A protein produced in the pituitary gland that stimulates the liver to produce somatomedins, which stimulate growth of bone and muscle. Human growth hormone is the most abundant hormone produced by the pituitary gland in the brain. It peaks during the rapid-growth phase of adolescence, then steadily declines with age. HGH stays in the bloodstream for only a few minutes. However, this is long enough to stimulate its uptake by the liver, causing the production of Insulin-like growth factor-1 (IGF-1). Growth hormone affects specific target tissues in its role as a chemical messenger, working with IGF-1 to affect uptake of nutrients into the cell. hGH helps boost the immune system, increase lean body mass, stimulate neuroendocrine system balance and promote optimal physical and mental performance.