Effect of severe energy restriction and refeeding on thyroid hormones in bulls.

Fifty-three Holstein-Friesian breeding bulls (944.99 +/- 14.59 kg) were fasted for 4 weeks. The influence of feeding on thyroid hormones was studied by comparing a starting point with a 4-week fasting period and a refeeding period. Blood samples were taken via a jugular vein catheter at 8:00 a.m. one day before, then once every week during, and two times after the fasting period. Plasma thyroxine (T4) and triiodothyronine (T3) levels were determined by direct radioimmuno-assay. The concentration of T4 and T3 decreased during fasting. The concentration of T3 increased after refeeding, but that of T4 did not. These data suggest that fasting is associated with a decrease in the peripheral conversion of T4 to T3 and, consequently, less T4 is converted into T3.