Placento-thyroidal relationship in normal pregnancy.

Abstract

Estimations of serum HCT, HTSH, T4, T3, PBI, ETR, Triosorb, TBG-binding capacity, BMR and urinary total estrogen were made simultaneously in 160 women in normal pregnancy. TRH stimulation tests were made in 20 cases in each trimester of pregnancy. HCT was detectable even in early pregnancy, tending to increase gradually toward the terminal stage of pregnancy as serum thyrotrophin bioactivity showed. On the other hand, serum TSH level measured by radio-immunoassay remained essentially the same throughout the course of pregnancy as in the nonpregnant state, moreover, it was suggested by the TRH stimulation test that pituitary TSH secreting function of pregnant women was similar to that of the non-pregnant. These findings suggest that thyroid hyperfunction during pregnancy which is shown by progressively increased T3, T4, and PBI may not be due to high estrogen-high TBG binding capacity-low free thyroxinenegative feed back-high TSH secretion but to HCT originating from placenta. In spite of thyroid hormone increase, it is true that the clinical picture of hyperthyroidism is not manifest among normal pregnant women, and ETR remained within the non-pregnant range throughout the course of pregnancy. We have also demonstrated that Triosorb decreased progressively. This may be interpreted to be due to the increase of TBG binding capacity which is increased progressively and binds more of free thyroxine during pregnancy. Such a change in TBG binding capacity is well known to be caused by the effect of estrogen which is progressively increased during pregnancy. In a word, it is possible to say that there is a placento-thyroidal system in pregnancy; HCT elevates thyroid function and TBG increased by estrogen carries thyroid hormone to target organ.