Thyroid Hormone Secretion Rates in Growing Jersey Cattle

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 72 h. 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 = aX ~ 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.4L 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 fo.r 197o 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 -6sG. When animals from 0 to 6 mo were excluded, the linear expression was Y = .0052 X'slL