craniosynostosis among children with XLH was 22 % (95 % confidence interval (CI) 9.0 % to 44 %)"
Meta-analysis supports the association and its non-universal frequency.
Limbs 1
Lower-Limb Deformity Bowing of the legs HP:0002979 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Bowing of the legs (HP:0002979). HP:0002979 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31104833 SUPPORT Human Clinical
"rickets, lower extremity bowing, and growth impairment"
The pediatric trial supports lower-extremity bowing as a characteristic manifestation.
Metabolism 1
Hypophosphatemia HP:0002148 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypophosphatemia (HP:0002148). HP:0002148 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:29947083 SUPPORT Human Clinical
"leading to lifelong renal phosphate wasting and hypophosphatemia"
This establishes lifelong low serum phosphate as the core biochemical phenotype.
Musculoskeletal 6
Rickets HP:0002748 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Rickets (HP:0002748). HP:0002748 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31104833 SUPPORT Human Clinical
"X-linked hypophosphataemia in children is characterised by elevated serum concentrations of fibroblast growth factor 23 (FGF23), hypophosphataemia, rickets, lower extremity bowing, and growth impairment."
The pediatric phase 3 trial identifies rickets as a cardinal childhood manifestation.
Osteomalacia HP:0002749 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Osteomalacia (HP:0002749). HP:0002749 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:29947083 SUPPORT Human Clinical
"fractures, and pseudofractures due to osteomalacia"
This directly supports active adult osteomalacia and its fracture manifestations.
Pseudofractures and Insufficiency Fractures Pathologic fracture HP:0002756 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Pathologic fracture (HP:0002756). HP:0002756 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:29947083 SUPPORT Human Clinical
"fractures, and pseudofractures due to osteomalacia"
The trial directly supports both fracture categories while the HPO mapping captures fractures arising in metabolically weakened bone.
Joint Stiffness HP:0001387 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Joint stiffness (HP:0001387). HP:0001387 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:29947083 SUPPORT Human Clinical
"Adults with XLH present with chronic musculoskeletal pain and stiffness"
This directly supports stiffness as a characteristic adult symptom.
Osteoarthritis Premature osteoarthritis HP:0003088 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Premature osteoarthritis (HP:0003088). HP:0003088 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"Adult patients with XLH may show pseudofractures due to osteomalacia as well as osteoarthritis, enthesopathies, spinal stenosis, hearing loss, depression and reduced quality of life"
The guideline supports osteoarthritis as part of adult XLH burden.
Spinal Canal Stenosis HP:0003416 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Spinal canal stenosis (HP:0003416). HP:0003416 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:22319799 SUPPORT Other
"Rarely, individuals with XLH can suffer from spinal stenosis, Chiari I malformation, syringomyelia, and/or raised intracranial pressure."
GeneReviews directly identifies spinal stenosis as a rare XLH manifestation.
Nervous System 1
Gait Disturbance and Impaired Mobility HP:0001288 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Gait disturbance (HP:0001288). HP:0001288 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:22319799 SUPPORT Other
"Adults may present with calcification of the tendons, ligaments, and joint capsules, joint pain, fatigue, insufficiency fractures, and impaired mobility."
GeneReviews explicitly includes impaired mobility among adult manifestations.
PMID:42429952 SUPPORT Human Clinical
"severe skeletal deformities with difficulty walking"
The untreated adult cohort corroborates clinically important walking difficulty.
Constitutional 1
Bone and Musculoskeletal Pain Bone pain HP:0002653 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Bone pain (HP:0002653). HP:0002653 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:29947083 SUPPORT Human Clinical
"Adults with XLH present with chronic musculoskeletal pain and stiffness"
This directly supports chronic musculoskeletal pain in adults.
Growth 1
Short Stature HP:0004322 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Short stature (HP:0004322). HP:0004322 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:29947083 SUPPORT Human Clinical
"Adults with XLH present with chronic musculoskeletal pain and stiffness, short stature, lower limb deformities"
The adult trial supports persistent short stature and lower-limb deformity.
Other 1
Enthesopathy
Show evidence (2 references)
PMID:29947083 SUPPORT Human Clinical
"accelerated osteoarthritis, dental abscesses, and enthesopathy"
The adult trial establishes the association without resolving causation.
PMID:39814982 SUPPORT Other
"enthesopathies and to what extent these are sequelae of certain primary manifestations of XLH is poorly understood"
The guideline explicitly marks the underlying causal interpretation as uncertain.
🧬

Genetic Associations

1
Germline PHEX Pathogenic Variants (Causative)
Gene: PHEX hgnc:8918 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is PHEX (hgnc:8918). hgnc:8918 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (3 references)
PMID:7550339 SUPPORT Human Clinical
"Intragenic non-overlapping deletions from four different families and three mutations (two splice sites and one frameshift) have been detected in HYP patients"
The original positional-cloning study documents multiple independent disruptive PHEX alleles in affected families.
PMID:11062477 SUPPORT Human Clinical
"Inactivating mutations of the gene PHEX, encoding a member of the neutral endopeptidase family of proteins, are responsible for XLH"
This independently states the inactivating PHEX disease mechanism.
CGGV:assertion_7b966545-e9dc-4a75-8d6a-91b328bc4854-2025-05-15T060000.000Z SUPPORT Other
"PHEX | HGNC:8918 | X-linked dominant hypophosphatemic rickets | MONDO:0010619 | XL | Definitive"
ClinGen classifies the PHEX-XLH gene-disease relationship as definitive with X-linked inheritance.
💊

Medical Actions

8
Burosumab
Category: Therapeutic Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: burosumab NCIT:C119744 NCI Thesaurus (NCIT) Relation: this treatment uses this therapeutic agent This treatment uses burosumab (NCIT:C119744). NCIT:C119744 is a therapeutic agent from the NCI Thesaurus.
Platform: Monoclonal antibody
Burosumab is a fully human monoclonal antibody that neutralizes FGF23. The comparative pediatric phase 3 evidence enrolled children aged 1-12 years with persistent rickets (RSS at least 2), hypophosphatemia and substantial prior conventional therapy; it did not directly establish superiority in mild rickets or adolescents. Updated recommendations support use in children and adolescents aged 1-17 years with signs of rickets. The adult placebo trial enrolled symptomatic adults with hypophosphatemia and pain and showed phosphate correction, greater fracture healing and reduced stiffness, while pain and physical-function comparisons did not survive multiplicity adjustment. A 2026 study in 16 infants younger than 12 months provides small, open-label, non-randomized safety evidence, not comparative efficacy or a universal authorization boundary; eligibility and labeling remain jurisdiction-specific. Burosumab is not co-administered with oral phosphate or active vitamin D and is avoided when baseline phosphate is normal or renal impairment is severe. Guidance recommends contraception during treatment for patients who could become pregnant and stopping burosumab when pregnancy is planned or identified. During lactation, the updated guideline does not treat burosumab as categorically contraindicated, but limits consideration to severe manifestations after individualized counselling.
Mechanism Target:
INHIBITS Excess Bone-Derived FGF23 Signaling — Burosumab binds circulating FGF23 and reduces its phosphaturic signaling.
Show evidence (1 reference)
PMID:29947083 SUPPORT Human Clinical
"Burosumab, a fully human monoclonal antibody, binds and inhibits FGF23 to correct hypophosphatemia."
This directly supports the on-target antibody mechanism.
Show evidence (16 references)
PMID:31104833 SUPPORT Human Clinical
"we enrolled children with X-linked hypophosphataemia aged 1-12 years. Key eligibility criteria were a total Thacher rickets severity score of at least 2.0, fasting serum phosphorus lower than 0.97 mmol/L (3.0 mg/dL), confirmed PHEX (phosphate-regulating endopeptidase homolog, X-linked) mutation..."
This records the pediatric comparative trial's actual severity, biochemical, genetic and prior-treatment enrollment boundaries.
PMID:31104833 SUPPORT Human Clinical
"Significantly greater clinical improvements were shown in rickets severity, growth, and biochemistries among children with X-linked hypophosphataemia treated with burosumab compared with those continuing conventional therapy."
This supports superiority within the enrolled pediatric trial population.
PMID:29947083 SUPPORT Human Clinical
"At week 24, 43.1% (burosumab) and 7.7% (placebo) of baseline active fractures were fully healed; the odds of healed fracture in the burosumab group was 16.8-fold greater than that in the placebo group"
This quantifies adult fracture healing without qualitative overstatement.
+ 13 more references
Oral Phosphate with Active Vitamin D
Category: Therapeutic Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: phosphate CHEBI:26020 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses phosphate (CHEBI:26020). CHEBI:26020 is a therapeutic agent from Chemical Entities of Biological Interest. calcitriol CHEBI:17823 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses calcitriol (CHEBI:17823). CHEBI:17823 is a therapeutic agent from Chemical Entities of Biological Interest. alfacalcidol CHEBI:31186 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses alfacalcidol (CHEBI:31186). CHEBI:31186 is a therapeutic agent from Chemical Entities of Biological Interest.
Platform: Small molecule
Conventional therapy combines divided oral phosphate with calcitriol or alfacalcidol. It can improve rickets, some manifestations of osteomalacia and pain but only partially corrects disease and does not make fasting serum phosphate the dosing target. Dose adjustment follows clinical and radiographic response, alkaline phosphatase and PTH while balancing gastrointestinal intolerance, hyperparathyroidism, hypercalciuria and nephrocalcinosis. Phosphate should not be given without active vitamin D; active vitamin D alone can cause hypercalcemia, hypercalciuria and nephrocalcinosis. It remains an important option when burosumab is unavailable or unsuitable and for selected symptomatic adults.
Mechanism Target:
MODULATES Reduced Circulating Phosphate — Divided phosphate doses transiently increase available phosphate without correcting upstream FGF23 excess.
MODULATES Skeletal Mineralization Failure — Active vitamin D promotes intestinal mineral absorption and, with phosphate, partially improves rickets and osteomalacia.
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"oral medication improves bone pain, radiological signs of rickets and serum ALP levels within 12 months"
The guideline supports partial downstream skeletal and biochemical improvement with conventional oral treatment.
Show evidence (4 references)
PMID:39814982 SUPPORT Other
"Serum phosphate levels remain low despite oral supplementation and are not a target for adjusting therapy, which requires the primary goal of achieving a satisfactory clinical response to be balanced with the risk of developing nephrocalcinosis and hyperparathyroidism"
This defines both the response target and principal monitoring trade-off for conventional therapy.
PMID:22319799 SUPPORT Other
"Treatment with phosphate without 1,25-dihydroxyvitamin D, because of the increased risk for secondary hyperparathyroidism."
GeneReviews explicitly warns against phosphate monotherapy.
PMID:22319799 SUPPORT Other
"Although 1,25-dihydroxyvitamin D has been used as a single agent, this may increase the risk for hypercalcemia, hypercalciuria, and nephrocalcinosis."
This supports the risks of active vitamin D monotherapy.
+ 1 more reference
Avoid Unindicated Bisphosphonate or Osteoporosis Medication
Category: Therapeutic Action: avoidance of contraindicated medicationsNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is avoidance of contraindicated medications, annotated with Supportive Care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. Ontology label: Supportive Care NCIT:C15747
Platform: Other
GeneReviews cautions that bisphosphonates and other osteoporosis medicines can worsen osteomalacia in some individuals with XLH. Low bone density or fractures should therefore not prompt empiric osteoporosis treatment without first distinguishing active osteomalacia and establishing a separate indication; this is a caution rather than a universal contraindication in every clinical context.
Show evidence (1 reference)
PMID:22319799 SUPPORT Other
"Bisphosphonates or osteoporosis medications may cause deterioration of osteomalacia in some individuals."
GeneReviews supplies the direct agents-to-avoid caution while retaining its conditional wording.
Multidisciplinary Surveillance
Category: Monitoring
Follow-up should be coordinated by a metabolic-bone team and include growth, lower-limb alignment, pain, mobility and dental review; serum phosphate, calcium, creatinine, ALP, PTH and vitamin D measures; and treatment-specific monitoring. Conventional therapy requires urinary calcium and renal ultrasonography for hypercalciuria/nephrocalcinosis. Burosumab dosing uses timed phosphate and TmP/GFR rather than FGF23 assays. Imaging is driven by rickets response, deformity, focal pain or neurological concern, and dental evaluation is recommended every six months.
Show evidence (4 references)
PMID:22319799 SUPPORT Other
"For individuals on burosumab therapy, regular monitoring of serum concentrations of phosphate, calcium, creatinine, alkaline phosphatase, and intact parathyroid hormone."
GeneReviews specifies the core biochemical surveillance during burosumab treatment.
PMID:22319799 SUPPORT Other
"For those on conventional treatment with active vitamin D and phosphate supplementation, additional testing includes urinary calcium and creatinine to assess for hypercalciuria; periodic renal ultrasound examination to assess for nephrocalcinosis."
This supports the additional renal safety surveillance required with conventional therapy.
PMID:22319799 SUPPORT Other
"dental evaluation every six months; hearing evaluation and evaluation for Chiari I malformation performed based on clinical suspicion"
This supports six-month dental review and symptom-directed hearing or neurological evaluation.
+ 1 more reference
Orthopedic and Joint Surgery
Category: Therapeutic Action: Orthopedic Surgical ProcedureNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Orthopedic Surgical Procedure (NCIT:C16186). NCIT:C16186 is a clinical intervention from the NCI Thesaurus. NCIT:C16186
Platform: Surgery
Persistent mechanically important lower-limb deformity or functional symptoms can require guided growth or corrective osteotomy after metabolic therapy has been optimized for at least 3-6 months. In growing children on burosumab, elective surgery may be delayed because alignment can continue to improve; guided growth requires adequate growth remaining, whereas later osteotomy has fewer recurrence concerns. Adults require the same metabolic optimization and expert multidisciplinary planning; joint replacement is reserved for function-limiting osteoarthritis despite medical treatment and rehabilitation. Emergency fracture fixation should not be delayed when necessary.
Show evidence (7 references)
PMID:39814982 SUPPORT Other
"elective surgical treatment should be performed only in children in whom medical treatment either with oral phosphate and active vitamin D or burosumab has been optimized for at least 3-6 months"
The guideline establishes preoperative metabolic optimization for children.
PMID:39814982 SUPPORT Other
"persisting deformity (mechanical axis deviation zone 2 or greater) despite optimized medical treatment and/or the presence of symptoms interfering with function should be considered for surgical treatment"
This defines the deformity and functional setting for elective pediatric surgery.
PMID:39814982 SUPPORT Other
"We recommend considering postponing surgery in children on burosumab, if possible, because sustained straightening of legs is expected for at least 3 years after initiation of this treatment in patients with growth potential"
This supports allowing time for continued alignment response in growing children receiving burosumab.
+ 4 more references
Preventive and Specialist Dental Care
Category: Therapeutic Action: Dental ProcedureNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Dental Procedure (NCIT:C38052). NCIT:C38052 is a clinical intervention from the NCI Thesaurus. NCIT:C38052
Platform: Other
Dental management complements systemic metabolic therapy. Children need standard prevention plus six-month visits and repeated pit-and-fissure sealing as needed; discoloration, fistula, swelling, abscess, cellulitis or pain prompts pulpal and periapical assessment. Adults need twice-yearly periodontal risk assessment and debridement when indicated. Systemic therapy should be optimized before orthodontic treatment or implant surgery, with prolonged healing plans for implants.
Target Phenotypes: Tooth abscess HP:0030757 Human Phenotype Ontology (HP) Relation: this treatment targets this phenotype This treatment targets Tooth abscess (HP:0030757). HP:0030757 is a phenotype from the Human Phenotype Ontology.
Show evidence (6 references)
PMID:39814982 SUPPORT Other
"In children, in addition to standard preventive care, we recommend dental visits every 6 months and suggest sealing pits and fissures with flowable resin composite on both temporary and permanent teeth as soon and as frequently as required"
This directly supports the pediatric preventive schedule and sealing intervention.
PMID:39814982 SUPPORT Other
"We suggest a thorough clinical investigation searching for pulp infection (colour changes, fistula, swelling, abscess, cellulitis or pain) and performing retrocoronal and/or periapical radiographs or orthopantomogram"
This directly supports symptom-triggered pulpal and periapical assessment.
PMID:39814982 SUPPORT Other
"In children, we suggest optimizing therapy with phosphate and active vitamin D or burosumab before initiation of orthodontic treatment"
This supports metabolic optimization before pediatric orthodontic treatment.
+ 3 more references
Rehabilitation and Symptom-Directed Support
Category: Therapeutic Action: Supportive CareNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Supportive Care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. NCIT:C15747
Platform: Behavioral / lifestyle
Physiotherapy, rehabilitation, adapted activity and analgesic or pain-clinic support address weakness, deconditioning, impaired mobility and chronic pain. These interventions are adjunctive and do not correct FGF23-mediated phosphate wasting.
Show evidence (1 reference)
PMID:22319799 SUPPORT Other
"rehabilitation, physiotherapy, and analgesics for musculoskeletal pain"
GeneReviews lists rehabilitation, physiotherapy and analgesia as supportive management.
Genetic Counseling and Cascade Evaluation
Category: Counseling / Informational Action: Genetic CounselingNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Genetic Counseling (NCIT:C15240). NCIT:C15240 is a clinical intervention from the NCI Thesaurus. NCIT:C15240
Counseling explains X-linked transmission, variable expressivity, testing of at-risk relatives, and reproductive options. Molecular testing of relatives is preferred when the familial PHEX variant is known; biochemical evaluation is an alternative for first-degree relatives at risk.
Show evidence (1 reference)
PMID:22319799 SUPPORT Other
"Molecular genetic testing (if the PHEX pathogenic variant has been identified in the family) or biochemical testing of first-degree relatives at risk to ensure early treatment for optimal outcome."
GeneReviews supports cascade molecular or biochemical evaluation of relatives at risk.
🔬

