T_4是甲状腺分泌的主要激素,但它只有极小的生物学活性。主要的生物学活性激素T_3也是由甲状腺产生的,但这种代谢物大多是产生于 T_4的5′-位或外环的脱碘作用。有两种碘化甲腺氨酸5′-脱碘酶同功酶对这一反应起催化作用。Ⅰ型碘化甲腺氨酸5′-脱 T_4 is the major hormone secreted by the thyroid gland, but it has only minimal biological activity. The main biologically active hormone T_3 is also produced by the thyroid gland, but most of this metabolite results from deiodination at the 5’-position of T_4 or the outer ring. Two iodinated thyronine 5’-deiodinase isoenzymes catalyze this reaction. Type I iodated thyronine 5’-deaza