Cellular entry of thyroid hormones by organic anion transporting polypeptides

Several transporter families that can transport thyroid hormones have been identified. One of these is represented by the organic anion transporting polypeptide (OATP) gene superfamily, classified within the solute carrier family SLCO. In humans, eleven different OATPs are expressed in various tissues, including the liver, kidney, brain, lung, intestine and placenta. They mediate sodium-independent transport of a variety of amphipathic organic compounds, including thyroid hormones, bile acids, steroid hormones and their conjugates, linear and cyclic peptides, prostaglandins, numerous drugs and other xenobiotics. OATP1C1, which is expressed in the blood–brain barrier and testes, is the thyroid hormone transporter with the highest apparent affinity, with Km values of 90.4 nM for thyroxine (T4) and 127.7 nM for reverse T3 (rT3) transport, and could be essential for thyroid hormone delivery to the developing brain. OATP4A1 is expressed in the placenta and could be important for maternal thyroid hormone transport to the developing fetus.