Human growth hormone (HGH) is a small protein that is made by the pituitary gland and secreted into the bloodstream. GH production is controlled by a complex set of hormones produced in the hypothalamus of the brain and the intestinal tract and pancreas.
The pituitary puts out GH in bursts; levels rise following exercise, trauma, and sleep. Under normal conditions, more GH is produced at night than during the day. This physiology is complex, but at a minimum, it tells us that sporadic blood tests to measure GH levels are meaningless since high and low levels alternate throughout the day. But scientists who carefully measure overall GH production report that it rises during childhood, peaks during puberty, and declines from middle age onward.