Thyroid function in early pregnancy. I: Thyroid-stimulating hormone response to thyrotropin-releasing hormone.

Maternal thyroid function in pregnancy is influenced by many factors. This study was undertaken to clarify the mechanism of thyroid regulation in the first trimester of normal pregnancy. We performed the thyrotropin-releasing hormone (TRH) test in eight women in the first trimester (week 6-9) of pregnancy and ten normal women in early follicular phase. Basal plasma levels of free triiodothyronine and free thyroxine were within the normal range in both groups, whereas thyroid-stimulating hormone (TSH) was at the lower limit of the normal range in pregnant women. TRH stimulation evoked a TSH response with a peak of 14.1 +/- 1.2 mIU/ml at 30 min. In control subjects TSH increased in response to TRH with a peak of 7.4 +/- 1.1 mIU/ml at 30 min. Statistical analysis with Student's t test revealed significantly higher TSH levels (p < 0.01) in pregnant women. The most striking finding was the enhanced responsiveness of TSH to TRH stimulation while the thyroid hormones, free triiodothyronine (fT3) and free thyroxine (fT4), remained in the normal range. This response was similar to that observed in central hypothyroidism. These results suggest that the reduction in maternal pituitary TSH levels is due to human chorionic gonadotropin (hCG) inhibition of TRH secretion.