To emphasize once more, this is an extremely simplified model. There are a great many interactions involved in HGH physiology. The pituitary is something of a master gland which affects many other glands in the endocrine system, and HGH is something of a "master hormone". As such it affects the levels of many other hormones in the body. Notwithstanding these other interactions, the pituitary's principal mode of regulation, vis a vis HGH release, is by the hypothalamus, as described.

If, with all the abbreviations used here, we seem to be thickening an alphabet soup already rich in letters, we might regard the two hormones from the hypothalamus that govern the HGH release from the pituitary merely as Stop and Go. In fact, we might look at the bloodstream as being a large parking lot with a certain number of cars in it, with the cars being molecules of HGH.

The hypothalamus would be represented by a counter that kept track of the number of cars (HGH) in the lot, and that also controlled the entrance to the lot with a Stop and a Go sign. As cars (HGH) left the lot at an exit some distance away, thereby making room for more cars (HGH) in the lot, the counter (hypothalamus) would detect this and would flash a Go sign at the entrance. When there were sufficient cars in the lot the counter (hypothalamus) would flash a Stop sign at the entrance. The main point is that the release of HGH by the pituitary is regulated by the hypothalamus as part of a more complex loop.

The part of the picture that interests us most, however, has to do with the effect of HGH on the rest of the body. Initially, this principally involves the liver. As large volumes of blood are pumped through the liver via the hepatic portal system, the HGH in the blood acts on the liver (amongst other tissues) to stimulate the production of a hormone called Insulin-like Growth Factor-1 (IGF-1) which consists of 70 amino acids, and Insulin-like Growth Factor-2 (IGF-2) which consists of 67 amino acids. It is through IGF-1 that HGH does most of its work.