CHANGES IN PLASMA PROTEIN-BOUND IODINE AND THYROXINE-LIKE ACTIVITY OF FECES OF DAIRY COWS BEING FED L-THYROXINE

Abstract

The effect of l-thyroxine, in relation to dose-level and length of time fed, upon plasma PBI and fecal thyroxine was determined in lactating cows. Two cows were fed varying daily dosages of l-thyroxine, with the level changed weekly. They were started at 60 mg. per day for the first week, with SO, 100, 120, 140, 160, 100, 80, and 80 mg. following in consecutive weeks. PBI concentration in both cows reached a maximum above initial levels the second week when 80 mg. were fed. The actual excretion of equivalent l-thyroxine in the feces generally increased as dose-level increased. However, the percentage of dose excreted decreased until 120 mg. of thyroxine were fed daily and then tended to remain constant through the 160 mg. daily dose. The changes of PBI concentration in two cows fed 110 mg. of l-thyroxine dailjr for 15 weeks followed parallel patterns, with a declining trend as treatment continued. In one cow the actual excretion of thyroxine and the percentage of dose excreted decreased as treatment progr...