Thyroid hormone (TH) is crucial for the development of different organs, in 
particular the brain, as disturbances in TH supply cause severe neurological 
abnormalities. TH transporters are necessary for the intracellular availability 
of TH to have access to the deiodinases and nuclear receptors inside the cell. 
The clinical importance of TH transporters is dramatically shown in patients 
with mutations in MCT8, suffering from severe X-linked psychomotor retardation 
in combination with disturbed TH levels, especially high serum T(3) levels, now 
referred as Allan-Herndon-Dudley Syndrome (AHDS). Worldwide >45 families have 
now been identified with MCT8 mutations. Most MCT8 mutations result in a 
complete loss of TH transport function when tested in vitro, but some mutations 
show significant residual activity and are associated with a somewhat milder 
clinical phenotype. It is difficult to identify MCT8 patients only on the basis 
of the clinical characteristics of X-linked mental retardation. Therefore, the 
criterion for MCT8 mutation screening in these patients is the profile of 
increased T(3) and low-normal to low FT(4) serum levels.