Ontogeny of thyrotrophin concentration in perinatal rats.

Negative feedback control by triiodothyronine (T3) of thyrotrophin (TSH) secretion from rat pituitary glands was studied during the perinatal period of rat development. Foetal serum TSH concentration declined significantly between 20 and 21 days of gestation, reached a low level at delivery, and remained low for several days after birth. T3 suppressed serum TSH concentration in a dose-responsive manner when given to foetuses on day 20 of gestation at 0.13 to 2.0 micrograms/100 g body weight of the estimated body weight. The responses of serum TSH levels and thyroid weights to PTU treatments differed with gestational age. We conclude that negative feedback control by T3 of serum TSH concentration exists in rat foetuses as early as day 20 of gestation and differs from that found in adult rats.