The Allan-Herndon-Dudley syndrome (AHDS;MIM 300523) of X-linked mental 
retardation and hypotonia is caused by mutations in a thyroid hormone 
transporter gene--the monocarboxylate transporter 8 (MCT8 also known as SLC16A2) 
gene. A 23-month-old boy with severe developmental delay, hypotonia, recurrent 
emesis, and irritability is described. He was diagnosed with hypothyroidism at 
the age of 4 months. However, T3 level was elevated. Molecular analysis of the 
MCT8 gene detected a single base duplication in exon 5 c.1614dupC (p.Ile539fs), 
consistent with a diagnosis of AHDS. While T3 is the best marker for this 
disorder, elevations in TSH should alert to the diagnosis.