Biochemical Markers

5
Low Serum Phosphate (Decreased below the age-related reference range)
Context: Interpret phosphate using age-appropriate reference intervals and sampling context; fasting sampling is recommended in adults.
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"serum levels of phosphate below the age-related reference range associated with isolated renal phosphate wasting"
This is the core biochemical diagnostic combination.
Reduced TmP/GFR and Renal Phosphate Wasting (Decreased tubular phosphate reabsorption)
Context: Calculate TmP/GFR from paired serum and urine phosphate and creatinine; interpret with age-adjusted reference values, especially in children.
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"urinary levels of calcium, phosphate and creatinine for calculation of the tubular maximum reabsorption of phosphate per glomerular filtration rate (TmP/GFR) and urinary calcium to creatinine ratio"
The guideline specifies the measurements used to establish renal phosphate wasting and assess calcium excretion.
Intact FGF23 (Inappropriately non-suppressed or elevated during hypophosphatemia)
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"If FGF23 levels are not suppressed, the differential diagnosis is limited to a few diseases, with XLH (OMIM#307800) being the most frequent"
The guideline supports inappropriately non-suppressed FGF23 as a discriminant for an FGF23-mediated differential, not as an XLH-specific test.
Alkaline Phosphatase (Elevated with active rickets or osteomalacia)
Context: Total ALP in children and total or bone-specific ALP in adults is used to assess mineralization disease activity and response; interpret against age-related ranges.
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"children and adolescents (aged 1-17 years) with X-linked hypophosphataemia (XLH) and signs of rickets including leg deformities, elevated total ALP, and/or radiological evidence of rickets"
The guideline identifies elevated ALP as one indicator of active pediatric rickets.
1,25-Dihydroxyvitamin D (Inappropriately low or normal for the degree of hypophosphatemia)
Context: FGF23 suppresses synthesis and increases degradation, blunting the usual calcitriol response to low phosphate.
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"a decrease in 1,25-dihydroxyvitamin D (1,25(OH)2D) synthesis and an increase in 1,25(OH)2D degradation in the proximal renal tubules"
This directly supports the direction of calcitriol dysregulation.
🔬

Diagnosis

2
Integrated Clinical, Biochemical and Radiographic Evaluation
In a growing child, consider XLH when rickets or impaired growth coexists with phosphate below the age-related reference range and isolated renal phosphate wasting after calcium and vitamin D deficiency are excluded. In adults, historical lower-limb deformity, osteomalacia or pseudofractures, early osteoarthritis, spinal disease, dental abscesses or enthesopathy can prompt the same biochemical evaluation.
Clinical Evaluation NCIT:C124351 NCI Thesaurus (NCIT)
Results: A compatible phenotype plus hypophosphatemia and renal phosphate wasting; non-suppressed intact FGF23 and an X-linked family history support the diagnosis but neither feature is individually specific.
Show evidence (3 references)
PMID:39814982 SUPPORT Other
"In children and growing adolescents, a diagnosis of X-linked hypophosphataemia (XLH) should be considered in the presence of or if there is a history of clinical, biochemical, and/or radiological signs of rickets, impaired growth velocity and serum levels of phosphate below the age-related..."
The guideline supplies the age-specific integrated diagnostic pattern for growing patients.
PMID:39814982 SUPPORT Other
"If molecular genetic analysis is not available, a family history of X-linked inheritance, as well as non-suppressed plasma levels of intact FGF23 in association with hypophosphataemia support the diagnosis of XLH"
This supports a clinical-biochemical diagnosis when molecular confirmation is unavailable.
PMID:39814982 SUPPORT Other
"In adults and adolescents with closed growth plates, diagnosis of XLH should be considered in the presence of or if there is a history of lower-limb deformities, clinical, biochemical and/or radiological signs of osteomalacia (including pseudofractures, early osteoarthritis, spinal degeneration..."
The guideline supplies the adult integrated diagnostic pattern and its major historical clues.
PHEX Molecular Genetic Testing
Molecular analysis of PHEX confirms the suspected diagnosis when a pathogenic hemizygous variant in a male or heterozygous variant in a female is identified. If PHEX testing is negative, reassess the phenotype and test for alternative genetic or acquired FGF23-mediated disorders rather than treating the negative result as exclusion of all hypophosphatemic disease.
Genetic Testing NCIT:C15709 NCI Thesaurus (NCIT)
Results: Pathogenic or likely pathogenic germline PHEX variant.
Show evidence (2 references)
PMID:22319799 SUPPORT Other
"The diagnosis is established in a proband with characteristic clinical, biochemical, and radiographic findings by identification of a hemizygous PHEX pathogenic variant in a male proband or a heterozygous PHEX pathogenic variant in a female proband on molecular genetic testing."
GeneReviews defines the molecular confirmation pattern in the context of compatible clinical findings.
PMID:39814982 SUPPORT Other
"We recommend that other causes of genetic (DMP1, ENPP1, FGF23) or acquired forms of hypophosphataemia (such as tumour-induced osteomalacia) be considered if the analysis of the PHEX gene yields a negative result"
The updated guideline defines the required next step after negative PHEX testing.
🩻

Imaging Findings

5
Radiographic Rickets
Wrist, knee or ankle radiographs demonstrate and grade active rickets; radiographic severity is supportive but not specific for XLH.
Xray Growing children with suspected or active rickets
Rickets HP:0002748 Human Phenotype Ontology (HP)
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"Wrist and/or knee and/or ankle radiographs (rickets) in growing patients"
The guideline specifies the radiographic sites used to evaluate pediatric rickets.
Lower-Limb Malalignment
A standardized standing anteroposterior limb-alignment radiograph defines the mechanical axis and documents varus or valgus deformity for longitudinal and surgical assessment.
Xray Standing alignment assessment in children or adults with deformity
Bowing of the legs HP:0002979 Human Phenotype Ontology (HP)
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"Standardized, well-positioned anterior-posterior standing limb alignment radiograph"
The guideline directly recommends standardized limb-alignment imaging.
Pseudofracture or Insufficiency-Fracture Line
Plain radiographs of symptomatic sites can reveal unhealed fracture or Looser-zone pseudofracture lines, indicating active osteomalacia rather than a pathognomonic XLH finding.
Xray Adults with focal bone pain or suspected osteomalacia
Show evidence (1 reference)
PMID:29947083 SUPPORT Human Clinical
"At week 24, 43.1% (burosumab) and 7.7% (placebo) of baseline active fractures were fully healed"
Trial radiographic assessment documents active baseline fracture and pseudofracture lesions and their healing.
Cranial-Suture Fusion
Skull or brain imaging confirms craniosynostosis and evaluates associated intracranial or craniocervical complications when clinical signs warrant it.
Abnormal skull shape or symptoms of intracranial hypertension
Craniosynostosis HP:0001363 Human Phenotype Ontology (HP)
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"We recommend a complete evaluation with fundoscopy and brain or skull imaging in any patient with XLH presenting with a skull morphology suggestive of craniosynostosis or clinical symptoms of intracranial hypertension"
The guideline limits cranial imaging to a clinically indicated setting rather than routine imaging of every patient.
Nephrocalcinosis on Renal Ultrasound
Renal ultrasonography detects calcium deposition as a complication relevant to long-term phosphate and active vitamin D exposure; it is not a diagnostic criterion for XLH.
Ultrasound Surveillance during conventional therapy or when hypercalciuria is present
Nephrocalcinosis HP:0000121 Human Phenotype Ontology (HP)
Show evidence (1 reference)
PMID:22319799 SUPPORT Other
"periodic renal ultrasound examination to assess for nephrocalcinosis"
GeneReviews supports renal ultrasound surveillance in conventionally treated patients.
📈

Progression

3
Infancy and early childhood
Age: Birth to approximately 2 years
Infants may be identified through an affected family before skeletal signs appear. In clinically presenting children, lower-extremity bowing commonly emerges after weight bearing begins during the first two years, accompanied by active rickets and impaired growth.
Show evidence (1 reference)
PMID:22319799 SUPPORT Other
"XLH typically manifests in the first two years of life with lower extremity bowing due to the onset of weight-bearing"
GeneReviews anchors the typical onset of skeletal manifestations to weight bearing in the first two years.
Childhood and adolescence
Age: Growing years
Persistent phosphate wasting sustains radiographic rickets, lower-limb deformity and disproportionate growth impairment. Dental infection can develop in structurally abnormal teeth, and a subset of children develop craniosynostosis or related intracranial complications.
Show evidence (2 references)
PMID:39814982 SUPPORT Other
"The clinical consequences of XLH can include rickets, osteomalacia, bone pain, leg deformities (Fig. 1), premature fusion of the cranial sutures and disproportionate short stature that usually develops during the first 2 years of life"
The recommendations summarize the major pediatric skeletal trajectory and early growth impairment.
PMID:40220947 SUPPORT Human Clinical
"The pooled prevalence of craniosynostosis among children with XLH was 22 % (95 % confidence interval (CI) 9.0 % to 44 %)"
Meta-analysis establishes craniosynostosis as an important but non-universal pediatric manifestation.
Adulthood and later life
Age: Adulthood
Adult disease is not simply residual childhood deformity: osteomalacia and pseudofractures can remain active while pain, stiffness, enthesopathy, osteoarthritis, dental-periodontal disease and impaired mobility accumulate. A 2026 untreated cohort illustrates substantial burden but should not be read as a population prevalence study or as proof of sex-determined severity.
Show evidence (3 references)
PMID:37752558 SUPPORT Human Clinical
"As a chronic disease resulting in progressive accumulation of musculoskeletal manifestations, it is important to understand the natural history of XLH over the patient's lifetime"
The international registry frames XLH as a lifelong disorder with accumulating musculoskeletal manifestations.
PMID:42429952 SUPPORT Human Clinical
"The phenotype in adult individuals with XLH was characterized by severe and disproportionate short stature, overweight/obesity, severe skeletal deformities with difficulty walking, osteoarticular pain, poor dental health, and reduced quality of life."
A contemporary untreated adult cohort documents the combined skeletal, mobility, pain, dental and quality-of-life burden.
PMID:42429952 SUPPORT Human Clinical
"Pseudofractures were documented in 17 patients (33%)."
The untreated cohort quantifies active pseudofracture burden without attributing it to treatment exposure.
📊

Prevalence

4
Populations in Southern Denmark, Japan, and Norway
Unknown 1.7–4.8 per 100,000 1–9 per 100,000
The 2025 recommendations derive this cross-setting range from studies in Southern Denmark, Japan, and Norway. The cited sentence does not identify a point or period denominator, so the measure subtype remains unknown.
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"The incidence of XLH is 3.9 per 100,000 live births and the prevalence ranges from 1.7 to 4.8 per 100,000 people"
The updated recommendations summarize the prevalence range across the three named study settings without specifying a point or period measure.
Live births in Southern Denmark
Birth Prevalence 3.9 per 100,000 1–9 per 100,000 (births)
The 2025 recommendations report 3.9 affected live births per 100,000, citing the Southern Denmark incidence and prevalence study. This live-birth denominator is represented as birth prevalence for structured comparison.
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"The incidence of XLH is 3.9 per 100,000 live births"
The live-birth denominator directly supports the normalized birth-prevalence record.
Australian pediatric population
Point Prevalence 1.33 per 100,000 (1.04–1.66) 1–9 per 100,000
Active pediatric surveillance estimated a minimum prevalence of 1.33 per 100,000 (95% CI 1.04-1.66); incomplete ascertainment probably made the true prevalence higher.
Show evidence (1 reference)
PMID:37263386 SUPPORT Human Clinical
"Estimated minimum prevalence based on these cases was 1.33 (1.04-1.66) per 100,000 and 1.60 per 100,000 (95%CI 0.97-2.58) in Australia and New Zealand respectively, with actual prevalence likely higher due to incomplete ascertainment."
This provides the Australian minimum pediatric point estimate and its confidence interval.
New Zealand pediatric population
Point Prevalence 1.6 per 100,000 (0.97–2.58) 1–9 per 100,000
Active pediatric surveillance estimated a minimum prevalence of 1.60 per 100,000 (95% CI 0.97-2.58); incomplete ascertainment probably made the true prevalence higher.
Show evidence (1 reference)
PMID:37263386 SUPPORT Human Clinical
"Estimated minimum prevalence based on these cases was 1.33 (1.04-1.66) per 100,000 and 1.60 per 100,000 (95%CI 0.97-2.58) in Australia and New Zealand respectively, with actual prevalence likely higher due to incomplete ascertainment."
This provides the New Zealand minimum pediatric point estimate and its confidence interval.
🔀

Differential Diagnoses

6

Conditions with similar clinical presentations that must be differentiated from X-Linked Hypophosphatemia:

Autosomal Dominant Hypophosphatemic Rickets Not Yet Curated MONDO:0008660
Overlapping Features ADHR is another FGF23-mediated phosphate-wasting rickets disorder and can overlap biochemically with XLH.
Distinguishing Features
  • Autosomal dominant rather than X-linked transmission.
  • Pathogenic FGF23 variants rather than PHEX variants.
  • Non-suppressed FGF23 does not distinguish ADHR from XLH, so pedigree and molecular testing are important.
Show evidence (1 reference)
PMID:11062477 SUPPORT Human Clinical
"We identified missense mutations in a gene encoding a new member of the fibroblast growth factor (FGF) family, FGF23"
The ADHR discovery study identifies FGF23 as its disease gene; it is not used as proof of the PHEX-to-FGF23 molecular link.
Autosomal Recessive Hypophosphatemic Rickets Not Yet Curated MONDO:0017324
Overlapping Features DMP1- and ENPP1-related recessive disorders can produce FGF23-mediated renal phosphate wasting, rickets and osteomalacia similar to XLH.
Distinguishing Features
  • Autosomal recessive pedigree rather than X-linked transmission.
  • Biallelic DMP1 or ENPP1 pathogenic variants rather than a PHEX variant.
  • Extra-skeletal features of the specific recessive disorder may redirect testing.
Show evidence (2 references)
PMID:39814982 SUPPORT Other
"autosomal-recessive hypophosphataemic rickets 1 (OMIM#241520)"
The updated recommendations explicitly include autosomal-recessive hypophosphataemic rickets in the non-suppressed-FGF23 differential.
PMID:39814982 SUPPORT Other
"other causes of genetic (DMP1, ENPP1, FGF23) or acquired forms of hypophosphataemia (such as tumour-induced osteomalacia) be considered if the analysis of the PHEX gene yields a negative result"
This directs gene-specific evaluation of DMP1- and ENPP1-related alternatives after negative PHEX testing.
Tumor-Induced Osteomalacia Not Yet Curated MONDO:0018124
Overlapping Features An acquired FGF23-secreting tumor can reproduce the adult biochemical and osteomalacic phenotype of XLH.
Distinguishing Features
  • Acquired FGF23-mediated presentation rather than X-linked transmission.
  • No causative germline PHEX variant.
  • Identification of an FGF23-secreting tumor establishes the acquired cause.
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"other causes of genetic (DMP1, ENPP1, FGF23) or acquired forms of hypophosphataemia (such as tumour-induced osteomalacia) be considered if the analysis of the PHEX gene yields a negative result"
The guideline specifically directs consideration of tumor-induced osteomalacia after negative PHEX testing.
Hereditary Hypophosphatemic Rickets with Hypercalciuria Not Yet Curated MONDO:0009431
Overlapping Features HHRH is a primary renal tubular phosphate-transport disorder rather than an FGF23-excess disorder.
Distinguishing Features
  • Low or suppressed FGF23 during hypophosphatemia.
  • Hypercalciuria and an increased calcitriol response favor HHRH over XLH.
  • Biallelic SLC34A3 variants establish the common genetic form.
Show evidence (2 references)
PMID:39814982 SUPPORT Other
"Hereditary hypophosphataemic rickets with hypercalciuria ... These diseases are characterized by low or suppressed levels of FGF23."
The guideline places HHRH among primary transporter-mediated disorders and describes the group as having low or suppressed FGF23.
PMID:39814982 SUPPORT Other
"urinary calcium excretion and 1,25(OH) 2D levels help to distinguish between FGF23-mediated diseases and primary tubular phosphate wasting ... increased levels of 1,25(OH)2D and hypercalciuria"
This directly supports the high-calcitriol, hypercalciuric and suppressed-FGF23 pattern of transporter-mediated phosphate wasting.
Fanconi Renotubular Syndrome MONDO:0001083
Overlapping Features Generalized proximal tubular dysfunction can cause hypophosphatemic rickets but is not isolated to phosphate handling.
Distinguishing Features
  • Metabolic acidosis or bicarbonate wasting.
  • Urinary glucose, amino acids and low-molecular-weight proteins in addition to phosphate.
  • Low or suppressed FGF23 during hypophosphatemia.
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"metabolic acidosis, hypercalciuria and renal Fanconi syndrome (that is, proximal renal tubular wasting that is not restricted to phosphate) should be excluded by measuring serum bicarbonate levels, and urinary excretion of calcium, amino acids, glucose and low-molecular-weight protein markers"
This provides the specific biochemical tests that distinguish generalized Fanconi wasting from isolated XLH phosphate wasting.
Calcipenic or Nutritional Rickets
Overlapping Features Vitamin D or calcium deficiency can produce rickets and elevated alkaline phosphatase without the isolated FGF23-mediated renal phosphate-wasting pattern of XLH.
Distinguishing Features
  • Vitamin D or calcium deficiency and high PTH activity favor calcipenic rickets.
  • Renal phosphate loss is secondary to PTH rather than inappropriately non-suppressed FGF23.
  • Correction of the nutritional or mineral deficiency separates it from inherited XLH.
Show evidence (1 reference)
PMID:39814982 SUPPORT Other
"The differential diagnosis of XLH is based on the mechanisms leading to hypophosphataemia ... high parathyroid hormone (PTH) activity (leading to calcipenic rickets)"
The guideline distinguishes calcipenic from phosphopenic mechanisms at the start of the diagnostic work-up.
{ }

