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Fig 14-6 The passage of T4 and T3 from the maternal to fetal circulation requires negotiation through the apical membrane (maternal-facing) and the basolateral membrane (fetal-facing) of syncytiotrophoblasts (ST), and in the first half of pregnancy (A) through the plasma membranes of cytotrophoblasts (CT) as well. The localisation and function of the six different TH transporters () present in the placenta may differ. These include monocarboxylate transporters (MCT) 8 and 10, system-L amnio acid transporters (LAT1 and LAT2) and organic anion transporting polypeptides (OATP) 1A2 and 4A1.There may also be other yet to be identified TH transporters. In addition, T4 and T3 are subject to metabolism by deioidnase type 2 (D2) and type 3 (D3) as they pass through the trophoblasts. (From 51)