Metabolism of thyroxine in hyperthyroidism in cattle.

Abstract

Mean biological half-life of thyroxine (t 1/2) in a group of 16 normal dairy cows was observed to be 2.47 days and a turnover rate of 28.4% per day(1). The present study was concerned with influence of hyperthyroidism on t 1/2 of thyroxine. Hyperthyroidism was defined in cattle in terms of 50 and 100% increase above their estimated winter thyroxine secretion rate (TSR). Methods and materials. TSR was determined in late winter in lactating cows of several breeds by replacement technique(2). All animals were maintained in dry lot with free access to iodized salt. In each experiment 300 μc of carrier-free l-thyroxine-I131 was injected intravenously and blood samples taken from jugular vein each day following injection of hormone. Measurements of radioactivity of plasma were made in a National Radiac scintillation well counter (Model SA-2D) and conventional corrections were made for decay of isotope and for background. Disappearance of radioactivity in plasma from day to day followed an exponential course. In all experiments 4 g Tapazole/1000 lb body weight/day was administered to each animal to prevent recycling of I131 from thyroxine-I131 metabolism(3). Plot of disappearance of I131 from plasma was extraploted back to zero time (time of injection) and biological half-time (t 1/2) of thyroxine was determined graphically or from slope of regression line(1). t 1/2 represents time required by animal body to eliminate one-half of I131 in blood. Eight animals, after they had attained peak milk production, were injected for several months with 1-thyroxine (T4) subcutaneously daily, 50% in excess of estimated winter TSR, and its effect on milk production noted. t 1/2 of 1-thyroxine-I131 was determined during this period of hyperthyroidism. In 2 animals, (a Guernsey and Jersey-Holstein cross) the level of T4 injection was further raised to equal twice normal estimated TSR of animal. t 1/2 of 1-thyroxine-I131 was again determined in these 2 animals.