Role of MCT8 in transporting thyroid hormone across cell membranes

In the CNS, MCT8 is expressed in the endothelial cells and has an important role in transporting T4 and T3 across the blood brain barrier. In peripheral tissues, other transporters are also expressed, providing redundancy in thyroid hormone transport. Thus, MCT8 deficiency is characterized by stigmata of thyroid hormone deficiency as well as excess. This disorder is known as thyroid hormone cell membrane transport defect (THCMTD), or Allan-Herndon-Dudley syndrome.