EFFECT OF HYPOTHYROIDISM ON THE PITUITARY-GONADAL AXIS IN THE ADULT FEMALE RAT

The pituitary-ovarian axis was studied after withdrawal of thyroid hor mone in I-131-radiothyroidectomized adult female rats. Oestrous cycles became prolonged and irregular within 2 weeks after the supply of thy roid 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 hy pothyroid rats with an LH surge on pro-oestrus, plasma progesterone sh owed a rise similar to that in controls at pro-oestrus; the ovulation rate was decreased in hypothyroid rats. About half of the rats from wh ich blood was sampled daily in the afternoon between 7 and 18 days aft er tri-iodothyronine (T-3) withdrawal had 1 day of pro-oestrus; on thi s 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 simi lar 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 peri od before and after these days. The other hypothyroid rats showed no p ro-oestrus and no LH surge during this period, while their plasma prog esterone levels were high on all days. On the morning of day 10 after T-3 withdrawal and 5 days after the preceding pro-oestrus, most hypoth yroid rats had high progesterone and low oestradiol plasma levels. In these rats, injection of gonadotrophin-releasing hormone caused a rela tively small increase in LH; it did not stimulate the secretion of oes tradiol or progesterone, and it did not induce ovulation. It was concl uded that hypothyroidism induces major changes in the secretion of ste roids by corpora lutea and growing follicles. Whether the changed ster oid metabolism is the primary cause of the observed prolongation of th e oestrous cycles, the increased pro-oestrous LH surge and the reduced ovulation rate remains to be investigated.