| Domain | Curated finding | Quantitative evidence | Suggested ontology/identifier | Evidence type |
|---|---|---:|---|---|
| Disease identity | Allan-Herndon-Dudley syndrome, also termed MCT8 deficiency, is an ultra-rare disorder of thyroid hormone transport caused by SLC16A2 dysfunction (pqac-00000000, pqac-00000004) | Open Targets disease-target score 0.853; ~320 clinical cases described worldwide in reviews (pqac-00000005) | MONDO:0010354; OMIM:300523; MeSH:C537047; SLC16A2 | Aggregated disease resource + review |
| Synonyms | Common synonyms include Allan-Herndon-Dudley syndrome, AHDS, MCT8 deficiency, monocarboxylate transporter 8 deficiency (pqac-00000003, pqac-00000004) | — | MONDO:0010354; OMIM:300523 | Review/guideline |
| Etiology | Caused by pathogenic loss-of-function variants in SLC16A2 encoding monocarboxylate transporter 8 (MCT8), a thyroid hormone transporter (pqac-00000001, pqac-00000004) | ~150 pathogenic variants reported; ~250 families in one review (pqac-00000009, pqac-00000019) | SLC16A2; MCT8 | Human genetics + review |
| Inheritance | Usually X-linked, predominantly affecting males; rare affected females occur with skewed X-inactivation or chromosomal rearrangements (pqac-00000009, pqac-00000033) | A carrier mother has a 50% risk of transmitting the mutant allele to sons; skewed X-inactivation described in rare females (pqac-00000033) | X-linked inheritance; SLC16A2 | Human clinical/genetic counseling review |
| Variant spectrum | Variant classes include large deletions, truncating/frameshift/nonsense variants, and missense variants; pathogenicity of many missense variants requires functional testing (pqac-00000019, pqac-00000029) | C-terminal missense variants beyond Met574 may be better tolerated in some cases (pqac-00000020) | SLC16A2 | Human genetics + functional in vitro |
| Core mechanism | Loss of MCT8 reduces T3/T4 transport across brain barriers and into neural cells, causing cerebral hypothyroidism with simultaneous peripheral thyrotoxicosis from elevated circulating T3 (pqac-00000001, pqac-00000015) | Postmortem human cortex showed ~50% reduction in cerebral T3/T4 in cited review summary (pqac-00000015, pqac-00000013) | SLC16A2; HP:0001252 Hypotonia; HP:0001290 Generalized hypotonia | Human postmortem + model + review |
| Laboratory signature | Characteristic thyroid hormone “fingerprint”: high T3, low/low-normal T4, low reverse T3, TSH normal or mildly elevated; elevated T3/rT3 ratio is especially characteristic (pqac-00000006, pqac-00000035) | Elevated T3 in 95%; low free T4 in 89%; low total T4 in 90%; low rT3 in 91%; TSH within age-specific range in 89% (pqac-00000019) | HP:0031508 Increased circulating triiodothyronine level; HP:0011787 Decreased circulating thyroxine level | Human cohort + guideline |
| Neonatal laboratory pattern | Standard newborn screening usually misses the disorder because T3 is not yet elevated in the neonatal period; low rT3 may be an earlier clue (pqac-00000034, pqac-00000035) | In 8 patients with T4-based newborn screening data, 88% had total T4 below the 20th percentile, but none were identified by newborn screening (pqac-00000019) | Newborn screening limitation; SLC16A2 | Human cohort + review |
| Major neurologic phenotype | Severe intellectual and motor disability with hypotonia, spasticity, dystonia, poor head control, lack of speech, and often inability to sit/walk (pqac-00000019, pqac-00000030) | In a 24-patient cohort: hypotonia 100%, spasticity 71%, dystonia 75%, MRI hypomyelination 19/24; in a larger cohort only 4/77 developed walking abilities (pqac-00000019, pqac-00000017) | HP:0001252 Hypotonia; HP:0001257 Spasticity; HP:0001332 Dystonia; HP:0001263 Global developmental delay | Human cohort |
| Seizures and movement phenomena | Seizures occur in a minority; exaggerated startle/paroxysmal nonepileptic events and choreiform/dystonic manifestations are recognized (pqac-00000009, pqac-00000035) | Seizures in ~25% in review summary; exaggerated startle/paroxysmal nonepileptic events in 11 patients in one movement-disorder summary (pqac-00000009, pqac-00000035) | HP:0001250 Seizure; HP:0001336 Myoclonus (if present); HP:0001332 Dystonia | Human cohort/review |
| MRI/myelin phenotype | Brain MRI often shows delayed myelination/hypomyelination, especially in early childhood, with uncertainty over delayed versus permanent hypomyelination (pqac-00000005, pqac-00000033) | Majority of patients abnormal in infancy; 19/24 with hypomyelination in one cohort (pqac-00000019) | HP:0002188 Delayed CNS myelination | Human MRI cohort + review |
