[Modification of endocrine function of trophoblasts by thyroid hormone]

Direct effects of L-triiodothyronine(T3) on placental endocrine function were investigated in vitro with an organ culture system for human placental tissues. Explants of trophoblastic tissues obtained from early and term placentas were cultured with or without graded doses of T3 in a serum-free condition. The addition of an optimal concentration of T3(10(-3) M T3) stimulated daily secretion of progesterone and estradiol from cultured early placental tissues by acting at the level of 3 beta-hydroxysteroid dehydrogenase and aromatase enzyme activity, together with the enhancement of hCG(alpha, beta) and hPL secretion. The addition of higher or lower concentrations of T3 gave attenuated effects and the addition of an excessive concentration of T3(10(-3) M T3) resulted in remarkable inhibition of progesterone and estradiol secretion by cultured early placental tissues. These results suggest that the optimal concentration of thyroid hormone acts as a biological amplifier of endocrine function of cultured trophoblasts obtained from early placentas. Unlike the early placental tissues, cultured term placental tissues did not respond to the addition of graded doses of T3 with increased endocrine function. Thus, the frequent occurrence of spontaneous abortion in early pregnancy during the state of hypothyroidism or hyperthyroidism may represent a direct consequence of inadequate thyroid hormone availability at the level of the trophoblast, followed by diminished endocrine function of early placental trophoblasts.