Human Growth Hormone 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. 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 medical mental and physical performance.

Growth factors
Small proteins produced by the human body that enable cells to communicate and effectively coordinate activities between one another. Growth factors in the body affect the individual cells by binding to growth-factor-specific receptors on the cell surface. A specific growth factor may have many cell sources and can use different signal transduction pathways at different times and with different cells. Growth factors are involved in complex feedback loops between the immune, nervous and endocrine systems, and have significant effects on RNA, DNA protein synthesis, and cell division.

Insulin like growth factor (IGF-1)
Polypeptides with considerable sequence similarity to insulin. They are capable of eliciting the same biological responses, including mitogenesis in cell culture. On the cell surface, there are two types of insulin like growth factor receptor, one of which closely resembles the insulin receptor (which is also present). IGF-1 is primarily secreted by the liver in response to a signal from growth hormone (hGH). It is also released by many different tissues throughout the body, and affects almost every cell to some degree. The major target tissues affected by IGF-1 are muscle, cartilage, bone, liver, kidney, nerves, skin, and lungs. IGF-1 additionally regulates cell growth by moving cells from a resting phase to an active phase of the cell cycle. IGF-1 also increases the cell's ability to complete DNA synthesis. IGF-1 acts within the nervous system and is critical for the growth and development of nerve cells. IGF-1 plays an active role at the neuromuscular junction, where interaction between nerve and muscle cells occurs.