DIETARY IODINE AND SELENIUM INTERACT TO AFFECT THYROID-HORMONE METABOLISM OF RATS

The interaction of dietary selenium and iodine on the activities of th e selenoenzymes, selenium-dependent glutathione peroxidase (GSH-Px), a nd type I deiodinase (DI-I), and the thyroid hormones thyroxine (T4) a nd triiodothyronine (T3) were studied. Male weanling Sprague-Dawley ra ts were fed an AIN-93G diet for 6 wk with modified selenium and iodine concentration as follows: three levels each of iodine and selenium (0 .03, 0.2 added and 1.0 added mg iodine/kg diet, and 0.05, 0.18 added a nd 1.0 added mg selenium/kg diet) were used in a 3 x 3 factorial desig n. Renal, but not hepatic, DI-I activity was lower in rats with low se lenium intake than in controls. Circulating T3 concentration was not a ffected by the dietary levels of iodine or selenium. Unlike in liver, kidney and erythrocytes, thyroidal GSH-Px activity was not lower than in controls in rats with low selenium intake, but was significantly hi gher when iodine intake was low. Significant interactions of iodine an d selenium on serum T4 and thyroidal GSH-Px activity were observed. Se rum T4 was maintained at control levels when both dietary iodine and s elenium were low, but not when iodine alone, or selenium alone, was lo w, Activity of thyroidal GSH-Px was lowest in rats fed a diet containi ng high iodine and low selenium. The results suggest that high iodine intake, when selenium is deficient, may permit thyroid tissue damage a s a result of low thyroidal GSH-Px activity during thyroid stimulation . A moderately low selenium intake normalized circulating T4 concentra tion in the presence of iodine deficiency.