Thyroxine 5-deiodinase (DIO3)

Responsible for the deiodination of T4 (3,5,3',5'- tetraiodothyronine) into RT3 (3,3',5'-triiodothyronine) and of T3 (3,5,3'-triiodothyronine) to T2 (3,3'-diiodothyronine).

RT3 and T2 are inactive metabolites.

May play a role in preventing premature exposure of developing fetal cells to mature levels of thyroid hormones. Can modulate circulating fetal thyroid hormone concentrations throughout gestation.