The one thing that stands out when you begin to study hormone action is the subtle interplay among seemingly disparate hormones in
the body. That is, it's difficult to alter the intricate balance of one hormone without affecting others. An example of this is the
relationship between thyroid hormones and insulin like growth factor-1.
Danish researchers recently examined this interplay, report in the results in the American Journal of Physiology (32;E840-E845, i995). Most hormones, including growth hormone, thyroid, testosterone and IGF-1, are carried in the blood bound to proteins. Such hormone and protein carriers are considered "storage" forms of the hormone, since only the unbound or "free" hormone can interact with cellular receptors and begin the process associated with that particular hormone.
Using rats as subjects, the researchers administered various doses of thyroid hormone to the animals, then quantified their IGF-1 responses using a newly developed sophisticated test. What they found was that larger doses of thyroid hormone caused an increase in IGF-1-binding proteins. This, in turn, caused a decrease in the free levels of circulating IGF-1. The amount of thyroid hormone, however, wasn't enough to cause a catabolic or muscle-protein breakdown sequence in the rats.
Since IGF-1 is considered the anabolic stimulus behind growth hormone, taking thyroid hormones may short-circuit the effects of this hormone in muscle by causing it to become tied up in its blood-binding proteins. This could also contribute to the general muscle catabolism often seen with the use of exogenous thyroid hormones.