Source YAML

click to show 
name: X-Linked Hypophosphatemia
synonyms:
- XLH
- X-linked hypophosphataemia
- X-linked dominant hypophosphatemic rickets
- X-linked hypophosphatemic rickets
creation_date: '2026-02-13T00:31:42Z'
category: Mendelian
description: >-
  X-linked hypophosphatemia (XLH) is a PHEX-related, X-linked renal
  phosphate-wasting disorder. Germline loss-of-function PHEX variants cause
  excess bone-derived FGF23 through an incompletely resolved molecular link;
  FGF23 reduces renal phosphate reabsorption and dysregulates calcitriol,
  producing lifelong hypophosphatemia, childhood rickets, adult osteomalacia,
  skeletal deformity, dental disease, and progressive musculoskeletal burden.
disease_term:
  preferred_term: X-linked dominant hypophosphatemic rickets
  term:
    id: MONDO:0010619
    label: X-linked dominant hypophosphatemic rickets
parents:
- Metabolic Bone Disorders
references:
- reference: PMID:22319799
  title: X-Linked Hypophosphatemia.
  tags:
  - GeneReviews
- reference: PMID:39814982
  title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
inheritance:
- name: X-linked dominant inheritance
  inheritance_term:
    preferred_term: X-linked dominant inheritance
    term:
      id: HP:0001423
      label: X-linked dominant inheritance
  expressivity: VARIABLE
  description: >-
    Hemizygous males and heterozygous females can both be clinically affected,
    and intrafamilial severity is variable rather than reliably predicted by
    sex. An affected male transmits the PHEX variant to all daughters and no
    sons; an affected female has a 50% chance of transmission to each child.
  evidence:
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "XLH is inherited in an X-linked manner; hemizygous males and heterozygous females are similarly affected."
    explanation: GeneReviews explicitly describes the X-linked inheritance pattern and clinical involvement of both sexes.
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Affected males transmit the PHEX pathogenic variant to all of their daughters (who will be heterozygotes and will be affected) and none of their sons."
    explanation: This directly supports father-to-daughter and absent father-to-son transmission.
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Affected females have a 50% chance of transmitting the pathogenic variant to each child: male and female offspring who inherit the pathogenic variant will be affected."
    explanation: This directly supports the one-in-two transmission risk from an affected heterozygous female to each child.
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The severity of manifestations can differ among family members who inherit a PHEX pathogenic variant; intrafamilial clinical variability does not correlate with the sex of the affected family member."
    explanation: This supports variable expressivity and avoids a categorical claim that males are always more severely affected.
classifications:
  isds_skeletal_category:
  - classification_value: abnormal_mineralization
    notes: >-
      ISDS Nosology and Classification of Genetic Skeletal Disorders, 2023
      revision (Unger et al., PMID:36779427), group 27 "Disorders of bone
      mineralisation"; listed as PHEX-related X-linked hypophosphataemic
      rickets.
prevalence:
- population: Populations in Southern Denmark, Japan, and Norway
  measure_type: UNKNOWN
  prevalence_class: BAND_1_9_PER_100000
  rate_low: 1.7
  rate_high: 4.8
  notes: >-
    The 2025 recommendations derive this cross-setting range from studies in
    Southern Denmark, Japan, and Norway. The cited sentence does not identify a
    point or period denominator, so the measure subtype remains unknown.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The incidence of XLH is 3.9 per 100,000 live births and the prevalence ranges from 1.7 to 4.8 per 100,000 people"
    explanation: The updated recommendations summarize the prevalence range across the three named study settings without specifying a point or period measure.
- population: Live births in Southern Denmark
  measure_type: BIRTH_PREVALENCE
  prevalence_class: BAND_1_9_PER_100000
  rate_per_100000: 3.9
  notes: >-
    The 2025 recommendations report 3.9 affected live births per 100,000, citing
    the Southern Denmark incidence and prevalence study. This live-birth
    denominator is represented as birth prevalence for structured comparison.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The incidence of XLH is 3.9 per 100,000 live births"
    explanation: The live-birth denominator directly supports the normalized birth-prevalence record.
- population: Australian pediatric population
  measure_type: POINT_PREVALENCE
  prevalence_class: BAND_1_9_PER_100000
  rate_per_100000: 1.33
  rate_low: 1.04
  rate_high: 1.66
  notes: >-
    Active pediatric surveillance estimated a minimum prevalence of 1.33 per
    100,000 (95% CI 1.04-1.66); incomplete ascertainment probably made the true
    prevalence higher.
  evidence:
  - reference: PMID:37263386
    reference_title: "Prevalence and characteristics of paediatric X-linked hypophosphataemia in Australia and New Zealand: Results from the Australian and the New Zealand Paediatric Surveillance Units survey."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Estimated minimum prevalence based on these cases was 1.33 (1.04-1.66) per 100,000 and 1.60 per 100,000 (95%CI 0.97-2.58) in Australia and New Zealand respectively, with actual prevalence likely higher due to incomplete ascertainment."
    explanation: This provides the Australian minimum pediatric point estimate and its confidence interval.
- population: New Zealand pediatric population
  measure_type: POINT_PREVALENCE
  prevalence_class: BAND_1_9_PER_100000
  rate_per_100000: 1.6
  rate_low: 0.97
  rate_high: 2.58
  notes: >-
    Active pediatric surveillance estimated a minimum prevalence of 1.60 per
    100,000 (95% CI 0.97-2.58); incomplete ascertainment probably made the true
    prevalence higher.
  evidence:
  - reference: PMID:37263386
    reference_title: "Prevalence and characteristics of paediatric X-linked hypophosphataemia in Australia and New Zealand: Results from the Australian and the New Zealand Paediatric Surveillance Units survey."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Estimated minimum prevalence based on these cases was 1.33 (1.04-1.66) per 100,000 and 1.60 per 100,000 (95%CI 0.97-2.58) in Australia and New Zealand respectively, with actual prevalence likely higher due to incomplete ascertainment."
    explanation: This provides the New Zealand minimum pediatric point estimate and its confidence interval.
progression:
- phase: Infancy and early childhood
  age_range: Birth to approximately 2 years
  notes: >-
    Infants may be identified through an affected family before skeletal signs
    appear. In clinically presenting children, lower-extremity bowing commonly
    emerges after weight bearing begins during the first two years, accompanied
    by active rickets and impaired growth.
  evidence:
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "XLH typically manifests in the first two years of life with lower extremity bowing due to the onset of weight-bearing"
    explanation: GeneReviews anchors the typical onset of skeletal manifestations to weight bearing in the first two years.
- phase: Childhood and adolescence
  age_range: Growing years
  notes: >-
    Persistent phosphate wasting sustains radiographic rickets, lower-limb
    deformity and disproportionate growth impairment. Dental infection can
    develop in structurally abnormal teeth, and a subset of children develop
    craniosynostosis or related intracranial complications.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The clinical consequences of XLH can include rickets, osteomalacia, bone pain, leg deformities (Fig. 1), premature fusion of the cranial sutures and disproportionate short stature that usually develops during the first 2 years of life"
    explanation: The recommendations summarize the major pediatric skeletal trajectory and early growth impairment.
  - reference: PMID:40220947
    reference_title: "Craniosynostosis among children with X-linked hypophosphatemia: A systematic review and meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The pooled prevalence of craniosynostosis among children with XLH was 22 % (95 % confidence interval (CI) 9.0 % to 44 %)"
    explanation: Meta-analysis establishes craniosynostosis as an important but non-universal pediatric manifestation.
- phase: Adulthood and later life
  age_range: Adulthood
  notes: >-
    Adult disease is not simply residual childhood deformity: osteomalacia and
    pseudofractures can remain active while pain, stiffness, enthesopathy,
    osteoarthritis, dental-periodontal disease and impaired mobility accumulate.
    A 2026 untreated cohort illustrates substantial burden but should not be
    read as a population prevalence study or as proof of sex-determined severity.
  evidence:
  - reference: PMID:37752558
    reference_title: "The International X-Linked Hypophosphatemia (XLH) Registry: first interim analysis of baseline demographic, genetic and clinical data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "As a chronic disease resulting in progressive accumulation of musculoskeletal manifestations, it is important to understand the natural history of XLH over the patient's lifetime"
    explanation: The international registry frames XLH as a lifelong disorder with accumulating musculoskeletal manifestations.
  - reference: PMID:42429952
    reference_title: Insight into Natural History and Phenotype in Untreated Adults with X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The phenotype in adult individuals with XLH was characterized by severe and disproportionate short stature, overweight/obesity, severe skeletal deformities with difficulty walking, osteoarticular pain, poor dental health, and reduced quality of life."
    explanation: A contemporary untreated adult cohort documents the combined skeletal, mobility, pain, dental and quality-of-life burden.
  - reference: PMID:42429952
    reference_title: Insight into Natural History and Phenotype in Untreated Adults with X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Pseudofractures were documented in 17 patients (33%)."
    explanation: The untreated cohort quantifies active pseudofracture burden without attributing it to treatment exposure.
pathophysiology:
- name: PHEX Loss of Function in Mineralizing Cells
  biological_scale: MOLECULAR
  description: >-
    Germline inactivating PHEX variants are the proximal defect. PHEX is
    expressed mainly in osteoblasts, osteocytes and odontoblasts, but the exact
    molecular route from PHEX deficiency to increased FGF23 production remains
    unresolved.
  genetic_context:
    gene:
      preferred_term: PHEX
      term:
        id: hgnc:8918
        label: PHEX
    functional_impact_category: LOSS_OF_FUNCTION
    description: Germline loss of PHEX function underlies the X-linked disorder.
  genes:
  - preferred_term: PHEX
    term:
      id: hgnc:8918
      label: PHEX
  cell_types:
  - preferred_term: osteoblast
    term:
      id: CL:0000062
      label: osteoblast
  - preferred_term: osteocyte
    term:
      id: CL:0000137
      label: osteocyte
  downstream:
  - target: Excess Bone-Derived FGF23 Signaling
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: PHEX loss is upstream of increased FGF23 synthesis, but the intervening molecular coupling is incompletely understood.
  evidence:
  - reference: PMID:11062477
    reference_title: Autosomal dominant hypophosphataemic rickets is associated with mutations in FGF23.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Inactivating mutations of the gene PHEX, encoding a member of the neutral endopeptidase family of proteins, are responsible for XLH"
    explanation: This establishes inactivating PHEX variants as the disease cause without using the ADHR finding as evidence for the unresolved PHEX-to-FGF23 coupling step.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "X-linked hypophosphataemia (XLH) is a rare metabolic bone disorder caused by pathogenic variants in the PHEX gene, which is predominantly expressed in osteoblasts, osteocytes and odontoblasts."
    explanation: The guideline supports the causal gene and its principal mineralizing-cell expression sites.
- name: Excess Bone-Derived FGF23 Signaling
  biological_scale: ORGANISM
  description: >-
    PHEX loss produces excess synthesis and circulating activity of the
    bone-derived phosphaturic hormone FGF23. This endocrine signal acts on the
    kidney to reduce phosphate reclamation and alter vitamin D metabolism.
  biological_processes:
  - preferred_term: phosphate ion homeostasis
    term:
      id: GO:0055062
      label: phosphate ion homeostasis
    modifier: ABNORMAL
  downstream:
  - target: Reduced Proximal Tubular Phosphate Reabsorption
    causal_link_type: DIRECT
    description: Excess FGF23 downregulates proximal tubular sodium-phosphate transport and causes phosphaturia.
  - target: FGF23-Driven Calcitriol Dysregulation
    causal_link_type: DIRECT
    description: Excess FGF23 decreases calcitriol synthesis and increases its degradation in proximal renal tubules.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "XLH is characterized by increased synthesis of the bone-derived phosphaturic hormone fibroblast growth factor 23 (FGF23), which results in renal phosphate wasting with consecutive hypophosphataemia"
    explanation: This directly supports the endocrine FGF23 excess and its renal phosphate-wasting consequence.
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "inherited loss-of-function mutations in the PHEX gene cause excess circulating levels of fibroblast growth factor 23 (FGF23), leading to lifelong renal phosphate wasting and hypophosphatemia"
    explanation: The adult trial background independently states the PHEX-FGF23-phosphate axis in XLH.
- name: Reduced Proximal Tubular Phosphate Reabsorption
  biological_scale: CELLULAR
  description: >-
    FGF23 signaling in renal proximal tubules downregulates the NPT2a and NPT2c
    sodium-phosphate transporters, lowering tubular phosphate reabsorption and
    causing persistent urinary phosphate loss despite hypophosphatemia.
  biological_processes:
  - preferred_term: phosphate ion transport
    term:
      id: GO:0006817
      label: phosphate ion transport
    modifier: DECREASED
  chemical_entities:
  - preferred_term: phosphate
    term:
      id: CHEBI:26020
      label: phosphate
    modifier: DECREASED
  cell_types:
  - preferred_term: epithelial cell of proximal tubule
    term:
      id: CL:0002306
      label: epithelial cell of proximal tubule
  downstream:
  - target: Reduced Circulating Phosphate
    causal_link_type: DIRECT
    description: Ongoing renal phosphate loss lowers the circulating phosphate concentration.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "downregulation of the sodium-dependent phosphate transporters NPT2a (encoded by SLC34A1) and NPT2c (encoded by SLC34A3)"
    explanation: The updated recommendations specify the proximal tubular transporter mechanism.
  - reference: PMID:7550339
    reference_title: "A gene (PEX) with homologies to endopeptidases is mutated in patients with X-linked hypophosphatemic rickets. The HYP Consortium."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "impaired phosphate uptake in the kidney, which is likely to be caused by abnormal regulation of sodium phosphate cotransport in the proximal tubules"
    explanation: The original PHEX report links XLH to abnormal proximal tubular sodium-phosphate cotransport.
- name: FGF23-Driven Calcitriol Dysregulation
  biological_scale: MOLECULAR
  description: >-
    Excess FGF23 decreases renal production and increases degradation of
    1,25-dihydroxyvitamin D. Calcitriol is therefore inappropriately low or
    normal for the degree of hypophosphatemia, limiting compensatory intestinal
    mineral absorption.
  biological_processes:
  - preferred_term: vitamin D metabolic process
    term:
      id: GO:0042359
      label: vitamin D metabolic process
    modifier: ABNORMAL
  downstream:
  - target: Skeletal Mineralization Failure
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - reduced vitamin D-dependent intestinal calcium and phosphate absorption
    description: An inadequate calcitriol response compounds the substrate deficit for mineralization.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "a decrease in 1,25-dihydroxyvitamin D (1,25(OH)2D) synthesis and an increase in 1,25(OH)2D degradation in the proximal renal tubules"
    explanation: This directly supports both directions of FGF23-mediated calcitriol dysregulation.
- name: Reduced Circulating Phosphate
  conforms_to: "defective_skeletal_mineralization#Phosphopenic Substrate Deficiency"
  biological_scale: ORGANISM
  description: >-
    Lifelong renal phosphate wasting produces serum phosphate below the
    age-adjusted reference range. This is a causally active systemic substrate
    deficit as well as the central biochemical finding used in diagnosis and
    treatment monitoring.
  chemical_entities:
  - preferred_term: phosphate
    term:
      id: CHEBI:26020
      label: phosphate
    modifier: DECREASED
  downstream:
  - target: Skeletal Mineralization Failure
    causal_link_type: DIRECT
    description: Insufficient extracellular phosphate impairs growth-plate and osteoid mineralization.
  - target: Dentoalveolar Mineralization Defect
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Chronic hypophosphatemia contributes to dental mineralization failure, with additional local PHEX-dependent mechanisms not fully resolved.
  - target: Hypophosphatemia
    causal_link_type: DIRECT
    description: The reduced circulating-phosphate state is expressed clinically as hypophosphatemia below the age-adjusted reference range.
    evidence:
    - reference: PMID:29947083
      reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "leading to lifelong renal phosphate wasting and hypophosphatemia"
      explanation: The adult trial background directly connects persistent renal phosphate wasting with the hypophosphatemic state.
  evidence:
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "leading to lifelong renal phosphate wasting and hypophosphatemia"
    explanation: The adult trial explicitly identifies the lifelong systemic phosphate deficit.
- name: Skeletal Mineralization Failure
  conforms_to: "defective_skeletal_mineralization#Impaired Hydroxyapatite Deposition at the Mineralization Front"
  biological_scale: TISSUE
  description: >-
    Chronic phosphate deficiency impairs mineralization at open growth plates
    and in bone osteoid. The age-dependent tissue expressions are rickets in
    growing children and osteomalacia, insufficiency fractures and
    pseudofractures after growth-plate closure.
  biological_processes:
  - preferred_term: bone mineralization
    term:
      id: GO:0030282
      label: bone mineralization
    modifier: DECREASED
  cell_types:
  - preferred_term: osteoblast
    term:
      id: CL:0000062
      label: osteoblast
  - preferred_term: chondrocyte
    term:
      id: CL:0000138
      label: chondrocyte
  downstream:
  - target: Rickets
    causal_link_type: DIRECT
    description: Failure to mineralize the growing skeleton produces rickets.
  - target: Osteomalacia
    causal_link_type: DIRECT
    description: Failure to mineralize osteoid produces adult osteomalacia.
  - target: Lower-Limb Deformity
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - weight bearing on rachitic bone
    description: Weight bearing on undermineralized growing bone promotes varus or valgus deformity.
  - target: Short Stature
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - growth-plate rickets and lower-limb shortening
    description: Persistent growth-plate disease impairs linear growth and disproportionately shortens the legs.
  - target: Pseudofractures and Insufficiency Fractures
    causal_link_type: DIRECT
    description: Osteomalacic bone develops non-traumatic pseudofractures and insufficiency fractures.
  - target: Bone and Musculoskeletal Pain
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - adult osteomalacia and insufficiency injury
    description: Active osteomalacia and insufficiency injury contribute to bone and musculoskeletal pain, although structural joint disease can add other pain generators.
    evidence:
    - reference: PMID:39814982
      reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Treatment with active vitamin D and phosphate supplements has been shown to improve musculoskeletal ... pain and stiffness, to ameliorate osteomalacia"
      explanation: Concurrent improvement of pain and osteomalacia supports a mineralization-mediated pain component but does not exclude structural pain generators.
  - target: Joint Stiffness
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: Mineralization disease can contribute to adult stiffness, but coexisting osteoarthritis and enthesopathy prevent assignment of a single settled intermediate mechanism.
    evidence:
    - reference: PMID:39814982
      reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "Treatment with active vitamin D and phosphate supplements has been shown to improve musculoskeletal ... pain and stiffness, to ameliorate osteomalacia"
      explanation: Treatment-associated improvement supports a mineralization-responsive stiffness component while the unknown-intermediate edge preserves competing structural causes.
  - target: Gait Disturbance and Impaired Mobility
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - lower-limb deformity, pain, stiffness and insufficiency-fracture burden
    description: The combined skeletal consequences restrict walking and mobility.
    evidence:
    - reference: PMID:42429952
      reference_title: Insight into Natural History and Phenotype in Untreated Adults with X-Linked Hypophosphatemia.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "severe skeletal deformities with difficulty walking"
      explanation: The untreated adult cohort directly links severe skeletal deformity with walking difficulty.
  evidence:
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "fractures, and pseudofractures due to osteomalacia"
    explanation: The adult trial directly attributes fractures and pseudofractures to osteomalacia.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "XLH is characterized by increased synthesis of the bone-derived phosphaturic hormone fibroblast growth factor 23 (FGF23), which results in renal phosphate wasting with consecutive hypophosphataemia, rickets, osteomalacia, disproportionate short stature"
    explanation: The guideline supports the systemic mechanism and principal skeletal consequences.
- name: Dentoalveolar Mineralization Defect
  biological_scale: TISSUE
  description: >-
    Impaired dentin and alveolar-bone mineralization creates structurally weak
    teeth with enlarged or exposed pulp pathways and susceptibility to
    endodontic infection, including abscesses in teeth that can appear sound.
  downstream:
  - target: Dental Abscesses and Periodontal Disease
    causal_link_type: DIRECT
    description: Intrinsic dental structural defects predispose to pulpal and periapical infection and early tooth loss.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Patients also show hypomineralization of teeth and are prone to tooth abscesses and periodontitis"
    explanation: The guideline directly links tooth hypomineralization with abscess and periodontal susceptibility.
  - reference: PMID:42409089
    reference_title: "Diagnosis and management of X-linked hypophosphatemia in dental practice: A scoping review."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "X-linked hypophosphatemia (XLH) is a PHEX-related disorder causing elevated FGF23, chronic hypophosphatemia, and impaired skeletal and dental mineralization, leading to frequent endodontic abscess in clinically sound teeth due to intrinsic structural failure, periapical lesions and early tooth loss."
    explanation: The dental scoping review supports the structural basis and characteristic infection pattern.
phenotypes:
- name: Rickets
  description: >-
    Active growth-plate mineralization failure in children, often accompanied by
    metaphyseal changes, leg bowing and impaired growth.
  phenotype_term:
    preferred_term: Rickets
    term:
      id: HP:0002748
      label: Rickets
  evidence:
  - reference: PMID:31104833
    reference_title: "Burosumab versus conventional therapy in children with X-linked hypophosphataemia: a randomised, active-controlled, open-label, phase 3 trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "X-linked hypophosphataemia in children is characterised by elevated serum concentrations of fibroblast growth factor 23 (FGF23), hypophosphataemia, rickets, lower extremity bowing, and growth impairment."
    explanation: The pediatric phase 3 trial identifies rickets as a cardinal childhood manifestation.
- name: Osteomalacia
  description: >-
    Persistent adult osteoid mineralization failure, clinically expressed by
    bone pain, insufficiency fractures and pseudofractures.
  phenotype_term:
    preferred_term: Osteomalacia
    term:
      id: HP:0002749
      label: Osteomalacia
  evidence:
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "fractures, and pseudofractures due to osteomalacia"
    explanation: This directly supports active adult osteomalacia and its fracture manifestations.
- name: Lower-Limb Deformity
  description: >-
    Varus or valgus lower-limb deformity develops under weight bearing and may
    persist despite medical treatment.
  phenotype_term:
    preferred_term: Bowing of the legs
    term:
      id: HP:0002979
      label: Bowing of the legs
  evidence:
  - reference: PMID:31104833
    reference_title: "Burosumab versus conventional therapy in children with X-linked hypophosphataemia: a randomised, active-controlled, open-label, phase 3 trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "rickets, lower extremity bowing, and growth impairment"
    explanation: The pediatric trial supports lower-extremity bowing as a characteristic manifestation.
- name: Short Stature
  description: >-
    Disproportionate short stature, especially lower-limb shortening, originates
    in the growing years and commonly persists in adults.
  phenotype_term:
    preferred_term: Short stature
    term:
      id: HP:0004322
      label: Short stature
  evidence:
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Adults with XLH present with chronic musculoskeletal pain and stiffness, short stature, lower limb deformities"
    explanation: The adult trial supports persistent short stature and lower-limb deformity.
- name: Pseudofractures and Insufficiency Fractures
  description: >-
    Non-traumatic Looser-zone pseudofractures and insufficiency fractures are
    manifestations of adult osteomalacia and can remain unhealed without
    effective metabolic treatment.
  phenotype_term:
    preferred_term: Pathologic fracture
    term:
      id: HP:0002756
      label: Pathologic fracture
  evidence:
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "fractures, and pseudofractures due to osteomalacia"
    explanation: The trial directly supports both fracture categories while the HPO mapping captures fractures arising in metabolically weakened bone.
- name: Bone and Musculoskeletal Pain
  description: >-
    Chronic pain is a major adult burden; etiologies can include active
    osteomalacia as well as structural osteoarthritis, enthesopathy and
    deformity, which should not be assumed to share one mechanism.
  phenotype_term:
    preferred_term: Bone pain
    term:
      id: HP:0002653
      label: Bone pain
  evidence:
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Adults with XLH present with chronic musculoskeletal pain and stiffness"
    explanation: This directly supports chronic musculoskeletal pain in adults.
- name: Joint Stiffness
  description: >-
    Stiffness is common in symptomatic adults and can reflect several coexisting
    skeletal and joint processes rather than a single modeled causal mechanism.
  phenotype_term:
    preferred_term: Joint stiffness
    term:
      id: HP:0001387
      label: Joint stiffness
  evidence:
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Adults with XLH present with chronic musculoskeletal pain and stiffness"
    explanation: This directly supports stiffness as a characteristic adult symptom.
- name: Gait Disturbance and Impaired Mobility
  description: >-
    Lower-limb deformity, pain, stiffness and fracture burden can produce an
    abnormal gait and reduced mobility across adult life.
  phenotype_term:
    preferred_term: Gait disturbance
    term:
      id: HP:0001288
      label: Gait disturbance
  evidence:
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Adults may present with calcification of the tendons, ligaments, and joint capsules, joint pain, fatigue, insufficiency fractures, and impaired mobility."
    explanation: GeneReviews explicitly includes impaired mobility among adult manifestations.
  - reference: PMID:42429952
    reference_title: Insight into Natural History and Phenotype in Untreated Adults with X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "severe skeletal deformities with difficulty walking"
    explanation: The untreated adult cohort corroborates clinically important walking difficulty.
- name: Dental Abscesses and Periodontal Disease
  description: >-
    Endodontic abscesses may occur in clinically sound teeth; periapical disease,
    periodontitis and early tooth loss contribute substantial lifelong burden.
  phenotype_term:
    preferred_term: Tooth abscess
    term:
      id: HP:0030757
      label: Tooth abscess
  evidence:
  - reference: PMID:42409089
    reference_title: "Diagnosis and management of X-linked hypophosphatemia in dental practice: A scoping review."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "frequent endodontic abscess in clinically sound teeth due to intrinsic structural failure, periapical lesions and early tooth loss"
    explanation: The dental review describes the characteristic infection pattern and consequences.
- name: Enthesopathy
  description: >-
    Calcification and ossification at tendon, ligament and joint-capsule
    attachments accumulate in adults. Its pathogenesis and relationship to
    primary XLH mechanisms remain unresolved, so it is not assigned a causal
    edge from osteomalacia.
  review_notes: >-
    NEEDS_TERM: HP:0100686 denotes inflammatory enthesitis and would misrepresent
    the calcific or ossifying enthesopathy of XLH; no suitably specific HPO term
    is assigned pending a dedicated ontology term.
  evidence:
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "accelerated osteoarthritis, dental abscesses, and enthesopathy"
    explanation: The adult trial establishes the association without resolving causation.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "enthesopathies and to what extent these are sequelae of certain primary manifestations of XLH is poorly understood"
    explanation: The guideline explicitly marks the underlying causal interpretation as uncertain.
- name: Osteoarthritis
  description: >-
    Premature and progressive degenerative joint disease is common in adults,
    but whether it is a primary PHEX/FGF23 effect or secondary to altered
    biomechanics and deformity remains uncertain.
  phenotype_term:
    preferred_term: Premature osteoarthritis
    term:
      id: HP:0003088
      label: Premature osteoarthritis
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Adult patients with XLH may show pseudofractures due to osteomalacia as well as osteoarthritis, enthesopathies, spinal stenosis, hearing loss, depression and reduced quality of life"
    explanation: The guideline supports osteoarthritis as part of adult XLH burden.
- name: Craniosynostosis
  description: >-
    Premature cranial-suture fusion occurs in a subset of children and can be
    associated with abnormal skull shape or intracranial complications.
  phenotype_term:
    preferred_term: Craniosynostosis
    term:
      id: HP:0001363
      label: Craniosynostosis
  evidence:
  - reference: PMID:40220947
    reference_title: "Craniosynostosis among children with X-linked hypophosphatemia: A systematic review and meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The pooled prevalence of craniosynostosis among children with XLH was 22 % (95 % confidence interval (CI) 9.0 % to 44 %)"
    explanation: Meta-analysis supports the association and its non-universal frequency.
- name: Hearing Impairment
  description: >-
    Sensorineural or other hearing impairment is reported, especially in adults;
    the direct inner-ear mechanism is not established here.
  phenotype_term:
    preferred_term: Hearing impairment
    term:
      id: HP:0000365
      label: Hearing impairment
  evidence:
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "sensorineural hearing loss has also been reported"
    explanation: GeneReviews supports hearing loss as an associated manifestation without implying a settled mechanism.
- name: Spinal Canal Stenosis
  description: >-
    Spinal canal stenosis is a rare adult structural complication. Because its
    causal relationship to osteomalacia, enthesopathy and degenerative change is
    not resolved, it is not assigned a single pathophysiology edge.
  phenotype_term:
    preferred_term: Spinal canal stenosis
    term:
      id: HP:0003416
      label: Spinal canal stenosis
  evidence:
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Rarely, individuals with XLH can suffer from spinal stenosis, Chiari I malformation, syringomyelia, and/or raised intracranial pressure."
    explanation: GeneReviews directly identifies spinal stenosis as a rare XLH manifestation.
- name: Hypophosphatemia
  description: >-
    Serum phosphate remains chronically below the age-related reference range
    because renal phosphate wasting persists throughout life.
  phenotype_term:
    preferred_term: Hypophosphatemia
    term:
      id: HP:0002148
      label: Hypophosphatemia
  evidence:
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "leading to lifelong renal phosphate wasting and hypophosphatemia"
    explanation: This establishes lifelong low serum phosphate as the core biochemical phenotype.
- name: Nephrocalcinosis
  description: >-
    Renal calcium deposition is observed in XLH cohorts and is particularly
    relevant as a treatment-associated complication during conventional oral
    phosphate and active vitamin D therapy. Cohort association alone does not
    establish that every case is treatment-caused.
  phenotype_term:
    preferred_term: Nephrocalcinosis
    term:
      id: HP:0000121
      label: Nephrocalcinosis
  evidence:
  - reference: PMID:29460029
    reference_title: Outcome of adult patients with X-linked hypophosphatemia caused by PHEX gene mutations.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Dental disease (63%), nephrocalcinosis (42%), and hearing impairment (14%) were also common"
    explanation: The adult cohort supports prevalence of nephrocalcinosis but is not used alone to infer treatment causality.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Oral phosphate supplements and active vitamin D can also promote hyperparathyroidism and cause nephrocalcinosis in a dose-dependent manner"
    explanation: The guideline specifically identifies nephrocalcinosis as an adverse effect associated with conventional therapy.
genetic:
- name: Germline PHEX Pathogenic Variants
  gene_term:
    preferred_term: PHEX
    term:
      id: hgnc:8918
      label: PHEX
  association: Causative
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  notes: >-
    Inactivating PHEX variants include deletions, splice variants, frameshifts,
    nonsense and missense alleles. Allelic heterogeneity is extensive. The
    causal gene-disease relationship is definitive, whereas the downstream
    molecular route to FGF23 excess remains incompletely resolved.
  evidence:
  - reference: PMID:7550339
    reference_title: "A gene (PEX) with homologies to endopeptidases is mutated in patients with X-linked hypophosphatemic rickets. The HYP Consortium."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Intragenic non-overlapping deletions from four different families and three mutations (two splice sites and one frameshift) have been detected in HYP patients"
    explanation: The original positional-cloning study documents multiple independent disruptive PHEX alleles in affected families.
  - reference: PMID:11062477
    reference_title: Autosomal dominant hypophosphataemic rickets is associated with mutations in FGF23.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Inactivating mutations of the gene PHEX, encoding a member of the neutral endopeptidase family of proteins, are responsible for XLH"
    explanation: This independently states the inactivating PHEX disease mechanism.
  - reference: CGGV:assertion_7b966545-e9dc-4a75-8d6a-91b328bc4854-2025-05-15T060000.000Z
    reference_title: PHEX / X-linked dominant hypophosphatemic rickets (Definitive)
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "PHEX | HGNC:8918 | X-linked dominant hypophosphatemic rickets | MONDO:0010619 | XL | Definitive"
    explanation: ClinGen classifies the PHEX-XLH gene-disease relationship as definitive with X-linked inheritance.
diagnosis:
- name: Integrated Clinical, Biochemical and Radiographic Evaluation
  diagnosis_term:
    preferred_term: Clinical Evaluation
    term:
      id: NCIT:C124351
      label: Clinical Evaluation
  description: >-
    In a growing child, consider XLH when rickets or impaired growth coexists
    with phosphate below the age-related reference range and isolated renal
    phosphate wasting after calcium and vitamin D deficiency are excluded. In
    adults, historical lower-limb deformity, osteomalacia or pseudofractures,
    early osteoarthritis, spinal disease, dental abscesses or enthesopathy can
    prompt the same biochemical evaluation.
  results: >-
    A compatible phenotype plus hypophosphatemia and renal phosphate wasting;
    non-suppressed intact FGF23 and an X-linked family history support the
    diagnosis but neither feature is individually specific.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "In children and growing adolescents, a diagnosis of X-linked hypophosphataemia (XLH) should be considered in the presence of or if there is a history of clinical, biochemical, and/or radiological signs of rickets, impaired growth velocity and serum levels of phosphate below the age-related reference range associated with isolated renal phosphate wasting and in the absence of vitamin D or calcium deficiency"
    explanation: The guideline supplies the age-specific integrated diagnostic pattern for growing patients.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "If molecular genetic analysis is not available, a family history of X-linked inheritance, as well as non-suppressed plasma levels of intact FGF23 in association with hypophosphataemia support the diagnosis of XLH"
    explanation: This supports a clinical-biochemical diagnosis when molecular confirmation is unavailable.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "In adults and adolescents with closed growth plates, diagnosis of XLH should be considered in the presence of or if there is a history of lower-limb deformities, clinical, biochemical and/or radiological signs of osteomalacia (including pseudofractures, early osteoarthritis, spinal degeneration and stenosis, dental abscesses and enthesopathies) in the context of serum levels of phosphate below the age-related reference range associated with isolated renal phosphate wasting"
    explanation: The guideline supplies the adult integrated diagnostic pattern and its major historical clues.
- name: PHEX Molecular Genetic Testing
  diagnosis_term:
    preferred_term: Genetic Testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  description: >-
    Molecular analysis of PHEX confirms the suspected diagnosis when a
    pathogenic hemizygous variant in a male or heterozygous variant in a female
    is identified. If PHEX testing is negative, reassess the phenotype and test
    for alternative genetic or acquired FGF23-mediated disorders rather than
    treating the negative result as exclusion of all hypophosphatemic disease.
  results: Pathogenic or likely pathogenic germline PHEX variant.
  evidence:
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The diagnosis is established in a proband with characteristic clinical, biochemical, and radiographic findings by identification of a hemizygous PHEX pathogenic variant in a male proband or a heterozygous PHEX pathogenic variant in a female proband on molecular genetic testing."
