Effect of thyroid state on lipid peroxidation, antioxidant defences, and susceptibility to oxidative stress in rat tissues.

The effects of altered thyroid states on lipid peroxidation, antioxidant capacity, and susceptibility to oxidative stress of rat tissues were examined. Hypothyroidism was induced by administering methimazole in drinking water for 15 days. Hyperthyroidism was elicited by a 10-day treatment of hypothyroid rats with tri-iodothyronine (10 micrograms/100 g body weight). In tissues of hypothyroid rats the lipid peroxidation was not modified, whereas in hyperthyroid rats lipid peroxidation increased in liver and heart but not in skeletal muscle. The glutathione peroxidase activity increased significantly in heart and muscle of hypothyroid rats and in muscle of hyperthyroid rats. The glutathione reductase activity was not modified in tissues of hypothyroid and hyperthyroid rats. In both rat groups the whole antioxidant capacity of tissues decreased, but significantly only in liver and heart. The results obtained studying the response to oxidative stress in vitro indicated that the susceptibility to oxidative challenge was increased in all tissues of hyperthyroid rats and in heart and muscle of hypothyroid animals. These results are explainable in terms of tissue variations in haemoprotein content and/or of antioxidant capacity. Since it has been reported that hypothyroidism offers in vivo protection against free radical damage, we suggest that such an effect could be due to greater effectiveness of cellular defence systems different from antioxidant ones.