Background: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.