1. This paper reports a study of the relationship between thyroid gland activity and rate of growth of lambs held in a controlled environment with feeding ad libitum.2. Thyroid activity was assessed by the rate of release of 131I from the thyroid gland over a period of 1-2 weeks following injection. Growth rates were simultaneously recorded on a total of thirty-one lambs during forty-six measurements of thyroid activity.3. The results demonstrated a highly significant curvilinear correlation between growth rate and the rate constant for thyroid 131I release (P < 0·001).4. Surgically thyroidectomized animals were demonstrated to have a much reduced rate of growth.5. It was concluded that thyroid activities considerably above or below the optimum have an adverse effect on growth rate.
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