Thyroid function as measured by131iodine release rate, weight and RNA/DNA in growing lambs, and its relation to growth rate

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Abstract
Thyroid activity was assessed in two groups of crossbred lambs and in Dorset Horn lambs fed ad libitum, by measuring the rate constant (K4) for the release of 131iodine from the gland. The results demonstrated a highly significant curvilinear correlation (P<0·001) between growth rate and the rate constant (K4) in experiments based on individual measurements in animals from three populations.Separate work carried out on the measurements of both thyroid size and RNA/DNA ratio suggests a need for caution when these are interpreted as parameters of thyroid activity. In the growing animal these may be more reflective of the growth of the thyroid gland itself, differences which may be governed by factors not directly related to variations in hormone secretion rate.The findings are discussed in terms of an explanation of the contradictory results obtained where attempts have been made to alter the thyroid status of growing animals by the use of thyroid hormone analogues and thyroid depressant drugs.