Insulin-like growth factors (IGFs) are peptide hormones that are primarily involved in stimulating the growth of many types of cells. They participate in the growth and function of almost every organ in the body and are also partially capable of reducing blood glucose levels.
The name “insulin-like growth factor” reflects the fact that these hormones are structurally related to insulin and can even bind to the insulin receptor, although at a lower affinity than insulin. IGFs are far less effective at decreasing blood glucose levels than insulin.
Although IGFs are mainly produced in the liver as endocrine hormones, they are also produced in many other tissues, where they exert paracrine (acting on nearby cells) and autocrine (self-stimulating) effects.
There are two forms of insulin-like growth factor: IGF-1 and IGF-2. Both are released when stimulated by growth hormone and their production may be decreased by growth hormone insensitivity or a lack of GH receptors. Malnutrition can also lead to IGF deficiency.
IGF-1 consists of a single chain of 70 amino acids with three intramolecular disulfide bridges and has a molecular weight of 7,649 Daltons. Most of the actions of pituitary growth hormone are moderated by IGFs, mainly IGF1. Growth hormone triggers tissues to make and secrete IGF-1, which then stimulates cells to increase in size (hypertrophy) and number (hyperplasia).
IGF-1 is essential to growth during childhood and has anabolic effects in adulthood. It is produced throughout life, with the highest rate of production seen during puberty and the lowest rate seen in infancy and old age.