. "Iodothyronine deiodinases 1 and 2 (DIO1/2) are the vertebrate enzymes responsible for the deiodination of the prohormone thyroxine (T4; 3,5,3',5'-tetraiodothyronine) into the biologically active hormone T3 (3,5,3'-triiodothyronine). DIO1/2 activity is critical for appropriate T3 levels in the brain during development."^^ . . .