Limitations of interpretation of age-related changes in hormone levels: illustration by effects of thyroid hormones on cardiac and skeletal muscle.

Studies on age-related changes in hormone secretion or circulation are frequently interpreted to indicate changes in the functions of target tissues for that hormone. Such interpretations are not necessarily correct. For instance, the levels of thyroid hormone change modestly with age, and responses to this change differ dramatically in cardiac and skeletal muscles. There is a substantial change in myosin isozyme content of the heart, but little or no change in skeletal muscle fiber type distribution in specific pathogen free (SPF) barrier-protected rats. These changes (or lack of them) are reflected in functional differences (or lack of them) in the target tissues. The most obvious difference between the recent studies showing no differences with age in skeletal muscle fiber distribution and previous reports of general decreases with age in content of fast fibers is in animal housing conditions; all of the animals showing an age-related change had been housed in conventional colonies, while those exhibiting little or no change were in SPF barrier-protected facilities.