The Relationship of Thyroid Activity to Lactation, Growth, and Sex in Sheep1

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Abstract
Shropshire ewes with a high thyroid activity as measured by thyroid output half-time tended to produce faster gaining twin lambs to three weeks of age than ewes with low thyroid activity. It is believed that the faster gains of these young lambs is due largely to increased milk consumption. Daily 1-thyroxine secretion rates per hundred pounds live weight were determined by the extrapolation technique in individual intact Shropshire lambs. The secretion rates by sex were: ewes, 0.08 mg.; rams, 0.07 mg.; and wethers, 0.05 mg. The ewe lambs were secreting significantly more 1-thyroxine than wether lambs (P>.05). The thyroxine secretion of ram lambs was not significantly different from ewe or wether lambs. Ewe lambs with a higher thyroid secretion rate tended to gain more rapidly during a 30-day feeding period. A correlation coefficient of 0.81 was obtained between daily 1-thyroxine secretion rate and 30-day gain which is significant at the 1% level. This relationship is expressed by the regression equation, Ŷ=−1.51+108.46x; where Y=lb. gained and x=mg. thyroxine secreted daily. Copyright © . .