Effect of Changes in Thyroid State on Metabolism of Thyroxine by Rat Placenta

Abstract

We studied the effect of the state of the thyroid on T4 monodeiodination in the rat placenta, and it was compared with those in the liver and kidney. The tissues, maternal serum, and amniotic fluid were obtained from pregnant rats. The tissues were homogenized in cold 50 mM Tris-HCl buffer, pH 7.5. The homogenate (1mg protein) was incubated at 37°C for 60 min with 1μg T₄ in the presence of 5mM DTT. The T₃ and reverse T₃ generated in the reaction mixture were extracted into cold ethanol and measured by RIAs. The conversion of T₄ to reverse T₃ in rat placenta was not significantly changed in MMI-induced hypothyroidism or T₄ induced hyperthyroidism. On the other hand, conversion of T₄ to T₃ in the liver and kidney were changed in parallel with the thyroid state. The concentration of reverse T₃ in the amniotic fluid was increased in accordance with the increase in the maternal serum T₄ concentration. These results indicate that the placental T₄ inner ring deiodination is not affected by the thyroid state, and that the change in the amniotic fluid reverse T₃ concentration in this study is mainly dependent upon the change in maternal thyroid function.