Secretion Rates of Thyroxine and Triiodothyronine in Dairy Cattle

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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 r,-thyroxine (L-T4) equivalents per 45.4 kg body weight per day. In the same animals, TSR for thyroxine (T4SR) and for triiodothyronine (T~SR) were deter- mined 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 r~-triiodo- thyronine (L-T3) when no goitrogen was given but were increased to 56.0 liters (12.2% of body size) for L-T 4 and 201.0 liters (41.5% of body size) for L-T a 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-T 4 and L-T 3 in the absence of goitrogen and 1.59 and 1.02 with goitrogen. Concentration of L-T4 in the plasma pool was 7.49 tLg per 100 ml without the goitrogen and 6.18 tzg 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 gg 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-T 3 is 2.1 times as effective as L-T 4 in suppressing 131I release by the thyroid, the combined TSI~ of L-T 3 and L-T 4 without a goitrogen would be .172 mg of L-T 4 equivalents per 45.4 kg body weight per day and .189 mg with the

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