| Peripheral/nutritional phenotype | Low body weight, muscle wasting/hypotrophic musculature, feeding problems, reflux, constipation, and swallowing impairment reflect peripheral thyrotoxicosis plus neurologic disability (pqac-00000030, pqac-00000033) | Underweight in 71%; hypotrophic musculature in 84%; impaired swallowing in 71% in the international cohort (pqac-00000019) | HP:0004325 Decreased body weight; HP:0002015 Dysphagia; HP:0002020 Gastroesophageal reflux | Human cohort |
| Cardiovascular phenotype | Tachycardia, PACs/arrhythmia, systolic hypertension, and conduction abnormalities are common and clinically important (pqac-00000030, pqac-00000017) | Resting tachycardia 31%; elevated systolic blood pressure 53%; premature atrial contractions 76% (pqac-00000017) | HP:0001649 Tachycardia; HP:0011675 Arrhythmia; HP:0000822 Hypertension | Human cohort |
| Skeletal/orthopedic phenotype | Scoliosis, hip subluxation, osteoporosis, and later spasticity contribute substantially to disability (pqac-00000030, pqac-00000033) | Quantitative prevalence not consistently reported in retrieved sources | HP:0002650 Scoliosis; HP:0002827 Hip dislocation; HP:0000939 Osteoporosis | Guideline + review |
| Onset and course | Symptoms usually become apparent after 2–4 months of age; developmental abilities plateau far below age expectations and disease is lifelong/progressive in disability burden (pqac-00000035, pqac-00000017) | First symptoms around 4 months; median age at diagnosis 24 months (IQR 12.0–60.0); median delay from symptoms to diagnosis 18 months (IQR 7.8–63.0) (pqac-00000009, pqac-00000035) | Congenital/pediatric neurodevelopmental disorder; HP:0001263 | Human cohort + review |
| Prognosis | Life expectancy is substantially reduced, with childhood deaths often related to pulmonary infection/aspiration and possibly sudden cardiac death (pqac-00000002, pqac-00000017) | Median survival 35 years; ~30% mortality in childhood in review summary; approximately 50% of severely affected patients may die in childhood (pqac-00000002, pqac-00000017) | OMIM:300523 | Human cohort + review |
| Mortality predictors | Early underweight and absent head control are major markers of poor prognosis (pqac-00000004, pqac-00000017) | Both linked to higher mortality; being underweight at 1–3 years and no head control before 1.5 years strongly associated with early death (pqac-00000017) | HP:0004325 Decreased body weight; HP:0001252 Hypotonia | Human cohort |
| Diagnosis | Diagnostic workup combines clinical phenotype, thyroid profile, MRI, and confirmatory SLC16A2 sequencing; VUS assessment should include segregation, functional assays, and structural modeling (pqac-00000029, pqac-00000035) | 2024 ETA recommends sequencing in any male with major biochemical criteria plus developmental delay, hypomyelination, movement disorder, primitive reflexes, or family history (pqac-00000029) | SLC16A2 sequencing; MONDO:0010354 | Guideline |
| Differential diagnosis | Key differentials include cerebral palsy, Pelizaeus-Merzbacher(-like) disease, MECP2 duplication, mitochondrial disease, and RTHα; tachycardia/failure to thrive favor MCT8 deficiency over RTHα (pqac-00000035) | RTHα is the closest biochemical mimic among thyroid disorders in retrieved review table (pqac-00000035) | THRA; MECP2 | Review |
| Genetic counseling/prevention | Cascade testing, carrier testing for at-risk female relatives, prenatal diagnosis, and preimplantation genetic testing are feasible once a family variant is known (pqac-00000029, pqac-00000034) | Prenatal SLC16A2 testing recommended/considered for at-risk male fetuses in positive families (pqac-00000029) | SLC16A2; prenatal testing | Guideline + review |
| Supportive care | Multidisciplinary care is essential: neurology, endocrinology, gastroenterology/nutrition, cardiology, physical/speech/occupational therapy, orthopedics, and social work/case management (pqac-00000033, pqac-00000030) | In one registry survey only 19% had a pediatric gastroenterologist, 31% had dietary advice, 12.5% of those with feeding problems had a feeding tube, and 1 in 5 lacked regular physical therapy (pqac-00000033) | NCIT supportive care terms not assigned here; HP:0011968 Feeding difficulties | Registry/review |
