Effects of hypothyroidism on the estrous cycle and reproductive hormones in mature female rat.

The present study was undertaken to examine the systemic role of thyroid hormones by analyzing changes in reproductive functions in hypothyroid female rats. Serum concentrations of triiodo-thyronine (T3) significantly decreased 1 week after the initiation of propyl-thiouracil treatment or thyroidectomy. The estrous cycle became irregular 3 and 2 weeks after the initiation of propyl-thiouracil treatment and thyroidectomy, respectively. Serum luteinizing hormone (LH) levels significantly reduced in both groups on the day of diestrus-1 about 1 month later. Hypothyroid rat shows the high progesterone and low testosterone levels. No significant changes in inhibin and estradiol levels were detected. The serum levels of FSH decreased in the thyroidectomy group. The irregular estrous cycle and the above changes in hormone levels were recovered by administration of T4. Compensatory secretions of FSH and LH induced by ovariectomy were enhanced by thyroidectomy and suppressed by T4 treatment. These results suggest that thyroid hormones play a role in the regulation of reproductive hormones secretion in the cyclic rat ovary.