Triiodothyronine (t₃) modulation of gonadotropin secretion in the female rat

The effect of T₃ upon gonadotropin secretion was examined in ovariectomized (Ovarx), Ovarx thyro-parathyroidectomized (Ovarx-TxPx), or proestrus rats. T₃ (50μg/-100gBW), administered late diestrus-2, abolished the LH surge during the critical period of proestrus in 7 out of 9 rats; the rise in sera FSH was not inhibited, although a distinct peak was absent. Administration of 5 or 50μg T₃/100gBW 2.5h before the critical period resulted in either a suppression or an alteration of the timing of LH release. In the 5μg T₃/100gBW treated animals the sera FSH peak was delayed in timing, whereas in the 50μg T₃/100gBW treated rats sera FSH demonstrated two separate peaks during the critical period. Treatment with various dosages of T₃ of Ovarx-TxPx rats resulted in significant suppressions (p < 0.05) of sera LH and FSH. Despite depressed concentrations of sera LH and FSH in T₃-treated rats pituitary sensitivity to a challenge of LHRH was enhanced. Hence, the pituitary was not the site of T₃ inhibition of gonadotropin secretion. Additionally, T₃ did not modify pituitary LH content or hypothalamic LH releasing activity (LHRH). Since T₃ did not inhibit gonadotropin secretion at the pituitary level, a neural site of T₃ action is suggested.