Responsible for the deiodination of T4 (3,5,3',5'-tetraiodothyronine) into T3 (3,5,3'-triiodothyronine) and of T3 into T2 (3,3'-diiodothyronine). Plays a role in providing a source of plasma T3 by deiodination of T4 in peripheral tissues such as liver and kidney. 3,3',5-triiodo-L-thyronine + A + H(+) + iodide = AH2 + L-thyroxine Belongs to the iodothyronine deiodinase family.