Effect of Estrogen and Progesterone on Thyroid Function of Cattle

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Abstract
A preliminary study is presented concerning the possible role of the ovarian hormones (estrogen and progesterone) upon secretion rate of thyroid hormone during estrus and pregnancy. For this purpose, a technique was developed consisting of the measurement of thyroidal-I131 release when recycling of I131 was prevented by a goitrogen (thiouracil) during a preliminary control period. During the subsequent period of ovarian hormone injection, the measurement of release rate is continued. If a change in release rate is noted, it is believed to indicate that a change in thyroxine secretion rate has been induced. Injection of 100 mg per day of progesterone alone or in combination with 100 ug of estradiol benzoate failed to alter the release rate of thyroidal I131 in groups of dairy cattle. While 1.15 mg estradiol benzoate alone also failed, when 2.5 or 3.0 mg were injected daily, the release rate was increased to a statistically significant extent. These observations are interpreted as indicating that the level of estrogen secreted in late pregnancy stimulates increased secretion of thyrotropin and thyroxine during early lactation.