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MCT8 DEFICIENCY DIAGNOSIS

Diagnosis of MCT8 deficiency (Allan-Herndon-Dudley syndrome [AHDS]) is frequently delayed (median of 14 months [range: 0 months-26 years] after first symptoms) due to nonspecific clinical features, limited clinician awareness, and incomplete thyroid hormone panel.1,2

Patients with MCT8 deficiency show early symptoms.1 Clinicians should suspect the disorder in patients with neurodevelopmental impairment, hypotonia, progressive deterioration of body weight, tachycardia, abnormal blood pressure, and feeding difficulties.1,2

Because T3 is not included in standard thyroid hormone panels, adding T3 testing is essential to prompt earlier evaluation and genetic confirmation.1

Testing graphic.

MCT8 Deficiency DIAGNOSIS PROCESS1

Diagnosis process graphic. Diagnosis process graphic.

This approach helps differentiate MCT8 deficiency from other causes of neurodevelopmental impairment and thyroid dysfunction, such as cerebral palsy or resistance to thyroid hormone.1 Incorporating T3 testing early in the diagnostic process is critical to avoid misdiagnosis.1

Thyroid Hormone Profile

Standard thyroid panels often do not include T3, delaying diagnosis.1 Including T3 testing in children with neurodevelopmental impairment and hypotonia is essential.1

Thyroid hallmark profile for MCT8 deficiency charts.

Genetic Testing

A definitive diagnosis of MCT8 deficiency requires identification of a pathogenic mutation in the SLC16A2 gene.1 This gene encodes MCT8, which is essential for transporting thyroid hormones—particularly triiodothyronine (T3)—into cells and across the bloodbrain barrier.3

Genetic counseling is recommended for families to explain inheritance patterns, carrier risks, and family planning.1 As an X-linked disorder, male children of carrier mothers have a 50% risk of being affected.1 De novo mutations and germline mosaicism can also occur, although are rare.1

Genetic testing is essential for definitive diagnosis and family counseling, enabling early intervention and informed family planning.1

Time to Diagnosis

MCT8 deficiency typically manifests early as symptoms usually present at 2 to 3 months of age.1 Yet diagnosis often lags behind symptom onset.2 Responses to patient and caregiver questionnaires show the median2

  • Diagnostic delay is 14 months (range: 0 months-26 years) from first symptoms

For patients born after 2017, this delay in diagnosis improved to 8 months compared with 19 months for those born before 2017, likely due to increased awareness and access to genetic testing.2

Late diagnosis remains a major challenge, with 57% of parents reporting delayed or alternative diagnoses and 43% citing the need to visit multiple specialists before reaching the correct diagnosis.2

Baby with parent talking to doctor.

Why diagnosis delays may occur:

Potential clinical consequences:

Delayed diagnosis postpones access to supportive care, management and investigational therapies, increasing risks of progressive deterioration of body weight, aspiration pneumonia, and cardiovascular complications.1

When evaluating infants with neurodevelopmental impairment and hypotonia, consider MCT8 as part of the the differential diagnosis. Includes T3 as part of thyroid hormone panel and/or order genetic testing.1

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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, 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. 3. 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.