Effect of Estrogen and Progesterone on Thyroid Function of Cattle

A preliminary study is presented concerning the possible role of the ovarian hormones (estrogen and progesterone) upon the rate of secretion of thyroid hormone during estrus and pregnancy.For this purpose, a technique was developed consisting of the measurement of thyroidal-F 1 release when recycling of 1 ~ was prevented by a goitrogen (thiouraeil) 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.The injection of 100 rag.per day of progesterone alone or in combination with 100 ~g. of estradiol benzoate failed to alter the release rate of thyroidal I ~ in groups of dairy cattle.While 1.15 rag.estradiol benzoate alone also failed, when 2.5 or 3.0 rag.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.