The Biological Half-Life of L-Thyroxine and L-Triiodothyronine in the Blood of the Dairy Cow

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Abstract
The biological half-life of thyroxine (t 1/2) 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 precentage 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 t1/2 and turnover rate. In 1957, during a period of average daily temperature of 47[degree]F, the mean tl/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[degree]F, the mean tl/2 was 2.65 days and a turnover rate of 26.7% was observed. In 1958, measurements were made at these 2 temperatures. Since these means did not differ significantly, it was concluded that seasonal variation in temperature did .not influence thyroxine tl/2 or turnover rate. The mean tl/2 of 16 observations was 2.47 days and turnover rate 28.4% per day. No breed difference in tl/2 injersey and Guernsey cows was observed. A significantly faster tl/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 buikb-up of thyroxine in the blood and total body space significantly higher than the daily thyroxine secretion rate.
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