PURPOSE OF REVIEW: To discuss the recent advances on thyroid hormone transport 
in the brain. A special attention is paid to the X-linked thyroid hormone cell 
transport (THCT) defect (also known as the Allan-Herndon-Dudley syndrome), 
caused by mutations of the specific thyroid hormone transporter MCT8 gene.
RECENT FINDINGS: MCT8 is involved in thyroid hormone transport in the brain. MRI 
of patients with THCT defect showed myelination delays, probably related to 
impaired thyroid hormone action on oligodendrocytes. MCT8 is also expressed in 
the thyroid and has an important role in thyroid hormone secretion. The altered 
circulating concentrations of thyroid hormone in the patients are partly because 
of impaired secretion and altered peripheral metabolism. Increased deiodinase 
activity is important in the pathophysiology of the syndrome. High D1 activity 
in liver and kidney increases T4 and rT3 deiodination, and contributes to the 
increased serum T3. High D2 activity in the brain contributes to compensate the 
deficient T3 transport by increasing local T3 production.
SUMMARY: Patients with suspected X-linked leukoencephalopathy should be screened 
for MCT8 gene mutations. Research on the brain pathophysiology of the THCT 
defect should focus on the specific role of Mct8 on oligodendrocytes and 
myelination.