Responsible for the deiodination of T4 (3,5,3',5'-tetraiodothyronine) into T3 (3,5,3'-triiodothyronine). Can also produce 3,3'-diiodothyronine from reverse T3 (rT3, 3,3',5'-triiodothyronine). Does not have significant inner-ring deiodination activity for T4, T3 or rT3. 3,3',5-triiodo-L-thyronine + A + H(+) + iodide = AH2 + L-thyroxine Inhibited by gold thioglucose (GTG) and iodoacetate (IAc). Optimum pH is 7. Optimum temperature is 20-30 degrees Celsius. Belongs to the iodothyronine deiodinase family.