T4 degradation rate and plasma levels of TSH and thyroid hormones in ten young bulls during feeding conditions and 48 h of starvation

The effect of starvation on thyroid hormones was studied during a starvation period of 48 h in 10 young bulls. Mean thyroxine degradation rate decreased from K/day 0.32 during feeding to 0.23 during fasting. Mean plasma concentration of T4 decreased to 75% of normal, and it was calculated that the mean thyroxine secretion rate during the starvation period was 24% of normal. Plasma concentration of free thyroxine decreased to 54% of normal, indicating that the concentration of binding protein was not decreased. T3 and reverse T3 decreased to about 60% of normal. This indicated a parallel decrease in the secretion of all 3 hormones. We did not find evidence of an inactivating pathway for T4 as has been shown in humans during starvation. TSH decreased to 85% of normal.