Effect of Thyroid Hormones on the Redox Balance of Broiler Chickens

In the present study, two trials were conducted to evaluate the effects of hyper- and hypothyroid status on the redox balance of broiler chickens. In Trial 1, 3 groups of broiler chickens were randomly subjected to one of the three treatments: subcutaneous administration of triiodothyronine (T3, $150{\mu}g/kg$ BW), methimazole (MMI, 150 mg/kg BW), or saline. The blood, liver and heart were sampled at 3 h after injection. In Trial 2, three groups of 20 broiler chickens were randomly fed with one of the three diets: control, dietary supplementation of T3 (1.5 mg/kg diet) or MMI (1 g/kg diet) for 7 days. In trial 1, the plasma concentrations of T3 and T3 to thyronine ratio (T3/T4) were significantly increased by T3 injection. Plasma levels of thiobarbituric acid reacting substances (TBARS) tended to be increased (p = 0.067) by both T3 and MMI treatments while the ferric reduced/antioxidant capacity (FRAP) was increased only by MMI treatment. Acute T3 treatment had no significant effect on the activities of superoxide dismutase (SOD) and the concentrations of FRAP and TBARS in either liver or heart tissue. In contrast, the hepatic activities of SOD were decreased (p<0.05) while the cardiac levels of FRAP were significantly increased (p<0.0001) by MMI treatment. In chronic treatments, the rectal temperature of chickens was significantly decreased (p<0.05) by MMI treatment. The circulating T3 levels were significantly increased (p<0.05) by long-term T3 treatment, and showed a trend to decrease in MMI treatment. The plasma concentrations of TBARS were significantly (p<0.05) increased by MMI treatment. All the redox parameters measured in either liver or heart were not significantly altered by either long-term T3 or MMI treatment except that the hepatic SOD activities were significantly augmented by T3 treatment. The result showed that neither acute nor long-term elevation of circulating T3 levels induced lipid peroxidation in broiler chickens. The enhanced enzymatic antioxidant system (SOD in cardiac tissue) may be involved in the protection of the bird to increased oxidative challenge. The responses of redox balance to changed thyroid state seem to be tissue specific.