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The Biological Half-Life of L-Thyroxine and L-Triiodothyronine in the Blood of the Dairy Cow^{1, 2,}

Department of Dairy Husbandry, University of Missouri, Columbia

ABSTRACT

The biological half-life of thyroxine (t ) is defined as the time in days for one-half of the biological activity of the hormone to be lost. An alternative measurement is the percentage turnover rate per day. By the injection of thyroxine-I¹³¹ into 16 dairy cows in which recycling of I¹³¹ was blocked by a goitrogen (thiouracil), the determination of the rate of disappearance of radioactivity from the blood permitted the calculation of t and turnover rate. In 1957, during a period of average daily temperature of 47° F., the mean t 1/2 was 2.54 days and a turnover rate of 27.6% per day was observed. During a period of average daily temperature of 71° F., the mean t was 2.65 days and a turnover rate of 26.7% was observed. In 1958, measurements were made at these two temperatures. Since these means did not differ significantly, it was concluded that seasonal variation in temperature did not influence thyroxine t 1/2 or turnover rate. The mean t 1/2 of 16 observations was 2.47 days and turnover rate 28.4% per day. No breed difference in t % in Jersey and Guernsey cows was observed. A significantly faster t 1/2 of 1.99 days and turnover rate of 35.3% was observed in a study of triiodothyronine. These data indicate that there is a build-up of thyroxine in the blood and total body space significantly higher than the daily thyroxine secretion rate.

¹ Contribution from the Missouri Agricultural Experiment Station, Journal Series No. 1991. Approved by the Director.

² Aided in part by a grant from the U. S. Atomic Energy Commission [Contract No. AT(11-1)-301].