Supplementary Material for: Clinical and Biochemical Monitoring of MCT8 Deficiency (Allan–Herndon–Dudley Syndrome) Across the Lifespan: Practical Considerations for Multidisciplinary Care

Pathogenic mutations in the SLC16A2 gene can result in partial or complete loss of function of the critical and highly specific monocarboxylate transporter 8 (MCT8), a thyroid hormone transporter. MCT8 deficiency (Allan-Herndon-Dudley syndrome) is a rare X-linked genetic disorder that causes profound neurodevelopmental delay, movement disorders, and peripheral thyrotoxicosis secondary to elevated serum triiodothyronine (T3). The condition is chronic and life-limiting, with patients requiring regular multidisciplinary monitoring to manage their symptoms. This guideline proposes a comprehensive, multidisciplinary management strategy for healthcare professionals caring for patients with MCT8 deficiency across all age groups, monitoring key symptoms and sequelae. It highlights the substantial heterogeneity in symptoms and long-term outcomes associated with the condition, underscoring the need for individualized patient care plans.