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.
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