About MCT8 Deficiency

Monocarboxylate transporter 8 (MCT8) deficiency (Allan–Herndon–Dudley syndrome [AHDS]) is a rare disorder.¹ MCT8 is a cell surface thyroid hormone transporter vital for moving thyroid hormones into cells and the brain.²

The condition leads to impaired neurodevelopment due to insufficient thyroid hormone in the brain and systemic complications from persistent excess of T3 hormone in peripheral tissues.¹ The disorder is characterized by elevated serum T3, low or low-normal free T4, and normal to mildly elevated thyroid stimulating hormone (TSH) levels.¹

What Is MCT8 Deficiency?

MCT8 deficiency is a rare X-linked disorder caused by pathogenic mutations in the SLC16A2 gene.¹ This gene encodes the MCT8, which is essential for transporting thyroid hormones—particularly triiodothyronine (T3)—into cells and across the blood-brain barrier.² Thyroid hormones regulate neurodevelopment and metabolism, making this transporter critical for normal development and function of many tissues, and crucial for the overall homeostasis of the hypothalamus-pituitary-thyroid axis.²

When MCT8 function is impaired, two distinct processes occur:

Central effect

Insufficient T3 in the brain

Severe neurodevelopmental impairment and intellectual disability¹

Peripheral effect

Persistent thyrotoxicosis

Systemic complications such as failure to thrive, irritability, tachycardia, progressive deterioration in body weight, and cardiovascular symptoms^1,3

Diagnostic hallmarks of MCT8 deficiency¹

Elevated serum T3
Low or low-normal free T4
Normal to mildly elevated TSH

Consider checking T3 when you see neurodevelopmental impairment with hypotonia and failure to thrive.¹

MECHANISM OF DISEASE

Key pathophysiology

MCT8 is a critical transporter for thyroid hormones (T3 and T4), especially in the brain and peripheral organs^3,4
Mutations in the SLC16A2 gene cause MCT8 deficiency, impairing cellular uptake of thyroid hormones^5,6
Despite elevated circulating T3, tissues dependent on MCT8 (notably the brain) have reduced intracellular thyroid hormone signaling, leading to severe neurodevelopmental impairment^2,5

Disrupted hormonal regulation

Dysfunctional MCT8 impairs negative feedback in the hypothalamic-pituitary-thyroid (HPT) axis, causing increased thyroid hormone production²
T4 secretion is compromised, resulting in low/low-normal free T4 and elevated T3 in serum—a hallmark of MCT8 deficiency¹
Peripheral tissues (kidney, liver) upregulate deiodinase activity, further increasing T3 levels²

Clinical consequences

Persistently elevated peripheral T3 causes thyrotoxicosis: failure to thrive, progressive deterioration in body weight, and increased risk of infections¹⁴
Cardiovascular complications (arrhythmias, hypertension) and recurrent pulmonary infections contribute to premature mortality^1,2

MCT8 deficiency presents with a unique biochemical profile and a combination of neurodevelopmental and peripheral thyrotoxic symptoms¹

SYMPTOMS OF MCT8 DEFICIENCY

First months of life:

Poor head control²
Neurodevelopmental impairment¹
Persistent thyrotoxicosis already present biochemically (elevated serum T3 at approximately 4 months of age), though systemic consequences may be subtle initially¹
Failure to thrive¹

Clinical features:

Failure to achieve motor milestones^3,7
- Sit independently
- Walking

Feeding difficulties^1,3
- Failure to thrive
- Aspiration risk
- Feeding tube required
- Dysphagia

Hypotonia^1,3

Low bone mineral density⁶

Spasticity disorders^1,3
- Hypokinesia
- Dystonia
- Paroxysmal dyskinesia

Cardiac complications^1,6
- Persistent tachycardia and arrhythmias
- Hypertension

Electroencephalogram-confirmed seizures⁶

Ongoing muscle wasting and hypoplasia^3,6

Sleep problems and irritability⁶

Recurrent infections⁶

Limited or no independence in activities of daily living and reduced quality of life^1,3,7

Increased risk of premature mortality¹

Diagnostic indicators:

Neurodevelopmental impairment begins first and is non-progressive but severe^1,8
Peripheral thyrotoxic effects accumulate progressively over the lifespan^1,6
Neuroimaging may show delayed myelination or hypomyelination¹
Thyroid function panel reveals elevated T3, low or low-normal free T4, and normal or slightly elevated TSH¹
Genetic confirmation of SLC16A2 mutations confirms the diagnosis¹

For infants with neurodevelopmental impairment and hypotonia, include MCT8 deficiency in the differential diagnosis. Consider a thyroid hormone panel, INCLUDING T3, and conduct genetic testing.¹

