Hyperthyroidism is a hypermetabolic state produced by excess thyroid hormones. Hyperthyroidism produced histological changes in the cardiac and skeletal muscles of rats, these changes appeared as hyalinosis, lymphocytic invasion, vacuolation, atrophy, dissociation, loss of the nuclei. In addition there were nuclear aggregation, fragmentation, disruption of muscle fibers, increased connective tissue amount in both types of muscles and decreased glycogen content in skeletal muscle. L- carnitine minimized these changes due to its action as peripheral antagonist to thyroid hormones. The present study showed that L- carnitine was capable of counteracting thyroid hormone driven changes in cardiac and skeletal muscles.