Thyroid hormone concentrations in the plasma of fed and fasted Brahman and Hereford steers

Abstract

Plasma concentration patterns of thyroxine (TJ, free T4 (FT4), triiodothyronine (T3), and free T3 (FT3) were determined in Brahman steers fed lucerne hay ad libitum and in Brahman and Hereford steers fed restricted intakes of lucerne hay at the rate of either 208 g/h before fasting for 72 h or 250 g/h before fasting for 96 h. In Brahmans fed ad libitum, the plasma concentrations of all thyroid hormone fractions were significantly (P<0.01) correlated with one another and with feed intake. Within breeds, the concentrations of thyroid hormones were higher (P<0.001) when animals were fed at 250 g k than at 208 g/h. During both hourly feeding regimes T4, FT4, T3, and FT3 concentrations were higher (P<0.001) in Brahmans than in Herefords. Fasting after both hourly feeding regimes lowered (P<0.001) the concentrations of T4 about 53% in Brahmans and 30% in Herefords, while FT4, T3, and FT3 were lowered about 68% in Brahmans and 50% in Herefords. Consequently, thyroid hormone concentrations were significantly lower in Brahmans than in Herefords after 72 h fasting but did not differ significantly between breeds after 96 h fasting. The present results, together with those of our previous work showing breed differences in rumen metabolism, support the concept that, in Hereford and Brahman steers fed the same amount of hay in a thermoneutral environment, breed differences in plasma concentrations of thyroid hormones originate from quantitative differences in the supply of nutrients from the rumen to body tissues.