Effect of Ovariectomy and Replacement Therapy Upon Thyroxine Secretion Rate of Rat. ∗

Summary

1) Effect of ovariectomy and replacement with 1 μg estradiol benzoate (EB) and/or 3 mg progesterone (P) upon daily thyroxine secretion rate (TSR) has been studied in mature rats. 2) Ovariectomy reduces TSR approximately 33% within 2-6 days postcastration. Increasing the interval to approximately 90 days between ovariectomy and TSR determination did not result in a further reduction in TSR. EB alone and with P prevented decrease in TSR following ovariectomy but P alone had no effect. 3) It is suggested that ovariectomy reduces the circulating estrogen level which, in turn. may reduce thyrotropin secretion and, subsequently, lower daily TSR.