Relation of Plasma Thyroxine and Triiodothyronine to Body Weight in Growing Male and Female Holstein Cattle

Thyroxine and triiodothyronine were measured in blood plasma to determine relationships with growth in male and female Holstein calves at 6, 8, 10, 14, 18, and 22 wk of age. Average initial body weights at 6 wk for male and female calves were 54 and 50 kg. Thyroxine and triiodothyronine were determined by specific double antibody radioimmunoassay on eight calves of each sex at each age. Sexes differed for body weight gains from 6 to 22 wk, average daily gains for males were .93 kg/day and for females .73 kg/day. Thyroxine and triiodothyronine concentrations increased gradually (38 and 96%) in both sexes during the 6 to 22 wk and were higher for males than females. Average concentrations of thyroxine and triiodothyronine were 53.3 and 1.21 ng/ml for males and 39.9 and 1.06 ng/ml for females. Thyroxine and triiodothyronine concentrations in blood plasma of male and female dairy calves were positively correlated with body weight during this initial growth period.