PLACENTAL 5-DEIODINASE ACTIVITY AND FETAL THYROID-HORMONE ECONOMY AREUNAFFECTED BY SELENIUM DEFICIENCY IN THE RAT

In adult male rats, selenium deficiency results in a near complete los s in the selenoprotein 5'-deiodinase in the liver, resulting in decrea sed peripheral deiodination of thyroxine (T4) and increased serum T4 c oncentrations. Serum 3,5,3'-triiodothyronine concentrations are normal or slightly decreased, and serum 3,3',5'-triiodothyronine concentrati ons are normal or slightly increased in selenium-deficient rats. We no w report the effects of selenium deficiency on maternal and fetal thyr oid hormone economy and on placental 5-deiodinase activity in the rat. Weanling female rats were fed either a selenium-deficient or selenium -supplemented diet for 4 wk before mating and then throughout gestatio n. Rats were killed at 21 d of gestation. Selenium deficiency was conf irmed by a 95 and 94% decrease in glutathione peroxidase and a 84 and 56% decrease in liver type I outer ring 5' deiodinase activity in the mother and the fetus, respectively. In contrast to the increase in cir culating T4 observed in selenium-deficient male and nonpregnant female adult rats, serum T4 was not affected by selenium deficiency in pregn ant rats, but there was a 3-fold increase in serum 3,3',5'-triiodothyr onine concentrations associated with a 70% decrease in maternal brain type II outer ring 5' deiodinase activity. Maternal serum 3,5,3'-triio dothyronine concentrations were decreased by 21%. Placental 5-deiodina se activity was unaffected by selenium deficiency. In the fetus, serum T4, 3,3',5'-triiodothyronine, and TSH concentrations were not affecte d by selenium deficiency. These data suggest that placental 5-deiodina se is not a selenoenzyme and that the failure of selenium deficiency t o increase serum T4 concentrations in the mother as well as the minor role played by liver type I outer ring 5' deiodinase in the fetus resu lts in the protection of fetal thyroid hormone economy against the pot entially deleterious effects of selenium deficiency.