Thyroid Hormone Secretion Rates in Growing Jersey Cattle

Abstract

Thyroid hormone secretion rates were determined in growing Jersey cattle ranging in age from 1 wk to 3 vr. The secretion method was radioactive thyroxine pool-turnover with samples taken every 12 to 72h. Seventy individual determinations were made in 14 females and 6 males with no difference between sexes. Change in logarithm of rate of secretion (mg) versus logarithm of body weight (kg) was expressed by the equation Y = aXb or In Y = a + b In X, where Y was thyroid hormone secretion rate. When all 70 determinations were included, the linear expression was In secretion rate (mg) = −3.05 + .403 In body weight (kg) or Y = .0475 X.403. Analysis of variance indicated that the linear equation accounted for 35% of the variation of Y and the quadratic effect accounted for an additional 15%. When the data were analyzed within-animal, the linear effect accounted for 19% of the variation and the quadratic effect for 26%. Calves had higher rates than older animals per unit body weight. When individual determinations of animals 0 to 3 mo of age were excluded, the relation of secretion of body weight was Y = .0142 X.686. When animals from 0 to 6 mo were excluded, the linear expression was Y = .0052 X.811.