Responsible for the deiodination of T4 (3,5,3',5'-tetraiodothyronine) into RT3 (3,3',5'-triiodothyronine) and of T3 (3,5,3'-triiodothyronine) into T2 (3,3'-diiodothyronine). RT3 and T2 are inactive metabolites. May play a role in preventing premature exposure of developing fetal tissues to adult levels of thyroid hormones. Can regulate circulating fetal thyroid hormone concentrations throughout gestation. Essential role for regulation of thyroid hormone inactivation during embryological development. 3,3',5'-triiodo-L-thyronine + A + H(+) + iodide = AH2 + L-thyroxine Homodimer. May undergo minor heretodimerization with DIO1 and DIO2 (By similarity). Belongs to the iodothyronine deiodinase family. It is uncertain whether Met-1 or Met-27 is the initiator. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus.