Thyroid hormones are known to be essential for growth, development, and 
metabolism. Recently, the monocarboxylate transporter 8 (MCT8) was identified as 
a thyroid hormone transporter, and MCT8 mutations have been associated with 
Allan-Herndon-Dudley syndrome, an X linked condition characterized by severe 
mental retardation, dysarthria, athetoid movements, muscle hypoplasia, and 
spastic paraplegia. Here we describe in detail the clinical and biochemical 
features and the response to thyroid hormone (L-thyroxine (LT4)) administration 
in a boy with an MCT8 mutation (c.1649delA) that truncates the protein in the 
twelfth transmembrane domain. It is of note that brain magnetic resonance 
imaging (MRI) revealed delayed myelination from infancy. Endocrine functions 
other than thyroid hormone regulation and metabolism were intact, resulting in 
normal hypothalamic/pituitary function tests. While LT4 administration 
suppressed thyrotropin (TSH) secretion, no significant changes in thyroid 
hormone values or clinical symptoms were observed.
CONCLUSION: the characteristic thyroid hormone function tests and brain MRI 
findings may allow screening of high-risk populations for a better understanding 
of MCT8 pathophysiology.