Monocarboxylate transporter 8 (MCT8 or SLC16A2) is important for the neuronal 
uptake of triiodothyronine (T3) in its function as a specific and active 
transporter of thyroid hormones across the cell membrane, thus being essential 
for human brain development. We report on a German male with 
Allan-Herndon-Dudley syndrome presenting with severe intellectual and motor 
disability, paroxysmal dyskinesia combined with truncal muscular hypotonia, and 
peripheral muscular hypertonia at his current age of 9 years. Additionally, the 
patient has a lesion in the left putamen region revealed by magnetic resonance 
imaging and elevated serum T3 levels. The male appeared to have a hemizygous 
mutation (R271H) in the MCT8 gene that was sequenced directly from genomic DNA 
and occurred de novo in the maternal germline, as both his mother and his sister 
were not carriers of the mutation. Ruling out a common polymorphism, 50 normal 
individuals of the same ethnic background did not harbour the mutation. The 
identified MCT8 gene mutation (R271H) is very likely to be the genetic cause for 
neuronal hypothyroidism despite elevated serum T3 levels.