Pituitary-Thyroid Function of Fetuses of Hypothyroid and Growth Hormone Treated Hypothyroid Rats

Abstract

Maternal hypothyroidism induced by surgical thyroidectomy (Tx) of the rat resulted in significantly higher fetal serum levels of thyroid stimulating hormone (TSH) and thyroxine (T4) on day 22 of gestation. Surprisingly, administration of growth hormone (GH) to hypothyroid mothers increased further the fetal serum T₄ and TSH. The in vitro uptake of ¹³¹I-T₄ by erythrocytes was elevated significantly when incubated with serum from fetuses of both hypothyroid and hypothyroid GH-treated mothers. Although the plasma protein levels of hypothyroid mothers and their fetuses are decreased significantly as compared to controls this is not true of hypothyroid GH-treated mothers and their fetuses. The T₄ levels of both groups of Tx mothers were significantly below that of controls. However, as in the case of their fetuses, the serum T₄ of GH-treated hypothyroid mothers was elevated from that Of Tx only animals. It is concluded that the pituitary-thyroid system of fetuses of hypothyroid mothers is activated excessively during late gestation, that considerable T₄ can be transported from the fetus to the mother during this period and that these fetuses are in fact born in a hyperthyroid state which is aggravated by maternal treatment with GH.

Key words

Fetal Hyperthyroidism - Maternal Hypothyroidism - Thyrotropin - Growth Hormone