MCT8 deficiency (that is also known as Allan-Herndon-Dudley syndrome) is a rare X-linked inherited disorder of brain development that causes severe intellectual disability and problems with movement. This condition, which occurs almost exclusively in males, disrupts development from before birth. There is no sucking reflex and the child has marked hypotonia. Developmentally, unlike normal infants, affected males are unable to turn over from belly to back. Individuals with identical mutations have identical phenotypes and all individuals, regardless of the phenotype have severe neuropsychological impairment. Diagnosis is confirmed by demonstration of a mutation in the MCT8 gene (1,2).~MCT8-specific thyroid hormone cell-membrane transporter deficiency is characterized by severe cognitive deficiency, infantile hypotonia, diminished muscle mass and generalized muscle weakness, progressive spastic quadriplegia, joint contractures, and dystonic and/or athetoid movement with characteristic paroxysms or kinesigenic dyskinesias. Seizures occur in about 25% of cases. Most affected males never sit or walk independently or lose these abilities over time; most never speak or have severely dysarthric speech (1). Brain MRI obtained in the first few years of life shows transient delayed myelination, which improves by age four years (3). Although psychomotor findings observed in affected males do not occur in heterozygous females, the latter often have thyroid test abnormalities intermediate between affected and normal individuals.
MCT8 deficiency (that is also known as Allan-Herndon-Dudley syndrome) is a rare X-linked inherited disorder of brain development that causes severe intellectual disability and problems with movement.