The protein encoded by the DIO1 gene belongs to the iodothyronine deiodinase family. It catalyzes the activation, as well as the inactivation of thyroid hormone by outer and inner ring deiodination, respectively. The activation reaction involves the conversion of the prohormone thyroxine (3,5,3',5'-tetraiodothyronine, T4), secreted by the thyroid gland, to the bioactive thyroid hormone (3,5,3'-triiodothyronine, T3) by 5'-deiodination.
This protein provides most of the circulating T3, which is essential for growth, and metabolism. This protein is a selenoprotein, containing the rare amino acid selenocysteine (Sec) at its active site.