Differential effects of thyroid hormones on growth and thyrotropic hormones in rat fetuses near term.

We have studied the effects of thyroid hormone deficiency and excess on GH and TSH economy in the rat fetus near term. Pregnant rats were either left untreated (C group) or treated with methimazole to block thyroid function and infused with placebo, T4, T3, or both, until 21 days of gestation. Two experiments were performed: the doses (per 100 g body wt/day) of T4 ranging from 2.4-21.6 micrograms, those of T3 from 1.5-13.5 micrograms, with groups on 2.4 micrograms T4 + 1.5 micrograms T3. Fetal plasma T4 levels varied between 6-160% of C values and T3 values between 52-770%. Both plasma and pituitary GH decreased in hypothyroid fetuses from methimazole dams, and their plasma TSH was elevated. When T4 and/or T3 were infused, plasma and pituitary GH increased as a function of fetal plasma T4 and T3, reaching normal values when plasma T3 levels became normal, then increasing further. The effects on GH economy were related to the plasma T3 level, with no appreciable difference if T3 had been infused or derived from T4. In contrast, the elevated plasma TSH of the hypothyroid fetus decreased toward normal values when fetal plasma levels of T4, and of T3 derived from T4, became normal, but was not affected by normal fetal plasma T3 when T3 was infused. In the absence of T4, T3 decreased plasma TSH only when infused in doses that increased fetal plasma T3 3-fold above C values or more. Thus, both GH and TSH economy are under thyroid hormone control in rat fetuses near term. Similarities and differences with respect to regulation in adult rats cannot, however, be attributed exclusively to differences in fetal somatotrophs and thyrotrophs, because of the possibility that control is exerted at regulatory sites which are unique to the fetus.