Effect of Thyroidectomy on Reproductive Performance of Ewes and Semen Quality of Rams

Abstract

A study was conducted to determine the effect of thyroidectomy on the reproductive performance of aged ewes and the semen quality of mature rams. Fourteen ewes, seven of which were thyroidectomized, were bred to a thyroidectomized ram. All intact ewes produced at least one healthy lamb and ten lambs were dropped by this group. Four lambs (all singles) resulted in the thyroidectomized group, and two of these were small and weak. Two thyroidectomized ewes failed to lamb, and one which underwent euthanasia early in the trial was carrying normal twin fetuses. Statistical analysis showed the intact group of ewes produced significantly (P<.05) more lambs per ewe than did the thyroidectomized group. The birth weight of lambs born to intact ewes was 11.0±.48 lb., while lambs born to thyroidectomized ewes averaged 7.1± 1.16 lb. This difference was highly significant (P<.01). Semen studies on three intact and three thyroidectomized rams revealed no difference in apparent fertility due to thyroid status. Libido was similarly unaffected by the operation. Thyroidectomy did not impair the ability to sire young. On the basis of these observations, it appears that optimal reproductive ability in the ewe is dependent to a variable degree upon thyroid hormones. Semen quality and breeding performance in the mature ram are apparently unchanged by thyroid removal.