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 The UGA codon in position 83 may either function as a selenocysteine codon or a translation termination codon. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. The UGA codon in position 34 may either function as a selenocysteine codon or a translation termination codon. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. The UGA codon in position 59 may either function as a selenocysteine codon or a translation termination codon. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. The UGA codon in position 59 may either function as a selenocysteine codon or a translation termination codon. Belongs to the iodothyronine deiodinase family. Deiodinase entry