Suppression and stimulation of TSH and thyroid hormones in bulls during starvation and refeeding

Sixteen bull calves were fasted during two periods, 31/2 and 7 months old. Blood samples were taken every 6 h during an experimental period of 9 days: 2 control days, 5 fasting days and 2 refeeding days. During the control days with ad libitum feeding, T3, T4 and TSH increased during the day. During the fasting period, T4 decreased with a half-life of 3.5 days and T3 with a half-life of 3 days. TSH decreased in a less regularly way to a mean of 60% of control mean. Six h after refeeding, TSH had increased to 300% of control mean. T3 and T4 had a maximum 12 h after refeeding. As the T4 decreased with a rate nearly similar to the T4 disappearance rate in starving bulls, the secretion of T4 from the thyroid gland must have been almost stopped during the starvation period. The morning values were below normal for all three hormones after 2 days of refeeding.