Effects of Breed, Stage of Lactation, and Season of Year on Thyroid Secretion Rate of Dairy Cows as Determined by the Chemical Thyroxine Turnover Method

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Abstract
A study of the thyroid secretion-rate in 12 dairy cows (total of 48 determinations) was made in which breed (six Holsteins and six Guernseys), stage of lactation (seven to 14 days, and three, six, and nine months), and season of year (winter, spring, summer, and fall) were the variables under study. The mean thyroid secretion rate per 100 lb body weight for all animals and all determinations was 0.142 mg of thyroxine per day. The secretion rate for the Holsteins and Guernseys, respectively, was 0.127 and 0.156 mg of thyroxine per 100 lb body weight per day. The breed sample of animals was, however, too limited to consider this a true breed effect. Thyroid secretion rate per 100 lb of body weight was highest at the beginning of lactation (0.155 mg thyroxine per day) and lowest at the ninth month of lactation (0.132 mg of thyroxine per day). In contrast, the trend for blood thyroxine level (PBI) shows PBI to be lowest at the beginning of lactation (2.72 [mu]g%) and to reach its height at six months of lactation (3.58 [mu]g%). Thyroid secretion rate per 100 lb body weight was highest in the spring (0.165 mg thyroxine per day) and lowest in the fall (0.129 mg of thyroxine per day).
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