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A Rapid Method for Estimation for Thyroxine Secretion Rate of Cattle¹

University of Missouri, Columbia

Abstract

A rapid method for estimation of thyroxine secretion rate (TSR) in cattle was described in detail. The TSR was calculated as the product of the plasma thyroxine (PT), the thyroxine disappearance rate (K) and the thyroxine distribution space (TDS).

Different methods were used for the estimation of the fraction K. Results indicated that using plasma for the estimation of K is satisfactory for all practical purposes. The K values obtained using 2 days were similar to the values obtained using 4 days. Thus, 2 days were recommended as sufficient to estimate the TSR of cattle instead of using the thyroxine replacement methods which require at least 2 weeks for one determination.

The average TSR at 18° C. temperature in non-lactating Holstein cows was found to be 1.58 mg. per day per cow or 0.316 mg./100 kg. bodyweight. The average TDS was 81.5 1. per cow or 13.7 1./100 kg. bodyweight. These values are similar to those reported by the chemical thyroxine turnover method.

The method reported in this paper has numerous advantages over the replacement method in short-term experiments.

¹ A contribution from the Mo. Agr. Exp. Sta. Journal Series No. 5071. Approved by the Director.

² Dairy Husbandry Department.