Effect of hypothyroidism on the pituitary-gonadal axis in the adult female rat.

The pituitary-ovarian axis was studied after withdrawal of thyroid hormone in 131I-radiothyroidectomized adult female rats. Oestrous cycles became prolonged and irregular within 2 weeks after the supply of thyroid hormone was stopped. If an LH surge occurred in hypothyroid rats on the day of vaginal pro-oestrus it was significantly greater in rats which had been made hypothyroid for 4-5 weeks than in controls; in hypothyroid rats with an LH surge on pro-oestrus, plasma progesterone showed a rise similar to that in controls at pro-oestrus; the ovulation rate was decreased in hypothyroid rats. About half of the rats from which blood was sampled daily in the afternoon between 7 and 18 days after tri-iodothyronine (T3) withdrawal had 1 day of pro-oestrus; on this day the LH surge was higher than in controls. On days 2 and 1 before and days 1 and 2 after this pro-oestrus, plasma progesterone was similar to that of controls on days 2 and 1 before and days 1 and 2 after pro-oestrus respectively. However, progesterone was higher in the period before and after these days. The other hypothyroid rats showed no pro-oestrus and no LH surge during this period, while their plasma progesterone levels were high on all days. On the morning of day 10 after T3 withdrawal and 5 days after the preceding pro-oestrus, most hypothyroid rats had high progesterone and low oestradiol plasma levels.(ABSTRACT TRUNCATED AT 250 WORDS)