    explanation: GeneReviews defines the molecular confirmation pattern in the context of compatible clinical findings.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "We recommend that other causes of genetic (DMP1, ENPP1, FGF23) or acquired forms of hypophosphataemia (such as tumour-induced osteomalacia) be considered if the analysis of the PHEX gene yields a negative result"
    explanation: The updated guideline defines the required next step after negative PHEX testing.
biochemical:
- name: Low Serum Phosphate
  presence: Decreased below the age-related reference range
  context: >-
    Interpret phosphate using age-appropriate reference intervals and sampling
    context; fasting sampling is recommended in adults.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "serum levels of phosphate below the age-related reference range associated with isolated renal phosphate wasting"
    explanation: This is the core biochemical diagnostic combination.
- name: Reduced TmP/GFR and Renal Phosphate Wasting
  presence: Decreased tubular phosphate reabsorption
  context: >-
    Calculate TmP/GFR from paired serum and urine phosphate and creatinine;
    interpret with age-adjusted reference values, especially in children.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "urinary levels of calcium, phosphate and creatinine for calculation of the tubular maximum reabsorption of phosphate per glomerular filtration rate (TmP/GFR) and urinary calcium to creatinine ratio"
    explanation: The guideline specifies the measurements used to establish renal phosphate wasting and assess calcium excretion.
- name: Intact FGF23
  presence: Inappropriately non-suppressed or elevated during hypophosphatemia
  specificity: >-
    Supports an FGF23-mediated process but does not by itself distinguish XLH
    from ADHR, ARHR, tumor-induced osteomalacia or other FGF23-mediated causes.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "If FGF23 levels are not suppressed, the differential diagnosis is limited to a few diseases, with XLH (OMIM#307800) being the most frequent"
    explanation: The guideline supports inappropriately non-suppressed FGF23 as a discriminant for an FGF23-mediated differential, not as an XLH-specific test.
- name: Alkaline Phosphatase
  presence: Elevated with active rickets or osteomalacia
  context: >-
    Total ALP in children and total or bone-specific ALP in adults is used to
    assess mineralization disease activity and response; interpret against
    age-related ranges.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "children and adolescents (aged 1-17 years) with X-linked hypophosphataemia (XLH) and signs of rickets including leg deformities, elevated total ALP, and/or radiological evidence of rickets"
    explanation: The guideline identifies elevated ALP as one indicator of active pediatric rickets.
- name: 1,25-Dihydroxyvitamin D
  presence: Inappropriately low or normal for the degree of hypophosphatemia
  context: >-
    FGF23 suppresses synthesis and increases degradation, blunting the usual
    calcitriol response to low phosphate.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "a decrease in 1,25-dihydroxyvitamin D (1,25(OH)2D) synthesis and an increase in 1,25(OH)2D degradation in the proximal renal tubules"
    explanation: This directly supports the direction of calcitriol dysregulation.
imaging_findings:
- name: Radiographic Rickets
  modality: XRAY
  imaging_finding_term:
    preferred_term: Rickets
    term:
      id: HP:0002748
      label: Rickets
  diagnostic: false
  context: Growing children with suspected or active rickets
  description: >-
    Wrist, knee or ankle radiographs demonstrate and grade active rickets;
    radiographic severity is supportive but not specific for XLH.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Wrist and/or knee and/or ankle radiographs (rickets) in growing patients"
    explanation: The guideline specifies the radiographic sites used to evaluate pediatric rickets.
- name: Lower-Limb Malalignment
  modality: XRAY
  imaging_finding_term:
    preferred_term: Bowing of the legs
    term:
      id: HP:0002979
      label: Bowing of the legs
  diagnostic: false
  context: Standing alignment assessment in children or adults with deformity
  description: >-
    A standardized standing anteroposterior limb-alignment radiograph defines
    the mechanical axis and documents varus or valgus deformity for longitudinal
    and surgical assessment.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Standardized, well-positioned anterior-posterior standing limb alignment radiograph"
    explanation: The guideline directly recommends standardized limb-alignment imaging.
- name: Pseudofracture or Insufficiency-Fracture Line
  modality: XRAY
  diagnostic: false
  context: Adults with focal bone pain or suspected osteomalacia
  description: >-
    Plain radiographs of symptomatic sites can reveal unhealed fracture or
    Looser-zone pseudofracture lines, indicating active osteomalacia rather than
    a pathognomonic XLH finding.
  evidence:
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "At week 24, 43.1% (burosumab) and 7.7% (placebo) of baseline active fractures were fully healed"
    explanation: Trial radiographic assessment documents active baseline fracture and pseudofracture lesions and their healing.
- name: Cranial-Suture Fusion
  imaging_finding_term:
    preferred_term: Craniosynostosis
    term:
      id: HP:0001363
      label: Craniosynostosis
  diagnostic: false
  context: Abnormal skull shape or symptoms of intracranial hypertension
  description: >-
    Skull or brain imaging confirms craniosynostosis and evaluates associated
    intracranial or craniocervical complications when clinical signs warrant it.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "We recommend a complete evaluation with fundoscopy and brain or skull imaging in any patient with XLH presenting with a skull morphology suggestive of craniosynostosis or clinical symptoms of intracranial hypertension"
    explanation: The guideline limits cranial imaging to a clinically indicated setting rather than routine imaging of every patient.
- name: Nephrocalcinosis on Renal Ultrasound
  modality: ULTRASOUND
  imaging_finding_term:
    preferred_term: Nephrocalcinosis
    term:
      id: HP:0000121
      label: Nephrocalcinosis
  diagnostic: false
  context: Surveillance during conventional therapy or when hypercalciuria is present
  description: >-
    Renal ultrasonography detects calcium deposition as a complication relevant
    to long-term phosphate and active vitamin D exposure; it is not a diagnostic
    criterion for XLH.
  evidence:
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "periodic renal ultrasound examination to assess for nephrocalcinosis"
    explanation: GeneReviews supports renal ultrasound surveillance in conventionally treated patients.
differential_diagnoses:
- name: Autosomal Dominant Hypophosphatemic Rickets
  disease_term:
    preferred_term: autosomal dominant hypophosphatemic rickets
    term:
      id: MONDO:0008660
      label: autosomal dominant hypophosphatemic rickets
  description: >-
    ADHR is another FGF23-mediated phosphate-wasting rickets disorder and can
    overlap biochemically with XLH.
  distinguishing_features:
  - Autosomal dominant rather than X-linked transmission.
  - Pathogenic FGF23 variants rather than PHEX variants.
  - Non-suppressed FGF23 does not distinguish ADHR from XLH, so pedigree and molecular testing are important.
  evidence:
  - reference: PMID:11062477
    reference_title: Autosomal dominant hypophosphataemic rickets is associated with mutations in FGF23.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We identified missense mutations in a gene encoding a new member of the fibroblast growth factor (FGF) family, FGF23"
    explanation: The ADHR discovery study identifies FGF23 as its disease gene; it is not used as proof of the PHEX-to-FGF23 molecular link.
- name: Autosomal Recessive Hypophosphatemic Rickets
  disease_term:
    preferred_term: autosomal recessive hypophosphatemic rickets
    term:
      id: MONDO:0017324
      label: autosomal recessive hypophosphatemic rickets
  description: >-
    DMP1- and ENPP1-related recessive disorders can produce FGF23-mediated renal
    phosphate wasting, rickets and osteomalacia similar to XLH.
  distinguishing_features:
  - Autosomal recessive pedigree rather than X-linked transmission.
  - Biallelic DMP1 or ENPP1 pathogenic variants rather than a PHEX variant.
  - Extra-skeletal features of the specific recessive disorder may redirect testing.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "autosomal-recessive hypophosphataemic rickets 1 (OMIM#241520)"
    explanation: The updated recommendations explicitly include autosomal-recessive hypophosphataemic rickets in the non-suppressed-FGF23 differential.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "other causes of genetic (DMP1, ENPP1, FGF23) or acquired forms of hypophosphataemia (such as tumour-induced osteomalacia) be considered if the analysis of the PHEX gene yields a negative result"
    explanation: This directs gene-specific evaluation of DMP1- and ENPP1-related alternatives after negative PHEX testing.
- name: Tumor-Induced Osteomalacia
  disease_term:
    preferred_term: Oncogenic osteomalacia
    term:
      id: MONDO:0018124
      label: Oncogenic osteomalacia
  description: >-
    An acquired FGF23-secreting tumor can reproduce the adult biochemical and
    osteomalacic phenotype of XLH.
  distinguishing_features:
  - Acquired FGF23-mediated presentation rather than X-linked transmission.
  - No causative germline PHEX variant.
  - Identification of an FGF23-secreting tumor establishes the acquired cause.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "other causes of genetic (DMP1, ENPP1, FGF23) or acquired forms of hypophosphataemia (such as tumour-induced osteomalacia) be considered if the analysis of the PHEX gene yields a negative result"
    explanation: The guideline specifically directs consideration of tumor-induced osteomalacia after negative PHEX testing.
- name: Hereditary Hypophosphatemic Rickets with Hypercalciuria
  disease_term:
    preferred_term: hereditary hypophosphatemic rickets with hypercalciuria
    term:
      id: MONDO:0009431
      label: hereditary hypophosphatemic rickets with hypercalciuria
  description: >-
    HHRH is a primary renal tubular phosphate-transport disorder rather than an
    FGF23-excess disorder.
  distinguishing_features:
  - Low or suppressed FGF23 during hypophosphatemia.
  - Hypercalciuria and an increased calcitriol response favor HHRH over XLH.
  - Biallelic SLC34A3 variants establish the common genetic form.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Hereditary hypophosphataemic rickets with hypercalciuria ... These diseases are characterized by low or suppressed levels of FGF23."
    explanation: The guideline places HHRH among primary transporter-mediated disorders and describes the group as having low or suppressed FGF23.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "urinary calcium excretion and 1,25(OH) 2D levels help to distinguish between FGF23-mediated diseases and primary tubular phosphate wasting ... increased levels of 1,25(OH)2D and hypercalciuria"
    explanation: This directly supports the high-calcitriol, hypercalciuric and suppressed-FGF23 pattern of transporter-mediated phosphate wasting.
- name: Fanconi Renotubular Syndrome
  disease_term:
    preferred_term: Fanconi renotubular syndrome
    term:
      id: MONDO:0001083
      label: Fanconi renotubular syndrome
  description: >-
    Generalized proximal tubular dysfunction can cause hypophosphatemic rickets
    but is not isolated to phosphate handling.
  distinguishing_features:
  - Metabolic acidosis or bicarbonate wasting.
  - Urinary glucose, amino acids and low-molecular-weight proteins in addition to phosphate.
  - Low or suppressed FGF23 during hypophosphatemia.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "metabolic acidosis, hypercalciuria and renal Fanconi syndrome (that is, proximal renal tubular wasting that is not restricted to phosphate) should be excluded by measuring serum bicarbonate levels, and urinary excretion of calcium, amino acids, glucose and low-molecular-weight protein markers"
    explanation: This provides the specific biochemical tests that distinguish generalized Fanconi wasting from isolated XLH phosphate wasting.
- name: Calcipenic or Nutritional Rickets
  description: >-
    Vitamin D or calcium deficiency can produce rickets and elevated alkaline
    phosphatase without the isolated FGF23-mediated renal phosphate-wasting
    pattern of XLH.
  distinguishing_features:
  - Vitamin D or calcium deficiency and high PTH activity favor calcipenic rickets.
  - Renal phosphate loss is secondary to PTH rather than inappropriately non-suppressed FGF23.
  - Correction of the nutritional or mineral deficiency separates it from inherited XLH.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The differential diagnosis of XLH is based on the mechanisms leading to hypophosphataemia ... high parathyroid hormone (PTH) activity (leading to calcipenic rickets)"
    explanation: The guideline distinguishes calcipenic from phosphopenic mechanisms at the start of the diagnostic work-up.
treatments:
- name: Burosumab
  action_category: THERAPEUTIC
  therapeutic_modality: MONOCLONAL_ANTIBODY
  description: >-
    Burosumab is a fully human monoclonal antibody that neutralizes FGF23. The
    comparative pediatric phase 3 evidence enrolled children aged 1-12 years
    with persistent rickets (RSS at least 2), hypophosphatemia and substantial
    prior conventional therapy; it did not directly establish superiority in
    mild rickets or adolescents. Updated recommendations support use in children
    and adolescents aged 1-17 years with signs of rickets. The adult placebo
    trial enrolled symptomatic adults with hypophosphatemia and pain and showed
    phosphate correction, greater fracture healing and reduced stiffness, while
    pain and physical-function comparisons did not survive multiplicity
    adjustment. A 2026 study in 16 infants younger than 12 months provides small,
    open-label, non-randomized safety evidence, not comparative efficacy or a
    universal authorization boundary; eligibility and labeling remain
    jurisdiction-specific. Burosumab is not co-administered with oral phosphate
    or active vitamin D and is avoided when baseline phosphate is normal or
    renal impairment is severe. Guidance recommends contraception during
    treatment for patients who could become pregnant and stopping burosumab
    when pregnancy is planned or identified. During lactation, the updated
    guideline does not treat burosumab as categorically contraindicated, but
    limits consideration to severe manifestations after individualized
    counselling.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: burosumab
      term:
        id: NCIT:C119744
        label: Burosumab
  target_mechanisms:
  - target: Excess Bone-Derived FGF23 Signaling
    treatment_effect: INHIBITS
    description: Burosumab binds circulating FGF23 and reduces its phosphaturic signaling.
    evidence:
    - reference: PMID:29947083
      reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Burosumab, a fully human monoclonal antibody, binds and inhibits FGF23 to correct hypophosphatemia."
      explanation: This directly supports the on-target antibody mechanism.
  evidence:
  - reference: PMID:31104833
    reference_title: "Burosumab versus conventional therapy in children with X-linked hypophosphataemia: a randomised, active-controlled, open-label, phase 3 trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we enrolled children with X-linked hypophosphataemia aged 1-12 years. Key eligibility criteria were a total Thacher rickets severity score of at least 2.0, fasting serum phosphorus lower than 0.97 mmol/L (3.0 mg/dL), confirmed PHEX (phosphate-regulating endopeptidase homolog, X-linked) mutation or variant of unknown significance in the patient or a family member with appropriate X-linked dominant inheritance, and receipt of conventional therapy for at least 6 consecutive months for children younger than 3 years or at least 12 consecutive months for children older than 3 years."
    explanation: This records the pediatric comparative trial's actual severity, biochemical, genetic and prior-treatment enrollment boundaries.
  - reference: PMID:31104833
    reference_title: "Burosumab versus conventional therapy in children with X-linked hypophosphataemia: a randomised, active-controlled, open-label, phase 3 trial."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Significantly greater clinical improvements were shown in rickets severity, growth, and biochemistries among children with X-linked hypophosphataemia treated with burosumab compared with those continuing conventional therapy."
    explanation: This supports superiority within the enrolled pediatric trial population.
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "At week 24, 43.1% (burosumab) and 7.7% (placebo) of baseline active fractures were fully healed; the odds of healed fracture in the burosumab group was 16.8-fold greater than that in the placebo group"
    explanation: This quantifies adult fracture healing without qualitative overstatement.
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Burosumab significantly reduced the Western Ontario and the McMaster Universities Osteoarthritis Index (WOMAC) stiffness subscale compared with placebo"
    explanation: This directly supports the adult stiffness outcome within the 24-week trial setting.
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Reductions in WOMAC physical function subscale (-4.9 +/- 2.48; p = 0.048) and Brief Pain Inventory worst pain (-0.5 +/- 0.28; p = 0.092) did not achieve statistical significance after Hochberg multiplicity adjustment."
    explanation: This preserves the adult trial's negative multiplicity-adjusted pain and physical-function results.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "As children with XLH and mild-moderate rickets (RSS <2.0) and adolescent patients with XLH were excluded from the RCT, whether burosumab treatment is also superior to oral phosphate and active vitamin D in these groups is unclear."
    explanation: The guideline explicitly limits extrapolation of pediatric comparative superiority.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "We recommend treating children and adolescents (aged 1-17 years) with X-linked hypophosphataemia (XLH) and signs of rickets including leg deformities, elevated total ALP, and/or radiological evidence of rickets with burosumab as soon as the diagnosis is established"
    explanation: This supplies the guideline's recommended pediatric age and disease-activity setting, distinct from the narrower RCT population.
  - reference: PMID:29947083
    reference_title: "A Randomized, Double-Blind, Placebo-Controlled, Phase 3 Trial Evaluating the Efficacy of Burosumab, an Anti-FGF23 Antibody, in Adults With X-Linked Hypophosphatemia: Week 24 Primary Analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Participants with hypophosphatemia and pain were assigned 1:1 to burosumab 1 mg/kg (n = 68) or placebo (n = 66) subcutaneously every 4 weeks (Q4W)"
    explanation: This records the symptomatic, hypophosphatemic adult setting of the placebo-controlled trial.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Burosumab must not be given in conjunction with oral phosphate or active vitamin D, when fasting phosphate levels are within the age-related normal reference range before initiation of treatment or when severe renal impairment is present"
    explanation: The guideline states the principal co-treatment, biochemical and renal boundaries for burosumab use.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "We recommend contraceptive measures in sexually active fertile women on burosumab treatment ... Treatment with burosumab should be stopped when a woman plans pregnancy or when pregnancy is identified"
    explanation: The guideline directly supplies contraception and planned or identified pregnancy boundaries.
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Burosumab is not recommended during pregnancy."
    explanation: GeneReviews independently advises against burosumab use during pregnancy.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Burosumab is not contraindicated in breast-feeding women. Burosumab treatment may be considered in breast-feeding women with severe manifestations including pseudofractures on an individual basis after counselling"
