Evidence That Control of Fetal Thyrotropin Secretion is Independent of Both the Fetal and Maternal Hypothalamus*

Propylthiouracil (PTU) administered to pregnant rats from day 18-21 of gestation caused a significant increase in maternal and fetal thyroid weight and plasma TSH. Fetal encephalectomy on day 18 did not significantly affect basal or PTU-stimulated pituitary-thyroid function. Destruction of the basal hypothalamus in the mother on day 13 or 16 markedly reduced maternal plasma TSH and thyroxine and prevented a PTU-induced increase in thyroid size, but did not affect fetal pituitary-thyroid function. Plasma PRL, was undetectable in both intact and encephalectomized fetuses at 21 days but was increased greater than 6-fold to approximately 2 microgram/ml in the mothers by maternal hypothalamic destruction. We conclude that fetal pituitary-thyroid function in the rat is not dependent on either fetal or maternal hypothalamic TRH.