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 [I-125]N-bromoacetyl reverse triiodothyronine, is also labeled with Se-75 by in vivo treatment of rats with (Se-Na2SeO3)-Se-75. 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.