    explanation: This records the guideline's conditional lactation position without generalizing it to routine use.
  - reference: PMID:42044650
    reference_title: "Safety, tolerability, pharmacokinetics, and efficacy of burosumab in infants with X-linked hypophosphataemia: an open-label, multicentre, non-randomised study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This open-label, non-randomised, phase 1/2 study included infants from clinical sites in Austria, France, Italy, Spain and the UK."
    explanation: This establishes the non-comparative design and European clinical setting of the infant study.
  - reference: PMID:42044650
    reference_title: "Safety, tolerability, pharmacokinetics, and efficacy of burosumab in infants with X-linked hypophosphataemia: an open-label, multicentre, non-randomised study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Of 17 participants assessed for eligibility, 16 were enrolled and assigned to cohorts 1, 2, and 3"
    explanation: The very small sample bounds inference about infant safety and does not establish comparative efficacy.
  - reference: PMID:42044650
    reference_title: "Safety, tolerability, pharmacokinetics, and efficacy of burosumab in infants with X-linked hypophosphataemia: an open-label, multicentre, non-randomised study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "No TEAEs led to treatment discontinuation or death."
    explanation: This supplies a bounded short-term safety observation from the 16-infant study without implying long-term or comparative safety.
  - reference: PMID:42044650
    reference_title: "Safety, tolerability, pharmacokinetics, and efficacy of burosumab in infants with X-linked hypophosphataemia: an open-label, multicentre, non-randomised study."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Burosumab, a human anti-fibroblast growth factor 23 monoclonal antibody, is approved in Europe for treating X-linked hypophosphataemia (XLH) in patients aged at least 1 year."
    explanation: The study background records the European age boundary while the description avoids universalizing it across jurisdictions.
- name: Oral Phosphate with Active Vitamin D
  action_category: THERAPEUTIC
  therapeutic_modality: SMALL_MOLECULE
  description: >-
    Conventional therapy combines divided oral phosphate with calcitriol or
    alfacalcidol. It can improve rickets, some manifestations of osteomalacia
    and pain but only partially corrects disease and does not make fasting serum
    phosphate the dosing target. Dose adjustment follows clinical and
    radiographic response, alkaline phosphatase and PTH while balancing
    gastrointestinal intolerance, hyperparathyroidism, hypercalciuria and
    nephrocalcinosis. Phosphate should not be given without active vitamin D;
    active vitamin D alone can cause
    hypercalcemia, hypercalciuria and nephrocalcinosis. It remains an important
    option when burosumab is unavailable or unsuitable and for selected
    symptomatic adults.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: phosphate
      term:
        id: CHEBI:26020
        label: phosphate
    - preferred_term: calcitriol
      term:
        id: CHEBI:17823
        label: calcitriol
    - preferred_term: alfacalcidol
      term:
        id: CHEBI:31186
        label: alfacalcidol
  target_mechanisms:
  - target: Reduced Circulating Phosphate
    treatment_effect: MODULATES
    description: Divided phosphate doses transiently increase available phosphate without correcting upstream FGF23 excess.
  - target: Skeletal Mineralization Failure
    treatment_effect: MODULATES
    description: Active vitamin D promotes intestinal mineral absorption and, with phosphate, partially improves rickets and osteomalacia.
    evidence:
    - reference: PMID:39814982
      reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
      supports: SUPPORT
      evidence_source: OTHER
      snippet: "oral medication improves bone pain, radiological signs of rickets and serum ALP levels within 12 months"
      explanation: The guideline supports partial downstream skeletal and biochemical improvement with conventional oral treatment.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Serum phosphate levels remain low despite oral supplementation and are not a target for adjusting therapy, which requires the primary goal of achieving a satisfactory clinical response to be balanced with the risk of developing nephrocalcinosis and hyperparathyroidism"
    explanation: This defines both the response target and principal monitoring trade-off for conventional therapy.
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Treatment with phosphate without 1,25-dihydroxyvitamin D, because of the increased risk for secondary hyperparathyroidism."
    explanation: GeneReviews explicitly warns against phosphate monotherapy.
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Although 1,25-dihydroxyvitamin D has been used as a single agent, this may increase the risk for hypercalcemia, hypercalciuria, and nephrocalcinosis."
    explanation: This supports the risks of active vitamin D monotherapy.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "We recommend beginning treatment with active vitamin D together with oral phosphorus (phosphate salts) in patients with biochemical and/or clinical signs of osteomalacia, musculoskeletal pain or stiffness"
    explanation: This directly defines the symptomatic adult setting for conventional phosphate and active-vitamin-D therapy.
- name: Avoid Unindicated Bisphosphonate or Osteoporosis Medication
  action_category: THERAPEUTIC
  therapeutic_modality: OTHER
  description: >-
    GeneReviews cautions that bisphosphonates and other osteoporosis medicines
    can worsen osteomalacia in some individuals with XLH. Low bone density or
    fractures should therefore not prompt empiric osteoporosis treatment
    without first distinguishing active osteomalacia and establishing a
    separate indication; this is a caution rather than a universal
    contraindication in every clinical context.
  treatment_term:
    preferred_term: avoidance of contraindicated medications
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Bisphosphonates or osteoporosis medications may cause deterioration of osteomalacia in some individuals."
    explanation: GeneReviews supplies the direct agents-to-avoid caution while retaining its conditional wording.
- name: Multidisciplinary Surveillance
  action_category: MONITORING
  description: >-
    Follow-up should be coordinated by a metabolic-bone team and include growth,
    lower-limb alignment, pain, mobility and dental review; serum phosphate,
    calcium, creatinine, ALP, PTH and vitamin D measures; and treatment-specific
    monitoring. Conventional therapy requires urinary calcium and renal
    ultrasonography for hypercalciuria/nephrocalcinosis. Burosumab dosing uses
    timed phosphate and TmP/GFR rather than FGF23 assays. Imaging is driven by
    rickets response, deformity, focal pain or neurological concern, and dental
    evaluation is recommended every six months.
  evidence:
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "For individuals on burosumab therapy, regular monitoring of serum concentrations of phosphate, calcium, creatinine, alkaline phosphatase, and intact parathyroid hormone."
    explanation: GeneReviews specifies the core biochemical surveillance during burosumab treatment.
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "For those on conventional treatment with active vitamin D and phosphate supplementation, additional testing includes urinary calcium and creatinine to assess for hypercalciuria; periodic renal ultrasound examination to assess for nephrocalcinosis."
    explanation: This supports the additional renal safety surveillance required with conventional therapy.
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "dental evaluation every six months; hearing evaluation and evaluation for Chiari I malformation performed based on clinical suspicion"
    explanation: This supports six-month dental review and symptom-directed hearing or neurological evaluation.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Measuring FGF23 concentrations to monitor treatment efficacy is not recommended because burosumab interferes with the analytical assays"
    explanation: This supports avoiding FGF23 assays for burosumab response monitoring.
- name: Orthopedic and Joint Surgery
  action_category: THERAPEUTIC
  therapeutic_modality: SURGERY
  description: >-
    Persistent mechanically important lower-limb deformity or functional
    symptoms can require guided growth or corrective osteotomy after metabolic
    therapy has been optimized for at least 3-6 months. In growing children on
    burosumab, elective surgery may be delayed because alignment can continue to
    improve; guided growth requires adequate growth remaining, whereas later
    osteotomy has fewer recurrence concerns. Adults require the same metabolic
    optimization and expert multidisciplinary planning; joint replacement is
    reserved for function-limiting osteoarthritis despite medical treatment and
    rehabilitation. Emergency fracture fixation should not be delayed when
    necessary.
  treatment_term:
    preferred_term: Orthopedic Surgical Procedure
    term:
      id: NCIT:C16186
      label: Orthopedic Surgical Procedure
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "elective surgical treatment should be performed only in children in whom medical treatment either with oral phosphate and active vitamin D or burosumab has been optimized for at least 3-6 months"
    explanation: The guideline establishes preoperative metabolic optimization for children.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "persisting deformity (mechanical axis deviation zone 2 or greater) despite optimized medical treatment and/or the presence of symptoms interfering with function should be considered for surgical treatment"
    explanation: This defines the deformity and functional setting for elective pediatric surgery.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "We recommend considering postponing surgery in children on burosumab, if possible, because sustained straightening of legs is expected for at least 3 years after initiation of this treatment in patients with growth potential"
    explanation: This supports allowing time for continued alignment response in growing children receiving burosumab.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Guided growth techniques depend on the remaining growth potential of the child and must therefore be carried out at least 2-3 years before skeletal maturity"
    explanation: This supports age-sensitive selection of guided-growth surgery rather than a blanket near-maturity rule.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Joint replacement by prosthesis should be performed in case of physical function impairment due to osteoarthritis despite optimal medical treatment and rehabilitation"
    explanation: This defines the adult joint-replacement setting.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "surgical treatment should be performed in adults in whom medical treatment either with oral phosphate and active vitamin D or burosumab has been optimized for at least several months ... surgery should be performed by a surgeon with expertise in metabolic bone diseases"
    explanation: This supports adult metabolic optimization and specialist surgical planning.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "We recommend that emergency surgical treatment such as fracture fixation should occur when necessary"
    explanation: This distinguishes urgent fracture fixation from elective deformity correction that can await metabolic optimization.
- name: Preventive and Specialist Dental Care
  action_category: THERAPEUTIC
  therapeutic_modality: OTHER
  description: >-
    Dental management complements systemic metabolic therapy. Children need
    standard prevention plus six-month visits and repeated pit-and-fissure
    sealing as needed; discoloration, fistula, swelling, abscess, cellulitis or
    pain prompts pulpal and periapical assessment. Adults need twice-yearly
    periodontal risk assessment and debridement when indicated. Systemic therapy
    should be optimized before orthodontic treatment or implant surgery, with
    prolonged healing plans for implants.
  treatment_term:
    preferred_term: Dental Procedure
    term:
      id: NCIT:C38052
      label: Dental Procedure
  target_phenotypes:
  - preferred_term: Tooth abscess
    term:
      id: HP:0030757
      label: Tooth abscess
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "In children, in addition to standard preventive care, we recommend dental visits every 6 months and suggest sealing pits and fissures with flowable resin composite on both temporary and permanent teeth as soon and as frequently as required"
    explanation: This directly supports the pediatric preventive schedule and sealing intervention.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "We suggest a thorough clinical investigation searching for pulp infection (colour changes, fistula, swelling, abscess, cellulitis or pain) and performing retrocoronal and/or periapical radiographs or orthopantomogram"
    explanation: This directly supports symptom-triggered pulpal and periapical assessment.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "In children, we suggest optimizing therapy with phosphate and active vitamin D or burosumab before initiation of orthodontic treatment"
    explanation: This supports metabolic optimization before pediatric orthodontic treatment.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "In adults, we recommend twice-yearly visits to perform conventional supportive periodontal therapy, which should include periodontal risk assessment and supragingival and subgingival debridement if necessary"
    explanation: This directly supports adult periodontal surveillance and treatment.
  - reference: PMID:42409089
    reference_title: "Diagnosis and management of X-linked hypophosphatemia in dental practice: A scoping review."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Key recommendations highlight that interdisciplinary management is critical to reducing complications and improving long-term oral health."
    explanation: The 2026 scoping review supports interdisciplinary dental management but is not treated as comparative trial evidence.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "In adults, we suggest that dental implant surgery should be performed after at least 3 months of medical treatment, which should be continued for 6 months following implant surgery; healing time should be extended up to 6 months"
    explanation: This supplies the guideline's medical-optimization and healing-time boundaries for adult implant surgery.
- name: Rehabilitation and Symptom-Directed Support
  action_category: THERAPEUTIC
  therapeutic_modality: BEHAVIORAL
  description: >-
    Physiotherapy, rehabilitation, adapted activity and analgesic or pain-clinic
    support address weakness, deconditioning, impaired mobility and chronic pain.
    These interventions are adjunctive and do not correct FGF23-mediated
    phosphate wasting.
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "rehabilitation, physiotherapy, and analgesics for musculoskeletal pain"
    explanation: GeneReviews lists rehabilitation, physiotherapy and analgesia as supportive management.
- name: Genetic Counseling and Cascade Evaluation
  action_category: COUNSELING_INFORMATIONAL
  description: >-
    Counseling explains X-linked transmission, variable expressivity, testing of
    at-risk relatives, and reproductive options. Molecular testing of relatives
    is preferred when the familial PHEX variant is known; biochemical evaluation
    is an alternative for first-degree relatives at risk.
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:22319799
    reference_title: X-Linked Hypophosphatemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Molecular genetic testing (if the PHEX pathogenic variant has been identified in the family) or biochemical testing of first-degree relatives at risk to ensure early treatment for optimal outcome."
    explanation: GeneReviews supports cascade molecular or biochemical evaluation of relatives at risk.
discussions:
- discussion_id: gap_xlh_phex_to_fgf23_coupling
  prompt: What molecular intermediates connect PHEX loss in mineralizing cells to increased FGF23 synthesis and secretion?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#PHEX Loss of Function in Mineralizing Cells
  - pathophysiology#Excess Bone-Derived FGF23 Signaling
  rationale: >-
    The causal PHEX gene and downstream FGF23 excess are well established, but
    treating PHEX as though it directly degrades FGF23 would overstate current
    evidence. Resolving the intervening osteocyte pathways could expose upstream
    targets that complement systemic FGF23 neutralization.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The pathogenesis of XLH is complex and incompletely understood. Many features of the disease can be explained by increased secretion of the phosphaturic hormone fibroblast growth factor 23 (FGF23) from bone."
    explanation: The guideline explicitly separates established FGF23 consequences from the incompletely understood upstream pathogenesis.
- discussion_id: gap_xlh_long_term_structural_response
  prompt: >-
    Does long-term correction of FGF23-mediated phosphate wasting prevent or
    reverse enthesopathy, osteoarthritis, adult dental disease or neurosurgical
    complications?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - treatments#Burosumab
  - phenotypes#Enthesopathy
  - phenotypes#Osteoarthritis
  rationale: >-
    Short-term trials establish biochemical correction and healing of active
    fractures, but irreversible or slowly evolving structural outcomes have not
    been shown to respond. This distinction matters when counseling adults about
    which symptoms are plausibly treatment-responsive.
  evidence:
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Evaluate the long-term efficacy and safety of treatment with burosumab in patients with XLH with respect to critical outcomes such as growth, body weight and composition, bone shape, physical function, hyperparathyroidism, tooth mineralization, hearing function, neurosurgical complications and prevention of pseudofractures, enthesopathies, oral manifestations (dental infections, periodontitis, implant loss), mental wellbeing, quality of life and osteoarthritis"
    explanation: The guideline's future-research agenda directly identifies these long-term structural and functional outcomes as unresolved.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "Thus, the trial showed no evidence of an effect of burosumab on enthesopathy burden"
    explanation: The pivotal adult trial did not demonstrate a short-term enthesopathy effect.
  - reference: PMID:39814982
    reference_title: Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "No data are available on the impact of treatment with burosumab or oral phosphate and active vitamin D on osteoarthritis structural damage on X-rays or the course of osteoarthritis."
    explanation: The guideline explicitly identifies effects on osteoarthritis structure and course as unproven.
- discussion_id: gap_xlh_infant_burosumab_comparative_outcomes
  prompt: >-
    What are the comparative efficacy, optimal dosing and long-term developmental
    outcomes of starting burosumab before 12 months of age?
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - treatments#Burosumab
  rationale: >-
    The first infant study included only 16 participants without randomization or
    a control group. It expands safety experience but cannot establish
    comparative clinical benefit, durable prevention of deformity or a universal
    regulatory age threshold.
  evidence:
  - reference: PMID:42044650
    reference_title: "Safety, tolerability, pharmacokinetics, and efficacy of burosumab in infants with X-linked hypophosphataemia: an open-label, multicentre, non-randomised study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This open-label, non-randomised, phase 1/2 study included infants from clinical sites in Austria, France, Italy, Spain and the UK."
    explanation: The design supports only preliminary non-comparative inference in infants.
datasets: []
📚

