Secretion Rates of Thyroxine and Triiodothyronine in Dairy Cattle

Abstract

Thyroid hormone secretion rates (TSR) were determined in 7 lactating dairy cows by the substitution method (suppression of thyroidal 131I release) and averaged .21mg L-thyroxine (L-T4) equivalents per 45.4kg body weight per day. In the same animals, TSR for thyroxine (T4SR) and for triiodothyronine (T3SR) were determined separately by the radioiodine labelled hormone pool turnover method. Pool sizes in the 7 cows were 36.9 liters (8.27% of body size) for L-T4 and 107.1 liters (22.1% of body size) for l-triiodothyronine (L-T3) when no goitrogen was given but were increased to 56.0 liters (12.2% of body size) for L-T4 and 201.0 liters (41.5% of body size) for L-T3 when the goitrogen methimazole (tapazole) was given orally at 4g per 454kg body weight per day (P<.05). Turnover rates as half-lives in days were 1.29 and .79 for L-T4 and L-T3 in the absence of goitrogen and 1.59 and 1.02 with goitrogen. Concentration of L-T4 in the plasma pool was 7.49μg per 100ml without the goitrogen and 6.18μg per 100ml with it while the concentration of L-T3 in the plasma pool, measured in the absence of goitrogen treatment only, was .149μg per 100ml. Mean T4SR in the 7 cows was .143mg per 45.4kg body weight per day without a goitrogen and .148mg in the presence of the goitrogen; T3SR was .0139mg without methimazole and .0193 with it. If L-T3 is 2.1 times as effective as L-T4 in suppressing 131I release by the thyroid, the combined TSR of L-T3 and L-T4 without a goitrogen would be .172mg of L-T4 equivalents per 45.4kg body weight per day and .189mg with the goitrogen. On this basis the substitution method overestimates TSR in cattle by 12 to 22%. Correlation coefficients for TSR with milk production were positive but low and nonsignificant, varying from .64 to zero and being .30 in 39 comparisons.