Studies on the Thyrotropic Activity of hCG and its Derivatives

Abstract

The placenta may be involved in the regulation of the maternal thyroid function during pregnancy. As human chorionic gonadotropin (hCG) is a primary protein from the placenta, we examined the thyrotropic activity of hCG and its derivatives using tissue culture and monolayer cell culture of human thyroid glands. The TSH receptor preparation was made from thyroid tissues, and its binding affinity with hCG and Asialo hCG (As-hCG) was examined. hCG did not bind to TSH receptor, but As-hCG did. In the experiment with tissue culture, TSH, hCG, hCG alpha, beta subunit and As-hCG were added to the culture medium. Secretions of L-thyroxine (T4) and L-triiodothyronine (T3) in the culture medium were measured at regular time intervals. While TSH showed increases of T4 and T3 secretion, other hCG derivatives did not show any differences from the control values. When TSH and hCG were added together, the secretion of thyroid hormone was the same as that obtained by TSH single administration. On the other hand, the secretion of T4 and T3 was inhibited with co-administration of TSH and As-hCG. Only TSH and none of the other hCG derivatives showed the dose-dependent stimulation of T4 and T3 secretion. A similar experiment was carried out in a monolayer cell culture obtained by trypsin treatment of human thyroid tissue. The secretions of cyclic AMP (c-AMP), T4 and T3 were measured. As in the previous tissue experiment, the thyrotropic activity of TSH was not modified by hCG, but the secretions of T4, T3 and especially c-AMP were inhibited by co-administration of As-hCG and TSH. These findings suggest that hCG and its subunits do not show thyrotropic activity in human thyroid glands, but As-hCG acts as an antagonist of TSH.