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

Summary.: On the 8th day of pregnancy, ova were flushed from the uteri of ovariectomized albino rats maintained on progesterone. Hyperthyroidism and even a dose of L-thyroxine as low as 8 µg/day counteracted the detrimental effects of progesterone 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.