Effect of Experimental Hypothyroidism on Period of Gestation in the Rabbit. ∗

In some unpublished data obtained in this laboratory, it was found that thyroid removal in the pregnant female albino rat resulted in abortion or resorption of the foetuses, provided this operation was done before the first two-thirds of the gestation period. 1 Because the rabbit possesses a somewhat different type of ovarian activity than does the rat, it was thought to be of interest to determine whether the pregnant female rabbit would react to thyroidectomy in a similar manner. The results of this preliminary study indicate that contrary to the reaction obtained in the rat, thyroid removal has little if any effect on the period of gestation in the rabbit.

Although 33 New Zealand does, ranging in age from 7 to 18 months, were used as experimental animals, complete data on only 15 are reported here. The results on the remaining 18 animals will be reported when the data are completed. Ten additional normal females served as controls. Each doe was mated, being allowed to copulate twice, and the thyroid glands removed at definite intervals after mating. This interval in some cases was as short as 2 hours, while in other cases the time was extended to 12 days. The length of the period of gestation was then noted. The controls were mated but were not subjected to operative procedures and the duration of gestation also noted. As can be seen from Table I, no apparent differences between the length of the gestation period occurred in operated and unoperated animals.

It appeared possible that all of the thyroidectomies might not have been complete, and that regeneration of thyroid remnants might have occurred. In 6 animals (Nos. 3, 7, 8, 34, 40, and 54) which were sacrificed, the entire thyroid region was removed, fixed, and a careful search made for traces of thyroid tissue under a binocular dissecting microscope.