Influence of hypothyroid state on ovulation in rats

At puberty in intact rats, vaginal cornification and ovulation occurred on the day of vaginal opening (average 34 days of age). In thyroidectomized rats, both vaginal cornification and ovulation (with small number of ova) were delayed, but vaginal opening occurred on the same day as in controls. On the other hand, when PMS was given to intact rats at 28 days of age, ovulation occurred on day 31. In thyroidectomized rats, erratic ovulation with a lower per cent ovulation and smaller number of ova count occurred on both day 30 and day 31. However, Lthyroxine administration in thyroidectomized rats treated with PMS allowed normal ovulation on day 31. Furthermore, anesthetization with pentobarbital during 2 to 4 PM on day 29 in thyroidectomized rats treated with PMS caused normal ovulation on day 31. Therefore, it is concluded that continuous exposure of the CNS to thyroid hormone is necessary for the regulation of gonadotrophin secretion.