Thyroxine Turnover Methods for Determining Thyroid Secretion Rates in Dairy Cattle

Abstract

Various radiothyroxine and normal thyroxine turnover methods for rapid determinations of thyroid secretion rate were compared within animals. The preferrred radiothyroxine method was an isotope dilution procedure based on the decline in specific activity of PBI following the injection of I131 -labeled L-thyroxine. The preferred normal thyroxine turnover method was a chemical procedure based on the decline in plasma PBI after the injection of nonradioactive thyroxine. Daily thyroid secretion rates (per 100 lb. body weight), determined by the isotope dilution and chemical PBI methods, respectively, averaged 0.40 and 0.39 mg in 14 young bull calves and 0.14 and 0.13 mg in 4 nonlactating cows. In the cows, daily thyroid secretion rates (per 100 lb. body weight), determined by the commonly used thyroxine replacement method based on the inhibition of thyroidal-I131 release, averaged 0.13 mg. A simple correlation coefficient of 0.93 was found between thyroid secretion rates determined by the isotope dilution and the chemical thyroxine turnover methods in bull calves. Error coefficients of variation of thyroid secretion rates obtained from duplicate determinations in 4 cows were 9.7 and 11.5% for the isotope dilution and chemical thyroxine turnover methods, respectively. Evidence is presented that the turnover rate of I131-labeled L-thyroxine is not influenced significantly by the recycling of I131 through the thyroid during the first 96 hr. after radiothyroxine injection or by the single large dose of normal thyroxine used in the chemical thyroxine turnover method.