References & Deep Research

References

2
PMID:22319799
X-Linked Hypophosphatemia.
No top-level findings curated for this source.
PMID:39814982
Clinical practice recommendations for the diagnosis and management of X-linked hypophosphataemia.
No top-level findings curated for this source.

Deep Research

2

Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.

Disorder ▸

Disorder

  • Name: X-Linked Hypophosphatemia
  • Category: Mendelian
  • Existing deep-research providers: falcon
  • Existing evidence reference count in YAML: 32

Key Pathophysiology Nodes

  • Renal Phosphate Wasting via FGF23 Excess
  • Defective Bone Mineralization
  • Deep research literature mapping

Citation Inventory (for evidence mapping)

  • DOI:10.1038/s41405-024-00223-6
  • DOI:10.1186/s13023-025-03952-5
  • DOI:10.23804/ejpd.2025.2348
  • DOI:10.3389/fendo.2024.1414509
  • DOI:10.3389/fendo.2025.1702656
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Disease Pathophysiology Research Report
Edison Scientific Literature 21 citations 2026-02-10T18:25:30.703332

Disease Pathophysiology Research Report

Target Disease - Disease Name: X-Linked Hypophosphatemia (XLH) - MONDO ID: MONDO_0010619 (X-linked dominant hypophosphatemic rickets) - Category: Mendelian

Overview and current understanding XLH is an X-linked dominant, phosphate-wasting osteomalacic/rickets disorder driven by excess osteocyte-derived FGF23 consequent to loss-of-function variants in PHEX. Elevated FGF23 reduces renal phosphate reabsorption and suppresses 1,25-dihydroxyvitamin D [1,25(OH)2D] synthesis, impairing skeletal and dental mineralization from infancy through adulthood. “Loss-of-function mutations in the PHEX gene … result in upregulated FGF23 serum levels and consequent hypophosphatemia” (Orphanet J Rare Dis, 2025; URL: https://doi.org/10.1186/s13023-025-03952-5; published Oct 2025) (brandi2025xlinkedhypophosphatemiaand pages 2-4). Conventional therapy (oral phosphate plus active vitamin D) improves some features but does not correct high FGF23, whereas the FGF23-neutralizing monoclonal antibody burosumab restores phosphate balance and increases endogenous 1,25(OH)2D (Front Endocrinol, 2024; URL: https://doi.org/10.3389/fendo.2024.1414509; published Aug 2024; BDJ Open, 2024; URL: https://doi.org/10.1038/s41405-024-00223-6; published May 2024) (wang2024metaanalysisandsystematic pages 1-2, arhar2024characteristicsoforal pages 8-9).

1) Core Pathophysiology - Primary mechanisms - PHEX loss-of-function in osteocytes/odontoblasts leads to increased circulating FGF23 and accumulation of matrix mineralization inhibitors (e.g., SIBLING-derived ASARM peptides, osteopontin), producing systemic hypophosphatemia and local hypomineralization in bone and teeth (BDJ Open, 2024; URL: https://doi.org/10.1038/s41405-024-00223-6; EJPD, 2025; URL: https://doi.org/10.23804/ejpd.2025.2348) (arhar2024characteristicsoforal pages 8-9, defabianis2025xlinkedhypophosphatemiain pages 1-2). - Excess FGF23 acts on FGFR1c–Klotho in renal proximal tubule and parathyroid to: decrease phosphate reabsorption (downregulating NaPi-IIa/SLC34A1 and NaPi-IIc/SLC34A3), suppress CYP27B1 (1α-hydroxylase), and induce CYP24A1 (24-hydroxylase), lowering 1,25(OH)2D and intestinal phosphate absorption (Front Endocrinol, 2024; Orphanet J Rare Dis, 2025; URLs above) (wang2024metaanalysisandsystematic pages 1-2, brandi2025xlinkedhypophosphatemiaand pages 2-4, wang2024metaanalysisandsystematic pages 18-18). - Dysregulated pathways - Osteocyte endocrine axis: PHEX–FGF23–Klotho/FGFR1c signaling (kidney, parathyroid) (Orphanet J Rare Dis, 2025; Front Endocrinol, 2024) (brandi2025xlinkedhypophosphatemiaand pages 2-4, wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18). - Mineralization regulators: MEPE/ASARM, DMP1, ENPP1-mediated pyrophosphate (PPi) balance, and osteopontin accumulation act locally to inhibit hydroxyapatite crystal growth (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11). - Affected cellular processes - Renal tubular phosphate transport (reduced TmP/GFR), vitamin D activation/catabolism (CYP27B1↓, CYP24A1↑), and matrix mineralization (inhibited nucleation/fusion of calcospherites) (Front Endocrinol, 2024; BDJ Open, 2024) (wang2024metaanalysisandsystematic pages 1-2, arhar2024characteristicsoforal pages 8-9).

2) Key Molecular Players - Genes/Proteins (HGNC) - PHEX (HGNC:8860): Causal gene; loss-of-function increases FGF23 and allows accumulation of ASARM peptides/osteopontin that inhibit mineralization (Orphanet J Rare Dis, 2025; BDJ Open, 2024) (brandi2025xlinkedhypophosphatemiaand pages 2-4, arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11). - FGF23 (HGNC:3689): Endocrine phosphatonin from osteocytes; reduces renal phosphate reabsorption and 1,25(OH)2D synthesis (Front Endocrinol, 2024; Orphanet J Rare Dis, 2025) (wang2024metaanalysisandsystematic pages 1-2, brandi2025xlinkedhypophosphatemiaand pages 2-4, wang2024metaanalysisandsystematic pages 18-18). - FGFR1 (HGNC:3688) and KLOTHO (HGNC:6353): Co-receptor complex confers FGF23 target specificity in kidney and parathyroid (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 18-18). - SLC34A1/NaPi-IIa (HGNC:11039), SLC34A3/NaPi-IIc (HGNC:11041): Proximal-tubule phosphate cotransporters downregulated by FGF23 (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 18-18). - CYP27B1 (HGNC:2593), CYP24A1 (HGNC:2594): Vitamin D metabolic enzymes suppressed/induced by FGF23 respectively (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 18-18). - MEPE (HGNC:13323), DMP1 (HGNC:2936), ENPP1 (HGNC:3352), SPP1/Osteopontin (HGNC:11255): Matrix regulators linked to impaired mineralization in XLH (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11). - Chemical entities (ChEBI) - Phosphate (ChEBI:18367), calcitriol/1,25(OH)2D3 (ChEBI:17933/28940), and burosumab (ChEBI:132958) are central to mechanism and therapy (Front Endocrinol, 2024; BDJ Open, 2024) (wang2024metaanalysisandsystematic pages 1-2, arhar2024characteristicsoforal pages 8-9). - Cell types (CL) - Osteocytes and osteoblasts (bone); renal proximal tubule epithelial cells; parathyroid chief cells; odontoblasts/ameloblasts/cementoblasts (dental) are primary cell actors (Orphanet J Rare Dis, 2025; BDJ Open, 2024) (brandi2025xlinkedhypophosphatemiaand pages 2-4, arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11). - Anatomical locations (UBERON) - Bone, kidney (proximal tubule), parathyroid, growth plate, tooth (dentin, cementum), and entheses are key sites (Orphanet J Rare Dis, 2025; BDJ Open, 2024) (brandi2025xlinkedhypophosphatemiaand pages 2-4, arhar2024characteristicsoforal pages 8-9).

3) Biological Processes (for GO annotation) - Hormone-mediated signaling: FGF23–FGFR1c–Klotho signaling (GO:0005179 ligand activity; GO:0007173 transmembrane receptor protein tyrosine kinase signaling) leading to ERK/MAPK activation in renal proximal tubule/parathyroid (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 18-18). - Phosphate homeostasis and transport: negative regulation of phosphate ion transmembrane transport (GO:0035435) by downregulation of SLC34A1/SLC34A3; decreased renal tubular phosphate reabsorption (reflected by low TmP/GFR) (Front Endocrinol, 2024; Orphanet J Rare Dis, 2025) (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4). - Vitamin D metabolic process: negative regulation of 1α-hydroxylase (CYP27B1) and positive regulation of 24-hydroxylase (CYP24A1) (GO:0038180 regulation of vitamin D biosynthetic process) (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 18-18). - Biomineralization: extracellular matrix organization and mineralization (GO:0030198; GO:0030282), inhibited by MEPE/ASARM, osteopontin, and altered PPi balance (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11).

4) Cellular Components (GO) - Sites of action: osteocyte lacunar–canalicular system and bone matrix (extracellular region; GO:0005576), renal proximal tubular apical membrane (plasma membrane; GO:0005886) hosting NaPi-IIa/IIc, parathyroid gland tissues, and dental hard tissues (dentin/cementum) where PHEX and mineralization inhibitors operate (BDJ Open, 2024; Orphanet J Rare Dis, 2025) (arhar2024characteristicsoforal pages 8-9, brandi2025xlinkedhypophosphatemiaand pages 2-4).

5) Disease Progression - Sequence of events 1) Genetic trigger: PHEX loss-of-function in osteocytes/odontoblasts → overproduction of FGF23 and accumulation of ASARM/osteopontin (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11). 2) Endocrine effects: FGF23→FGFR1c–Klotho signaling in kidney/parathyroid → phosphaturia (NaPi-IIa/IIc↓), low 1,25(OH)2D (CYP27B1↓, CYP24A1↑), disordered PTH feedback (Front Endocrinol, 2024; Orphanet J Rare Dis, 2025) (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4). 3) Tissue consequences: impaired mineralization in growth plates (rickets) and bone (osteomalacia); dental dentin/enamel/cementum hypomineralization → abscesses; adult enthesopathy, pseudofractures, osteoarthritis (BDJ Open, 2024; Orphanet J Rare Dis, 2025) (arhar2024characteristicsoforal pages 8-9, brandi2025xlinkedhypophosphatemiaand pages 2-4, arcidiacono2025potentialpredictorsof pages 16-20). - Phases - Childhood: limb deformities, rickets, short stature, delayed motor development; “typical childhood manifestations include limb growth retardation, abnormal walking patterns, bone pain, and rickets” (Orphanet J Rare Dis, 2025) (brandi2025xlinkedhypophosphatemiaand pages 2-4). - Adulthood: persistent osteomalacia, musculoskeletal pain, pseudofractures, enthesopathies, early osteoarthritis; complications from prior therapy (nephrocalcinosis, hyperparathyroidism) (Unknown journal, 2025) (arcidiacono2025potentialpredictorsof pages 16-20).

6) Phenotypic Manifestations (with HP terms) - Biochemical hallmark: Hypophosphatemia (HP:0002148) with decreased TmP/GFR due to FGF23 excess (Orphanet J Rare Dis, 2025; Front Endocrinol, 2024) (brandi2025xlinkedhypophosphatemiaand pages 2-4, wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18). - Skeletal: Rickets (HP:0002748) in children and Osteomalacia (HP:0002649) in adults; bowing of long bones (HP:0002970), short stature (HP:0004322), pseudofractures (HP:0002757), enthesopathy (HP:0002657) (Orphanet J Rare Dis, 2025; Unknown journal, 2025) (brandi2025xlinkedhypophosphatemiaand pages 2-4, arcidiacono2025potentialpredictorsof pages 16-20). - Dental/craniofacial: spontaneous periapical dental abscesses (HP:0001088), hypomineralized dentin, enamel cracks, aberrant cementum; craniosynostosis reported in some cohorts (BDJ Open, 2024; EJPD, 2025) (arhar2024characteristicsoforal pages 8-9, defabianis2025xlinkedhypophosphatemiain pages 1-2, arhar2024characteristicsoforal pages 10-11). - Renal/endocrine complications (often therapy-related): nephrocalcinosis (HP:0000121), secondary/tertiary hyperparathyroidism (HP:0000829) (Front Endocrinol, 2024; BDJ Open, 2024) (wang2024metaanalysisandsystematic pages 1-2, arhar2024characteristicsoforal pages 8-9).

Recent developments and latest research (prioritized 2023–2024) - Pathophysiology-focused updates - Contemporary reviews reinforce the centrality of FGF23 as the driver of renal phosphate wasting and reduced 1,25(OH)2D in XLH and emphasize standardized biochemical assessment including intact FGF23 and TmP/GFR (Orphanet J Rare Dis, 2025; Front Endocrinol, 2025 review of biochemical evaluation; URLs: https://doi.org/10.1186/s13023-025-03952-5; https://doi.org/10.3389/fendo.2025.1702656) (brandi2025xlinkedhypophosphatemiaand pages 2-4, brandi2025xlinkedhypophosphatemiaand pages 14-14). - Dental mechanistic literature in 2024 highlights that PHEX is expressed in teeth and that mineralization defects reflect “inadequate mineralisation, uneven dentin tubules, and cracks and chipping in the enamel,” with FGF23 mRNA detected in ameloblasts and odontoblasts (BDJ Open, 2024; URL: https://doi.org/10.1038/s41405-024-00223-6; published May 2024) (arhar2024characteristicsoforal pages 8-9). - Therapeutics and outcomes (2024) - Meta-analyses/systematic reviews in 2024 show burosumab improves serum phosphorus, TmP/GFR, 1,25(OH)2D, alkaline phosphatase, rickets severity scores, and functional capacity (6-minute walk test) in children; “burosumab’s superiority in managing XLH in pediatric populations” is supported, though long-term growth and quality-of-life effects require further study (Front Endocrinol, 2024; URL: https://doi.org/10.3389/fendo.2024.1414509; published Aug 2024) (wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18).

Current applications and real-world implementations - Diagnostic practice: Routine panels include serum phosphate, calcium, ALP, PTH, 25(OH)D, 1,25(OH)2D, creatinine, intact FGF23, and estimation of TmP/GFR to document renal phosphate wasting; genetic testing confirms PHEX variants (Orphanet J Rare Dis, 2025; URL above; EJPD, 2025; URL: https://doi.org/10.23804/ejpd.2025.2348) (brandi2025xlinkedhypophosphatemiaand pages 2-4, defabianis2025xlinkedhypophosphatemiain pages 1-2). - Conventional therapy: divided-dose oral phosphate plus active vitamin D analogs remains in use but may raise FGF23 and carries risks (hypercalciuria, nephrocalcinosis, hyperparathyroidism) (BDJ Open, 2024; Front Endocrinol, 2024) (arhar2024characteristicsoforal pages 8-9, wang2024metaanalysisandsystematic pages 1-2). - Targeted therapy: Burosumab (anti-FGF23 mAb) is approved; it “neutralises elevated FGF23 levels, resulting in better phosphate reabsorption in the renal tubules … increased serum phosphate levels and the synthesis of endogenous 1,25(OH)2 vitamin D” (BDJ Open, 2024; URL above) (arhar2024characteristicsoforal pages 8-9). Randomized and cohort evidence summarized in 2024 meta-analyses demonstrates consistent biochemical and radiographic improvements in pediatric XLH (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18).

Expert opinions and analysis - Narrative review and expert opinion (Orphanet J Rare Dis, 2025) emphasizes that anti-FGF23 therapy “has revolutionized the traditional approach” to XLH/TIO management and urges standardized biochemical evaluation including intact FGF23 and TmP/GFR to guide care (URL above) (brandi2025xlinkedhypophosphatemiaand pages 2-4, brandi2025xlinkedhypophosphatemiaand pages 14-14). - Dental experts underscore that XLH dental disease is frequent and persists despite conventional therapy, reinforcing early, multidisciplinary management and consideration of burosumab’s potential oral-health benefits reported in emerging studies (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11).

Relevant statistics and data (recent where available) - Epidemiology: XLH affects up to ~1 in 20,000 individuals and accounts for ~80% of hypophosphatemic rickets (Orphanet J Rare Dis, 2025; URL above) (brandi2025xlinkedhypophosphatemiaand pages 2-4). - Dental involvement: Reports range widely (e.g., “dental involvement in 23–67% of patients; 40–50% of children and 60–85% of adults affected”), with histology showing “reduced dentin mineralisation … cracks and chipping in the enamel” (EJPD, 2025; BDJ Open, 2024; URLs above) (defabianis2025xlinkedhypophosphatemiain pages 1-2, arhar2024characteristicsoforal pages 8-9). - Therapeutic outcomes: Pediatric burosumab meta-analysis indicates superiority over conventional therapy for improving serum phosphorus, TmP/GFR, ALP, rickets severity scores, and 6MWT; long-term height/QOL data remain limited (Front Endocrinol, 2024; URL above) (wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18).

Direct quotes (for key statements) - “Loss-of-function mutations in the PHEX gene … result in upregulated FGF23 serum levels and consequent hypophosphatemia.” (Orphanet J Rare Dis, 2025) (brandi2025xlinkedhypophosphatemiaand pages 2-4). - “Burosumab neutralises elevated FGF23 levels, resulting in better phosphate reabsorption in the renal tubules … increased serum phosphate levels and the synthesis of endogenous 1,25(OH)2 vitamin D.” (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9). - Dental histology: “Inadequate mineralisation, uneven dentin tubules, and cracks and chipping in the enamel were observed, indicating mineralisation deviations.” (BDJ Open, 2024) (arhar2024characteristicsoforal pages 8-9).

