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

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-I 1~' * into 16 dairy cows in which recycling of I ~31 was blocked by a goitrogen (thiouracil), the determination of the rate of disappearance of radioactivity from the blood permitted the calculation of t 1~ and turnover rate. In ].957, during a period of average daily temperature of 47 F., the mean t l/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 tC, the mean t ~/2 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 seasonM variation in temperature did not influence thyroxine t ~/2 or turnover rate. The mean t 1~ of 16 observations was 2.47 days and turnover rate 28.4% per day. No breed difference in 1 1/2 in Jersey and Guernsey cows w~s observed. A significantly faster t ~/2 of 1.99 days ,~nd turnover rate of 35.3% was observed in a study of triiodothyronine. These data indicate that there is a build-up o thyroxine in the blood and total body space significantly higher than the daily thyroxine secretion rate.