The Thyroid Secretion Rate of Sheep as Affected by Season, Age, Breed, Pregnancy and Lactation1

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Abstract
A technique is described for measuring the daily thyroid secretion rate in intact, individual sheep. From 24 experimental ewes divided into four groups and using the extrapolation technique, the following results were observed. The season of the year has a great effect upon thyroid secretion in sheep. In July a daily thyroxine secretion of 0.04 mg. was significantly lower than at any other month readings were taken. The next lowest secretion was observed in September (0.14 mg.). There was a significant difference (P less than .05) between September and March (0.20 mg.). Pregnancy in sheep does not cause a significant difference in thyroid secretion. Lactation creates increased demands upon the thyroid gland as lactating 2-year old ewes had a higher thyroid output than either bred or open 2-year old ewes. Four-year old ewes secreted significantly less thyroxine daily than 2-year old ewes during January, March, May and December. However, differences during July and September were not significantly different. Significant breed differences in thyroid secretion rate between Hampshire and Shropshire ewes were observed only in January and December.