STRATEGIES TO REDUCE DIAGNOSTIC DELAY

Onset of symptoms¹

2 to 3 months of age

Diagnostic delay⁹

Median time: 14 months (range: 0 months-26 years)

Why it may be underdiagnosed²

Early symptoms—hypotonia, feeding difficulties, neurodevelopmental impairment—are nonspecific and may be similar to other disorders

Strategies to improve detection

Look at family history because MCT8 deficiency is X-linked¹⁰

Consider pedigree analysis to assist other physicians in diagnosing MCT8 deficiency in other genetically related individuals¹⁰

Consider genetic testing in the presence of global neurodevelopmental impairment/intellectual disability¹¹

CHILDHOOD MORTALITY

1 in 3 affected children die^9,a

^aMedian overall survival was 35 years in a large natural history study of individuals with MCT8 deficiency⁶

ADDING T3 TO THE THYROID HORMONE PANEL and early genetic testing can reduce time to diagnosis.¹

Prognosis

MCT8 deficiency is associated with high morbidity and mortality.²

Key mortality data:

Median population life expectancy: ~35 years in a large natural history study⁶
Childhood mortality: 1 in 3 patients will die in childhood¹
High-risk indicators: absence of head control by 18 months; severe underweight at ages 1 to 3 years²

Quality of life:

Patients may experience irritability and sleep disturbances^1,10

Patients generally remain nonverbal, wheelchair-bound, experience progressive deterioration in body weight, and are fully dependent on caregivers^2,3,9

Frequent hospitalizations from cardiovascular complications and recurrent pulmonary infections impact quality of life^2,12,13

Treatment outlook:

Currently, there are no approved treatments for MCT8 deficiency in the United States, creating a high unmet need²

While the prognosis is challenging, a multidisciplinary approach may optimize care for patients.⁹

Multidisciplinary team specialists graphic.

Review more about the importance of a multidisciplinary team

LEARN MORE

Don’t miss the latest information and resources on MCT8 deficiency

References: 1. Bauer AJ, Auble B, Clark AL, et al. Unmet patient needs in monocarboxylate transporter 8 (MCT8) deficiency: a review. Front Pediatr. 2024:12:1444919. 2. van Geest FS, Gunhanlar N, Groeneweg S, Visser WE. Monocarboxylate transporter 8 deficiency: from pathophysiological understanding to therapy development. Front Endocrinol (Lausanne). 2021:12:723750. 3. Groeneweg S, van Geest FS, Peeters RP, Heuer H, Visser WE. Thyroid hormone transporters. Endocr Rev. 2020;41(2):146-201. 4. van Geest FS, Groeneweg S, Visser WE. Monocarboxylate transporter 8 deficiency: update on clinical characteristics and treatment. Endocrine. 2021;71:689–695. 5. Groeneweg S, Lima de Souza EC, Meima ME, Peeters RP, Visser WE, Visser TJ. Outward-open model of thyroid hormone transporter monocarboxylate transporter 8 provides novel structural and functional insights. Endocrinology. 2017;158(10):3292-3306. 6. Groeneweg S, van Geest FS, Abacı A, et al. Disease characteristics of MCT8 deficiency: an international, retrospective, multicentre cohort study. Lancet Diabetes Endocrinol. 2020;8:594-605. 7. Schwartz CE, Stevenson RE. The MCT8 thyroid hormone transporter and Allan–Herndon–Dudley syndrome. Best Pract Res Clin Endocrinol Metab. 2007;21(2):307-321. 8. Groeneweg S, van Geest FS, Abacı A, et al. Disease characteristics of MCT8 deficiency: an international, retrospective, multicentre cohort study. Lancet Diabetes Endocrinol. 2020;8(suppl):594-605. 9. van Geest FS, Groeneweg S, Popa VM, Stals MAM, Visser WE. Parent perspectives on complex needs in patients with MCT8 deficiency: an international, prospective, registry study. J Clin Endocrinol Metab. 2024;109(1):e330–e335. 10. National Organization for Rare Disorders (NORD). MCT8-specific thyroid hormone cell transporter deficiency. Updated January 8, 2025. Accessed January 18, 2026. 11. Rodan LH, Stoler J, Chen E, Geleske T; Council on Genetics. Genetic evaluation of the child with intellectual disability or global developmental delay: clinical report. Pediatrics. 2025;156(1):e2025072219. 12. De La Rosa Poueriet KA. Report of a severe case of Allan-Herndon-Dudley syndrome. Genet Clin Genom. 2023;1(2):43-49. 13. Boelaert K, Bauer A, Cappola AR, et al. Thyrotoxicosis in MCT8 deficiency. J Clin Endocrinol Metab. Published online January 9, 2026. doi:10.1210/clinem/dgaf707