Very active and specific thyroid hormone transporter. Stimulates cellular uptake of thyroxine (T4), triiodothyronine (T3), reverse triiodothyronine (rT3) and diidothyronine. Does not transport Leu, Phe, Trp or Tyr. Homodimer. Highly expressed in liver and heart. The disease is caused by variants affecting the gene represented in this entry. Abnormal brain development associated with MCT8 deficiency may be the consequence of either decreased or increased intracellular T3 concentrations. Belongs to the major facilitator superfamily. Monocarboxylate porter (TC 2.A.1.13) family. Extended N-terminus.