EFFECTS OF SELENIUM AND IODINE DEFICIENCY ON TYPE-I, TYPE-II AND TYPE-III IODOTHYRONINE DEIODINASES AND CIRCULATING THYROID-HORMONES IN THERAT

The effects of nutritional selenium (Se) deficiency over a period of t hree generations and of a combined selenium and iodine deficiency on h epatic and cerebrocortical iodothyronine deiodinases and on circulatin g thyroid hormone levels were examined in the rat. Se deficiency stron gly decreased hepatic type I iodothyronine 5'- and 5-deiodinase to 6-1 3% of that in controls. Iodine depletion had only a marginal decreasin g effect on the type I activity. Cerebrocortical type II 5'-deiodinase was decreased in Se-deficient, iodine-replete rats. Its 5-6fold eleva tion in iodine-deficient rats was not reversed by additional selenium deficiency. Cortex type III 5-deiodinase was modestly decreased in all groups with insufficient trace element supply. Long-term Se deficienc y has only limited effects on serum T4 and T3 levels. Two months of io dine deficiency decreased serum T4 to less than 10% of that in control s, but did not significantly affect serum T3 levels. The strong decrea se of hepatic outer- and inner-ring deiodination of T4 in Se deficienc y obviously reflects the reduced tissue concentration of the type I de iodinase which was recently identified as a selenoenzyme. The maintena nce of increased cerebrocortical type II deiodinase in iodine-depleted animals irrespective of adequate or deficient selenium supply suggest s that the type II isoenzyme does not contain selenium in its catalyti c site. Further studies are necessary to clarify whether the weak, but repeatedly confirmed decrease of cortex type III deiodinase is the di rect effect of Se deficiency or the indirect consequence of the multil evel change in thyroid hormone metabolism. Though peripheral T4 deiodi nation in the rat greatly depends on the trace element selenium, the e ffects of its long-term deficiency on circulating thyroid hormones and thyroid function are apparently limited.