Human growth hormone (hGH) is a hormone that is naturally produced by the body. It is synthesized and secreted by cells in the anterior pituitary gland located at the base of the brain. Its secretion into the circulation follows a pulsatile pattern resulting in widely fluctuating blood levels that are influenced by multiple factors such as age, gender, sleep, physical activity, diet, stress, fever, steroids and the environment.
In serum, hGH exists as a complex combination of different molecular forms (isoforms), including the major 22-kDa form and minor isoforms such as the 20-kDa form. Furthermore, hGH also exists as aggregates of these isoforms (dimers and oligomers, forming both homo- and heterodimers). Following secretion into the blood circulation, the 22-kDa hGH has a short half-life of 10-20 minutes.
hGH stimulates many metabolic processes in cells. hGH affects protein, fat, carbohydrate and mineral metabolism. The major role of hGH is to stimulate the liver to secrete Insulin-like Growth Factor-I (IGF-I). IGF-I stimulates production of cartilage cells, resulting in bone growth and also plays a key role in muscle protein synthesis and organ growth.
hGH is prohibited both in- and out-of-competition under section S2 of WADA’s List of Prohibited Substances and Methods.