The endocrine system consists of various ductless glands that produce hormones. They are called ductless glands because they produce substances - in this case, hormones - that are secreted directly into the blood. These hormones, which act as chemical messengers, are generally quite specific as to the effects they bring about. The thyroid, for example, through its hormones thyroxine (T4) and triiodothyronine (T3), regulates the general metabolic level at which the body's cells operate. The adrenal glands produce adrenalin and steroids which girds the body for action during fight or flight situations.

HGH, on the other hand, operates in a much broader fashion, and on several fronts at once. One very important function of HGH is that it increases the efficiency of receptor sites on a great many cells in the body. These receptor sites are where hormones attach to a cell to stimulate it to effect a particular activity. By making these receptor sites more "receptive", HGH improves the efficacy of other hormones in achieving their intended results.

We might compare the body's hormones to tradesmen or contractors on a construction site. These hormones, like plumbers or carpenters, perform fairly specific tasks. In contrast, HGH might be likened to a master builder or general contractor. Just as a general contractor facilitates the work of plumbers or carpenters at various sites on the construction project, so too HGH facilitates the work of the other hormones at the sites of their operation.