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). 3,3',5'-triiodo-L-thyronine + A + H(+) + iodide = AH2 + L-thyroxine Belongs to the iodothyronine deiodinase family.