CHOP's PolicyLab Market Share Analysis for April 2026

Allan-Herndon-Dudley syndrome (AHDS) — also known at MCT8 deficiency — is a rare genetic disorder that affects a child’s cognition, mobility and overall health. What is MCT8 deficiency?Allan-Herndon-Dudley syndrome (AHDS) — also known at MCT8 deficiency — is a rare genetic disorder that affects a child’s cognition, mobility and overall health. In individuals with AHDS, thyroid hormone is unable to enter cells in the brain because of a defect in a thyroid hormone transporter called MCT8. Thyroid hormones are particularly important for proper brain development during pregnancy and early childhood.At the same time, excess thyroid hormone, specifically T3 (triiodothyronine), enter tissues in the rest of the body, which typically leads to life-threatening complications. The result is that patients are born with permanent and severe developmental delay from hypothyroidism (not enough thyroid hormone) in the brain while at the same time they experience hyperthyroidism (too much thyroid hormone) in the rest of their body, leading to intellectual disability, decreased muscle tone, disorganized movements, restless sleep, increased heart rate (tachycardia), failure to thrive/inability to gain weight, and, ultimately, severe infection and cardio-respiratory failure.AHDS is an X-linked genetic disorder with a 50% chance that male babies will have the disorder if their mother is a carrier of the abnormal gene that codes for the MCT8 thyroid hormone transporter protein. Although not well described, female carriers of the MCT8 gene may also have symptoms, often more mild although several females have had more severe symptoms similar to males. The disorder may also develop spontaneously during fetal development, where the baby is the first person in the family to have AHDS.Signs and symptoms of MCT8 deficiencyThere are several signs/symptoms of MCT8 deficiency. They include:Appearance – Patients may have a longer and more narrow face compared to other family members. Patients may al