Ontology-annotated knowledge elements - Embedded artifact with HGNC (genes/proteins), GO (processes), CL (cell types), UBERON (anatomy), HPO (phenotypes), and ChEBI (chemicals): | Category | Entity (preferred name) | Identifier (prefix:ID) | Role in XLH pathophysiology (1–2 lines) | Supporting sources | |---|---|---|---|---| | Gene/Protein | PHEX | HGNC:8860 | Loss-of-function mutations in PHEX (osteocytes/odontoblasts) lead to elevated FGF23 and accumulation of mineralization inhibitors (e.g., ASARM peptides), causing hypomineralization. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | FGF23 | HGNC:3689 | Osteocyte-derived hormone that reduces renal phosphate reabsorption and suppresses 1,25(OH)2D synthesis; central driver of XLH hypophosphatemia. | (wang2024metaanalysisandsystematic pages 18-18, wang2024metaanalysisandsystematic pages 1-2, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | FGFR1 | HGNC:3688 | FGFR1c (with Klotho) mediates FGF23 signalling in kidney/parathyroid, altering phosphate and vitamin D handling. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | KLOTHO | HGNC:6353 | Co-receptor for FGF23 that confers tissue specificity (renal proximal tubule and parathyroid) for FGF23 effects. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | SLC34A1 (NaPi-IIa) | HGNC:11039 | Renal proximal-tubule sodium-phosphate cotransporter downregulated by FGF23 → decreased phosphate reabsorption (low TmP/GFR). | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | SLC34A3 (NaPi-IIc) | HGNC:11041 | Alternate proximal-tubule phosphate transporter contributing to renal phosphate handling; mutations cause other hypophosphatemias and are relevant to renal phenotype. | (brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | CYP27B1 | HGNC:2593 | Encodes 1α-hydroxylase; FGF23 suppresses CYP27B1, lowering 1,25(OH)2D and reducing intestinal phosphate/calcium absorption. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | CYP24A1 | HGNC:2594 | Encodes 24‑hydroxylase; FGF23 upregulates CYP24A1, increasing catabolism of 1,25(OH)2D and contributing to low calcitriol. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Gene/Protein | MEPE | HGNC:13323 | Matrix protein whose ASARM-derived peptides (when not degraded by PHEX) inhibit hydroxyapatite formation and impair mineralization. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Gene/Protein | DMP1 | HGNC:2936 | Osteocyte matrix protein; loss-of-function forms cause FGF23 dysregulation in hereditary hypophosphatemias and modulate mineralization. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Gene/Protein | ENPP1 | HGNC:3352 | Regulates extracellular pyrophosphate (PPi); dysregulation affects mineralization balance and can interact with FGF23-related pathways. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Gene/Protein | SPP1 (Osteopontin) | HGNC:11255 | Accumulates when PHEX activity is reduced; binds mineral and can inhibit crystal growth, contributing to defective mineralization. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Cell type | Osteocyte | CL:0000121 | Principal source of FGF23 and site of PHEX expression; dysfunctional osteocytes drive endocrine and local mineralization defects. | (arhar2024characteristicsoforal pages 8-9, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Cell type | Osteoblast | CL:0000062 | Bone-forming cell interacting with osteocytes and matrix SIBLING proteins; contributes to defective mineral deposition in XLH. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Cell type | Chondrocyte | CL:0000138 | Growth-plate chondrocytes are affected by phosphate deficiency, causing rickets and growth-plate abnormalities in children. | (brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Cell type | Renal proximal tubule epithelial cell | CL:0002306 | Primary renal target where FGF23–Klotho–FGFR1c signalling downregulates NaPi transporters, causing phosphaturia. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Cell type | Parathyroid chief cell | CL:0000772 | Parathyroid is a target of FGF23; altered FGF23/PTH interplay contributes to mineral metabolism dysregulation and secondary hyperparathyroidism risk. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Cell type | Odontoblast | CL:0000115 | Dental cell expressing PHEX and FGF23 mRNA; PHEX dysfunction leads to dentin hypomineralization and increased dental abscess risk. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11, defabianis2025xlinkedhypophosphatemiain pages 1-2) | | Cell type | Ameloblast | CL:0002494 | Enamel-forming cell where FGF23 expression has been detected; contributes to dental phenotype alongside odontoblast defects. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Cell type | Cementoblast | CL:0009011 | Cementum-forming cell implicated in abnormal cementum observed in XLH models, contributing to periodontal/dentoalveolar pathology. | (arhar2024characteristicsoforal pages 10-11) | | Anatomy | Bone | UBERON:0001474 | Primary affected tissue: impaired mineralization (rickets in children, osteomalacia in adults) due to systemic hypophosphatemia and local inhibitors. | (brandi2025xlinkedhypophosphatemiaand pages 2-4, arhar2024characteristicsoforal pages 8-9) | | Anatomy | Kidney | UBERON:0002113 | Site of phosphate wasting via proximal-tubule transporter downregulation and altered vitamin D metabolism from FGF23 action. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Anatomy | Renal proximal tubule | UBERON:0001285 | Anatomical location of NaPi transporters and Klotho expression where FGF23 exerts phosphaturic effects. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Anatomy | Growth plate | UBERON:0003616 | Phosphate-dependent cartilage mineralization occurs here; deficiency leads to widening/irregularity and rickets. | (brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Anatomy | Parathyroid gland | UBERON:0001132 | Interacts with FGF23/vitamin D axis; PTH disturbances (secondary/tertiary hyperparathyroidism) are clinical concerns with therapy. | (wang2024metaanalysisandsystematic pages 18-18, arcidiacono2025potentialpredictorsof pages 16-20) | | Anatomy | Tooth | UBERON:0001091 | Dentition displays hypomineralized dentin, enamel defects, and increased periapical pathology in XLH. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11, defabianis2025xlinkedhypophosphatemiain pages 1-2) | | Anatomy | Dentin | UBERON:0001750 | Site of defective mineralization (reduced dentin mineral density, abnormal tubules) linked to PHEX/ASARM effects. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11) | | Anatomy | Cementum | UBERON:0001751 | Abnormal cementum reported in hyp mouse and human studies, contributing to periodontal/dental sequelae. | (arhar2024characteristicsoforal pages 10-11) | | Anatomy | Enthesis | UBERON:0002185 | Enthesopathies (calcific enthesopathy) are common adult complications related to chronic mineral imbalance and mechanical stress. | (arcidiacono2025potentialpredictorsof pages 16-20, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Hypophosphatemia | HPO:HP:0002148 | Biochemical hallmark caused by FGF23-driven renal phosphate wasting; central diagnostic feature (low serum phosphate, low TmP/GFR). | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Rickets | HPO:HP:0002748 | Pediatric manifestation from impaired growth-plate mineralization leading to bowing, deformity, and delayed motor milestones. | (brandi2025xlinkedhypophosphatemiaand pages 2-4, arcidiacono2025potentialpredictorsof pages 16-20) | | Phenotype | Osteomalacia | HPO:HP:0002649 | Adult manifestation of defective bone mineralization with bone pain, fractures, and pseudofractures. | (brandi2025xlinkedhypophosphatemiaand pages 2-4, arcidiacono2025potentialpredictorsof pages 16-20) | | Phenotype | Short stature | HPO:HP:0004322 | Growth impairment from chronic phosphate deficiency and rickets in pediatric patients. | (arcidiacono2025potentialpredictorsof pages 16-20, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Bowing of long bone | HPO:HP:0002970 | Classic orthopedic deformity in untreated/under-treated pediatric XLH due to growth-plate pathology. | (brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Dental abscess | HPO:HP:0001088 | Frequent spontaneous periapical infections linked to hypomineralized dentin and pulp exposures. | (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11, defabianis2025xlinkedhypophosphatemiain pages 1-2) | | Phenotype | Craniosynostosis | HPO:HP:0001363 | Reported comorbidity in some cohorts; linked to abnormal skull growth/mineralization in XLH. | (arcidiacono2025potentialpredictorsof pages 16-20, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Enthesopathy | HPO:HP:0002657 | Calcific enthesopathies and joint problems arise in adults with long-standing disease. | (arcidiacono2025potentialpredictorsof pages 16-20, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Pseudofractures | HPO:HP:0002757 | Insufficiency fractures due to impaired bone strength from osteomalacia. | (arcidiacono2025potentialpredictorsof pages 16-20, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Phenotype | Nephrocalcinosis | HPO:HP:0000121 | Potential complication of conventional phosphate/vitamin D therapy (and monitoring concern). | (wang2024metaanalysisandsystematic pages 18-18, wang2024metaanalysisandsystematic pages 1-2) | | Phenotype | Hyperparathyroidism | HPO:HP:0000829 | Secondary/tertiary hyperparathyroidism can arise from therapy or chronic mineral dysregulation. | (wang2024metaanalysisandsystematic pages 18-18, arcidiacono2025potentialpredictorsof pages 16-20) | | Phenotype | Hearing impairment | HPO:HP:0000365 | Reported extracranial manifestation in XLH cohorts (sensorineural/conductive elements described). | (arcidiacono2025potentialpredictorsof pages 16-20) | | Chemical | Phosphate | ChEBI:18367 | Essential mineral depleted in XLH due to renal wasting; low serum phosphate drives clinical manifestations. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Chemical | 1,25-dihydroxyvitamin D3 (calcitriol) | ChEBI:28940 / ChEBI:17933 | Active vitamin D reduced in XLH (FGF23 suppresses synthesis/increases catabolism), lowering intestinal phosphate/calcium absorption. | (wang2024metaanalysisandsystematic pages 18-18, brandi2025xlinkedhypophosphatemiaand pages 2-4) | | Chemical | Calcitriol (therapeutic) | ChEBI:17933 | Used therapeutically with oral phosphate to boost mineralization but does not target FGF23 and carries risk of hypercalciuria. | (wang2024metaanalysisandsystematic pages 18-18, wang2024metaanalysisandsystematic pages 1-2) | | Chemical | Burosumab (anti-FGF23 antibody) | ChEBI:132958 | FGF23-neutralizing monoclonal antibody that restores renal phosphate reabsorption, raises serum phosphate and 1,25(OH)2D, improving rickets/osteomalacia. | (wang2024metaanalysisandsystematic pages 18-18, wang2024metaanalysisandsystematic pages 1-2, brandi2025xlinkedhypophosphatemiaand pages 2-4) |

Table: A compact mapping of key genes, cell types, anatomical sites, phenotypes, and chemicals for X-linked hypophosphatemia (XLH), linked to standard ontology identifiers and primary supporting evidence from gathered sources. This table is intended for knowledge‑base annotation and mechanistic reference (arhar2024characteristicsoforal pages 8-9, brandi2025xlinkedhypophosphatemiaand pages 2-4).

Mechanism of action and outcomes of burosumab - Mechanism: Anti-FGF23 monoclonal antibody that restores renal phosphate handling (NaPi-IIa/IIc expression indirectly via FGF23 blockade) and increases endogenous 1,25(OH)2D by relieving CYP27B1 suppression/CYP24A1 induction (BDJ Open, 2024; Front Endocrinol, 2024) (arhar2024characteristicsoforal pages 8-9, wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18). - Outcomes: In children, improved serum phosphate, TmP/GFR, ALP, rickets severity scores, and functional capacity; favorable safety compared with conventional therapy, with ongoing assessment of long-term growth and QOL (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18).

Evidence items (with PMIDs/DOIs/URLs/dates) - Brandi ML et al. X-linked hypophosphatemia and tumor-induced osteomalacia: narrative review/expert opinion. Orphanet J Rare Dis. Published Oct 2025. DOI: 10.1186/s13023-025-03952-5. URL: https://doi.org/10.1186/s13023-025-03952-5 (brandi2025xlinkedhypophosphatemiaand pages 2-4, brandi2025xlinkedhypophosphatemiaand pages 14-14). - Wang K et al. Meta-analysis and systematic review: burosumab in children with XLH. Front Endocrinol. Published Aug 2024. DOI: 10.3389/fendo.2024.1414509. URL: https://doi.org/10.3389/fendo.2024.1414509 (wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18). - Arhar A et al. Characteristics of oral health of patients with XLH. BDJ Open. Published May 2024. DOI: 10.1038/s41405-024-00223-6. URL: https://doi.org/10.1038/s41405-024-00223-6 (arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11). - Defabianis P et al. XLH in childhood: dental involvement, diagnosis, and treatment. Eur J Paediatr Dent. Published Dec 2025. DOI: 10.23804/ejpd.2025.2348. URL: https://doi.org/10.23804/ejpd.2025.2348 (defabianis2025xlinkedhypophosphatemiain pages 1-2). - Arcidiacono GP. Potential predictors of response to burosumab in adults with XLH. 2025. (arcidiacono2025potentialpredictorsof pages 16-20).

Limitations - Some 2025 sources are narrative/expert reviews. Where randomized and meta-analytic evidence is summarized, we prioritized 2024 systematic reviews/meta-analyses. Long-term outcomes (adult skeletal structure, enthesopathy reversal, dental organ regeneration) require further longitudinal data (Front Endocrinol, 2024) (wang2024metaanalysisandsystematic pages 1-2).

Pathophysiology description (knowledge-base ready) - XLH results from PHEX (HGNC:8860) loss-of-function in osteocytes/odontoblasts, elevating FGF23 (HGNC:3689), which activates FGFR1 (HGNC:3688) with KLOTHO (HGNC:6353) in renal proximal tubule (UBERON:0001285) and parathyroid (UBERON:0001132), reducing NaPi-IIa (SLC34A1; HGNC:11039)/NaPi-IIc (SLC34A3; HGNC:11041) activity and suppressing CYP27B1 (HGNC:2593) while inducing CYP24A1 (HGNC:2594). Systemic effects are hypophosphatemia (HP:0002148) and low/normal 1,25(OH)2D, with local matrix inhibition by MEPE/ASARM (HGNC:13323) and osteopontin/SPP1 (HGNC:11255), and contributions from DMP1 (HGNC:2936) and ENPP1 (HGNC:3352) to PPi balance, leading to rickets (HP:0002748) and osteomalacia (HP:0002649), dental abscesses (HP:0001088), and adult enthesopathy (HP:0002657) (Orphanet J Rare Dis, 2025; Front Endocrinol, 2024; BDJ Open, 2024) (brandi2025xlinkedhypophosphatemiaand pages 2-4, wang2024metaanalysisandsystematic pages 1-2, wang2024metaanalysisandsystematic pages 18-18, arhar2024characteristicsoforal pages 8-9, arhar2024characteristicsoforal pages 10-11).

References

  1. (brandi2025xlinkedhypophosphatemiaand pages 2-4): Maria Luisa Brandi, Cristina Eller Vainicher, Danilo Fintini, Andrea Giusti, Andrea Magnolato, Salvatore Minisola, and Sandro Giannini. X-linked hypophosphatemia and tumor-induced osteomalacia: a narrative review and expert opinion on the diagnostic and therapeutic challenges in the era of burosumab. Orphanet Journal of Rare Diseases, Oct 2025. URL: https://doi.org/10.1186/s13023-025-03952-5, doi:10.1186/s13023-025-03952-5. This article has 0 citations and is from a peer-reviewed journal.

  2. (wang2024metaanalysisandsystematic pages 1-2): Kangning Wang, Runze Zhang, Ziyi Chen, Yi Bai, and Qing He. Meta-analysis and systematic review: burosumab as a promising treatment for children with x-linked hypophosphatemia. Frontiers in Endocrinology, Aug 2024. URL: https://doi.org/10.3389/fendo.2024.1414509, doi:10.3389/fendo.2024.1414509. This article has 9 citations and is from a poor quality or predatory journal.

  3. (arhar2024characteristicsoforal pages 8-9): Ana Arhar, Alenka Pavlič, and Luka Hočevar. Characteristics of oral health of patients with x-linked hypophosphatemia: case reports and literature review. BDJ Open, May 2024. URL: https://doi.org/10.1038/s41405-024-00223-6, doi:10.1038/s41405-024-00223-6. This article has 5 citations and is from a peer-reviewed journal.

  4. (defabianis2025xlinkedhypophosphatemiain pages 1-2): P. Defabianis, N. Bocca, and R. Ninivaggi. X-linked hypophosphatemia in childhood: dental involvement, diagnosis, and treatment. European journal of paediatric dentistry, pages 1, Dec 2025. URL: https://doi.org/10.23804/ejpd.2025.2348, doi:10.23804/ejpd.2025.2348. This article has 0 citations and is from a peer-reviewed journal.

  5. (wang2024metaanalysisandsystematic pages 18-18): Kangning Wang, Runze Zhang, Ziyi Chen, Yi Bai, and Qing He. Meta-analysis and systematic review: burosumab as a promising treatment for children with x-linked hypophosphatemia. Frontiers in Endocrinology, Aug 2024. URL: https://doi.org/10.3389/fendo.2024.1414509, doi:10.3389/fendo.2024.1414509. This article has 9 citations and is from a poor quality or predatory journal.

  6. (arhar2024characteristicsoforal pages 10-11): Ana Arhar, Alenka Pavlič, and Luka Hočevar. Characteristics of oral health of patients with x-linked hypophosphatemia: case reports and literature review. BDJ Open, May 2024. URL: https://doi.org/10.1038/s41405-024-00223-6, doi:10.1038/s41405-024-00223-6. This article has 5 citations and is from a peer-reviewed journal.

  7. (arcidiacono2025potentialpredictorsof pages 16-20): GP Arcidiacono. Potential predictors of response to burosumab treatment in adult patients with x-linked hypophosphatemia. Unknown journal, 2025.

  8. (brandi2025xlinkedhypophosphatemiaand pages 14-14): Maria Luisa Brandi, Cristina Eller Vainicher, Danilo Fintini, Andrea Giusti, Andrea Magnolato, Salvatore Minisola, and Sandro Giannini. X-linked hypophosphatemia and tumor-induced osteomalacia: a narrative review and expert opinion on the diagnostic and therapeutic challenges in the era of burosumab. Orphanet Journal of Rare Diseases, Oct 2025. URL: https://doi.org/10.1186/s13023-025-03952-5, doi:10.1186/s13023-025-03952-5. This article has 0 citations and is from a peer-reviewed journal.