EFFECT OF SEVERE ENERGY RESTRICTION AND REFEEDING ON THYROID-HORMONESIN BULLS

Fifty-three Holstein-Friesian breeding bulls (944.99 +/- 14.59 kg) wer e fasted for 4 weeks. The influence of feeding on thyroid hormones was studied by comparing a starting point with a 4-week fasting period an d a refeeding period. Blood samples were taken via a jugular vein cath eter at 8:00 a.m. one day before, then once every week during, and two times after the fasting period. Plasma thyroxine (T-4) and triiodothy ronine (T-3) levels were determined by direct radioimmuno-assay. The c oncentration of T-4 and T-3 decreased during fasting. The concentratio n of T-3 increased after refeeding, but that of T-4 did not. These dat a suggest that fasting is associated with a decrease in the peripheral conversion of T-4 to T-3 and, consequently, less T-4 is converted int o T-3.