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

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Abstract
Thyroxine [T4] and triiodothyronine [T3] 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. T4 and T3 were determined by specific double antibody radioimmunoassay on 8 calves of each sex at each age. Sexes differed for body weight gains from 6 to 22 wk, average daily gains for males were 0.93 kg/day and for females 0.73 kg/day. T4 and T3 concentrations increased gradually (38 and 96%) in both sexes during the 6-22 wk and were higher for males than females. Average concentrations of T4 and T3 were 53.3 and 1.21 ng/ml for males and 39.9 and 1.06 ng/ml for females. T4 and T3 concentrations in blood plasma of male and female dairy calves were positively correlated with body weight during this initial growth period.
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