Impact of vitamin C on concentrations of thyroid stimulating hormone and thyroid hormones in lambs under short-term acute heat stress

The present study evaluated the effect of vitamin C on alteration in thyroid hormones induced by short-term acute heat stress. Eight male lambs were divided into two groups of 4 animals each. Both groups were placed in an environment with hyper-acute heat stress based on the temperature – humidity index (THI). Groups I and II were injected intramuscularly normal saline and vitamin C (20 mg/kg), respectively, for the first five consecutive days of the experiment. All lambs were fed ad libitum. Blood samples were collected from both groups on days one, two, four, six and eight. Thyroxine and free thyroxine numerically increased (91.03 vs. 70.78 nmol L-1, P=0.080 and 29.8 vs. 24.8 pmol L-1, P=0.080; respectively) in heat stressed lambs supplemented with vitamin C compared to control group. Respiration rates and heart rates were elevated until day five of the experiment and then decreased. Mechanism for increasing the levels of thyroxine and free thyroxine by vitamin C is not well known. However, it may occur in part because of vitamin C antioxidant properties. The present study revealed that vitamin C might ameliorate the adverse effect of heat stress in lambs.