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Secretion Rates of Thyroxine and Triiodothyronine in Dairy Cattle

Department of Dairy Husbandry, University of Missouri, Columbia 65201

ABSTRACT

Thyroid hormone secretion rates (TSR) were determined in 7 lactating dairy cows by the substitution method (suppression of thyroidal ¹³¹I release) and averaged .21 mg L-thyroxine (L-T₄) equivalents per 45.4 kg 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 4 g per 454 kg 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 7.49 µg per 100 ml without the goitrogen and 6.18 µg per 100 ml with it while the concentration of L-T₃ in the plasma pool, measured in the absence of goitrogen treatment only, was .149 µg per 100 ml. Mean T₄SR in the 7 cows was .143 mg per 45.4 kg body weight per day without a goitrogen and .148 mg in the presence of the goitrogen; T₃SR was .0139 mg 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 .172 mg of L-T₄ equivalents per 45.4 kg body weight per day and .189 mg 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.