The role of selenium in thyroid hormone metabolism and effects of selenium deficiency on thyroid hormone and iodine metabolism

Selenium deficiency impairs thyroid hormone metabolism by inhibiting the synthesis and activity of the iodothyronine deiodinases, which convert thyroxine (T₄) to the more metabolically active 3,3′–5 triiodothyronine (T₃). Hepatic type I iodothyronine deiodinase, identified in partially purified cell fractions using affinity labeling with ¹²⁵I]N-bromoacetyl reverse triiodothyronine, is also labeled with⁷⁵Se by in vivo treatment of rats with⁷⁵Se−Na₂SeO₃. Thus, the type I iodothyronine 5′-deiodinase is a selenoenzyme. In rats, concurrent selenium and iodine deficiency produces greater increases in thyroid weight and plasma thyrotrophin than iodine deficiency alone. These results indicate that a concurrent selenium deficiency could be a major determinant of the severity of iodine deficiency.