Suppression of human chorionic gonadotropin in the human placenta at term by human thyroid-stimulating hormone in vitro.

Human chorionic gonadotropin (hCG) exerts a clinically apparent negative feedback on the secretion of human thyroid-stimulating hormone (hTSH) in pregnancy, and the two have cross-reactivity for the TSH receptor in membrane preparations of the thyroid. We examined whether hTSH, in turn, has an influence on the secretion and synthesis of hCG in short-term cultures of human placenta at term. A dose- and time-dependent decrease in the extracellular hCG concentration caused by hTSH was demonstrated. To examine whether hTSH inhibits de novo synthesis of hCG or decreases hCG depletion, we determined the amount of hCG secreted and the size of the intracellular pool by using an enzyme immunoassay. By incorporating a radiolabeled amino acid in the hCG molecule, we measured the amount of hCG synthesized de novo. We concluded that hTSH acts by decreasing the rate of de novo synthesis of placental hCG.