PROGESTERONE-DEPENDENT BLASTOCYST SURVIVAL DURING ALTERED THYROID ACTIVITY IN THE RAT

Summary. On the 8th day ofpregnancy, ova were flushed from the uteri of ovariectomized albino rats maintained on progesterone. Hyperthy-roidism and even a dose of l-thyroxine as low as 8 \g=m\g/daycounteracted the detrimental effects ofprogesterone deficiency upon blastocyst survival during this delay period, while thyroidectomy contributed to the detrimental effect. This study clarifies the earlier reported effect of the thyroid upon delayed implantation of blastocysts by showing that (1) the effect occurs during the progesterone-dependent maintenance period rather than during the implantation process, and (2) the effect can be obtained with a dose of l-thyroxine which is within the physiological hypersecretory potential of the rat thyroid. There are conflicting reports in the literature concerning the effects of thyroid hormone upon pregnancy. Investigators have reported beneficial effects, disadvantageous effects and some have reported that it exerts no effect. Exam¬ ples of these investigations and possible reasons for the conflicting results wer