Variation in the Thyroxine-Secretion Rate of Cattle

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Abstract
Individual estimations (104) of thyroxine secretion rates in 29 animals was made in dairy cattle during various months of the year, along with the determinations of the rate constants for hormone release normally and when the recycling of iodine was blocked with thiouracil. The relationship of thyroxine secretion rates with hormonal release rates was rather poor. This is believed to be due to the variation in the size of the thyroid gland with identical or different release rates. None of the indices of thyroid activity satisfactorily measured the actual amount of thyroxine secretion. The average thyroxine secretion rate for the cows was 0.56 (range 0.2 - 1.0) L-thyroxine/100 lb. body weight in winter, and 0.6 mg (range 0.2 - 1.0) L-rthyroxine/100 lb. body weight for the calves during the same period. Thyroxine-secretion rates were reduced 3-fold in the summer, but the extent of depression varied with the individual. There was an average 3-fold variation in thyroxine-secretion rate between individuals during the same season. The rate of change in thyroxine secretion was not in direct proportion to the rate of seasonal change under decreasing ambient temperature, and it is thought that an increasingly warm environment may be a greater stimulus to a reduction in thyrotropic hormone secretion than a gradually colder environment for increased thyrotropic activity.