BLOCKAGE OF GONADOTROPIN-INDUCED FIRST OVULATION CAUSED BY THYROIDECTOMY AND ITS POSSIBLE MECHANISMS IN RATS

To determine the role of the thyroid gland on the ovarian functions du ring the initiation process of puberty, we examined the effects and it s mechanisms of hypothyroidism on the first ovulation induced by equin e chorionic gonadotropin (eCG) in immature female rats. Animals were t hyroidectomized on day 22 and were injected with 5 IU of eCG on clay 2 6 to induce the first ovulation on day 29. The number of antral follic les that secrete inhibin and the ovarian weight were significantly inc reased in thyroidectomized rats (Tx rats) 48 h after eCG treatment com pared with those in non-Tx rats. However, thyroidectomy (Tx) significa ntly suppressed the rates of ovulating animals on day 29. The blockage of ovulation in Tx rats was recovered by administration of human chor ionic gonadotropin or luteinizing hormone (LH)-releasing hormone (LHRH ) on day 28. Inhibition of serum LH (not follicle-stimulating hormone) levels induced by Tx was almost restored to control levels by injecti on of LHRH. A significant increment in prolactin levels in Tx rats was also observed on day 28. The present data indicate that Tx before pub erty in female rats causes the blockage of the first ovulation and tha t the inhibitory effects on ovulation are mainly due to the reduction in the preovulatory LH surge, which is partially mediated through an i nhibition of LHRH action on the secretion of LH.