| Tiratricol/TRIAC evidence | TRIAC is the leading targeted therapy; it enters cells independently of MCT8 and improves peripheral thyrotoxicosis, with possible greater neurodevelopmental benefit if started very early (pqac-00000006, pqac-00000036) | Phase 2 Triac Trial I: 46 enrolled, 45 with follow-up, 40 completed 12 months; long-term cohort n=67 (27 trial + 40 compassionate use), 90% achieved target T3 range; improvements in weight, heart rate/rhythm, blood pressure, SHBG/creatinine markers (pqac-00000036, pqac-00000028) | Tiratricol / TRIAC / 3,3',5-triiodothyroacetic acid | Human clinical trial + real-world follow-up |
| Tiratricol status and current implementation | 2024 ETA recommends TRIAC (strong recommendation); expanded access and ongoing early-life/withdrawal studies support real-world implementation (pqac-00000006, pqac-00000021, pqac-00000022, pqac-00000024) | Target serum T3 1.4–2.5 nmol/L; Triac Trial II enrolled 22 children ≤30 months; ReTRIACt phase 3 completed with actual enrollment 20; US expanded access posted 2023-06-22 (pqac-00000006, pqac-00000022, pqac-00000024) | NCT02396459; NCT05579327; NCT05911399 | Guideline + ClinicalTrials.gov |
| DITPA evidence | DITPA lowers serum T3/TSH and can cross the BBB independently of MCT8, but published human neurologic benefit has been limited (pqac-00000016, pqac-00000036) | Compassionate use in 4 children normalized T3 and TSH but no clear neurocognitive improvement; median treatment 38.5 months (range 26–40) in one review summary (pqac-00000016) | DITPA / 3,5-diiodothyropropionic acid | Human compassionate use + preclinical |
| LT4/PTU and conventional thyroid therapy | Postnatal levothyroxine monotherapy is not recommended; LT4+PTU can reduce peripheral thyrotoxicosis but not neurologic deficits and PTU has important toxicity risks (pqac-00000006, pqac-00000017) | LT4+PTU reported in 5 patients; PTU carries severe hepatotoxicity/agranulocytosis risk (pqac-00000017, pqac-00000033) | Levothyroxine; propylthiouracil | Human case series + guideline |
| Chemical chaperone approach | Sodium/glycerol phenylbutyrate aims to rescue selected misfolded MCT8 mutants by improving membrane trafficking; human efficacy remains unproven (pqac-00000034, pqac-00000026) | Prospective GPB trial planned for up to 6 genetically confirmed patients (NCT05019417) (pqac-00000026) | NCT05019417 | Preclinical + trial registration |
| Gene therapy | AAV-mediated SLC16A2/Mct8 delivery is a promising preclinical strategy, especially targeting BBB/endothelium, but remains preclinical (pqac-00000034, pqac-00000011) | IV AAV9-hMCT8 and AAV-BR1-Mct8 increased brain T3 in mice; no human trial identified in retrieved sources (pqac-00000034) | SLC16A2 gene therapy | Preclinical animal |
| Principal animal model | Mct8/Oatp1c1 double-knockout mouse best recapitulates human disease, unlike Mct8 single knockout which lacks the full cerebral phenotype (pqac-00000010, pqac-00000016) | dKO mice show decreased life expectancy, central hypothyroidism, peripheral hyperthyroidism, impaired myelination, impaired motor abilities, and peripheral tissue thyrotoxicosis (pqac-00000010) | Mct8/Oatp1c1 dKO mouse | Animal model |
| Additional models | Zebrafish and patient-derived iPSC/cerebral organoid systems are useful for mechanism and therapeutic screening (pqac-00000016, pqac-00000013) | TRIAC completely rescued myelination in mct8-/- zebrafish larvae in review summary; human organoids showed D3 activity ~18.4 ± 7.7 vs 5.3 ± 3.1 pmol/mg/h in MCT8-deficient COs (~30% of WT) (pqac-00000016, pqac-00000012) | Zebrafish mct8-/-; iPSC cerebral organoid | Animal + human in vitro |
| Advanced human model insight | 2024 cerebral organoids directly showed impaired T3 uptake/action, smaller rosettes, thinner cortical units, and rescue of T3-responsive genes by DITPA/TRIAC (pqac-00000013, pqac-00000012) | MCT8-deficient CO D3 activity 5.3 ± 3.1 pmol/mg/h vs control 18.4 ± 7.7; selective MCT8 inhibitor reduced WT D3 activity to 4.4 ± 3.0 (pqac-00000012) | Human iPSC-derived cerebral organoid | Human in vitro |
| Quality-of-life and caregiver burden | Disease burden is high for patients and families; caregiver QoL/economic-burden study has been completed but results were not retrieved here (pqac-00000023, pqac-00000032) | Caregiver study enrolled 21 participants across multiple countries (pqac-00000023) | NCT06060197 | Observational study/registry |


*Table: This table condenses high-confidence disease facts for Allan-Herndon-Dudley syndrome/MCT8 deficiency, including identifiers, molecular cause, clinical signature, prognosis, diagnostics, therapies, and key research models. It is designed for rapid knowledge-base curation with quantitative evidence and cited evidence types.*