Effects of selenium deficiency on thyroid hormone economy in rats.

In selenium-deficient rats, peripheral T4 to T3 conversion is markedly decreased due to the loss of the selenoprotein, type I iodothyronine 5'-deiodinase (5'D-I). Despite the marked increase in circulating T4 that results from this loss of 5'D-I, serum T3 concentrations in selenium-deficient rats remain in the normal range. To determine the physiological mechanism(s) that maintains circulating T3 when peripheral T4 to T3 conversion is impaired, we examined the interrelationships between selenium intake and the metabolism of T3 and T4 in the rat. In euthyroid rats, selenium deficiency caused the expected loss of 5'D-I, with a 52% increase in serum T4, which paralleled an increase in the T4 biological half-life. Consistent with the prolonged t1/2 of T4, short term thyroidectomy (48 h) in selenium-deficient rats failed to decrease serum T4 concentrations to the levels observed in short term thyroidectomized, selenium-supplemented rats. Short term thyroidectomy also caused an expected 33% decrease in liver 5'D-I and a 44% increase in brain type II iodothyronine 5'-deiodinase (5'D-II) activities in selenium-supplemented rats. However, in selenium-deficient rats, short term thyroidectomy did not affect 5'D-I or 5'D-II activities. In contrast to the selenium-dependent changes in circulating T4 levels, little or no change in circulating T3 concentrations occurred. There was a 20% increase in the T3 half-life in selenium-deficient rats. The serum T3 sulfate concentration was increased, and T3 deiodination was reciprocally decreased in the selenium-deficient rats. These data suggest that increased T3 sulfate generation in selenium-deficient rats may lead to greater T3 availability through enterohepatic recycling of the iodothyronine and may explain why there are only minor changes in serum T3 concentrations in selenium-deficient rats.