Cellular entry is an important step preceding intracellular metabolism and 
action of thyroid hormone (TH). Transport of TH across the plasma membrane does 
not take place by simple diffusion but requires transporter proteins. One of the 
most effective and specific TH transporters identified to date is 
monocarboxylate transporter 8 (MCT8), the gene of which is located on the X 
chromosome. Although MCT8 is expressed in many tissues, its function appears to 
be most critical in the brain. Hemizygous MCT8 mutations in males cause severe 
psychomotor retardation, known as the Allan-Herndon-Dudley syndrome (AHDS), and 
abnormal serum TH levels. AHDS thus represents a type of TH resistance caused by 
a defect in cellular TH transport.