Thyroid hormone is essential for the proper development and function of the 
brain. The active form of thyroid hormone is T(3), which binds to nuclear 
receptors. Recently, a transporter specific for T(3), MCT8 (monocarboxylate 
transporter 8) was identified. MCT8 is highly expressed in liver and brain. The 
gene is located in Xq13 and mutations in MCT8 are responsible for an X-linked 
condition, Allan-Herndon-Dudley syndrome (AHDS). This syndrome is characterized 
by congenital hypotonia that progresses to spasticity with severe psychomotor 
delays. Affected males also present with muscle hypoplasia, generalized muscle 
weakness, and limited speech. Importantly, these patients have elevated serum 
levels of free T(3), low to below normal serum levels of free T(4), and levels 
of thyroid stimulating hormone that are within the normal range. This 
constellation of measurements of thyroid function enables quick screening for 
AHDS in males presenting with cognitive impairment, congenital hypotonia, and 
generalized muscle weakness.