Insulin-like growth factors, or IGFs, are hormones which help promote growth and which are produced by a number of different cells in the body. There are two main types of insulin-like growth factor, known as IGF-1 and IGF-2. When the pituitary gland in the brain secretes growth hormone, the liver releases IGF-1 and IGF-2 into the blood. The insulin-like growth factors then bind to special receptors on cells and stimulate growth. IGF-1 promotes growth of bone and cartilage after birth, while IGF-2 is responsible for development in the fetus and placenta.
Formerly known as somatomedins, insulin-like growth factors are so named because they show a weak insulin-like activity in addition to their growth-promoting properties. Insulin is a hormone which lowers blood sugar levels by increasing the transfer of glucose from the blood into cells. While there are only two main insulin-like growth factors in humans, both IGF-1 and IGF-2 exist in a number of slightly different forms.
Both insulin-like growth factors circulate in the blood attached to special IGF-binding proteins before they join to receptors on cells. There are three main types of receptors. Both IGF-1 and IGF-2 bind to what is known as the type 1 receptor, while the type 2 receptor mainly binds IGF-2. The third receptor binds IGF-1 and closely resembles an insulin receptor. High concentrations of IGF-1 binding to this insulin receptor can produce an effect similar to insulin activity.
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Pituitary disorders can lead to increased or decreased levels of growth hormone, and tests measuring the amount of IGF-1, which is the insulin-like growth factor most affected by growth hormone, can help assess pituitary function. Sometimes a tumor of the pituitary gland produces excessive amounts of growth hormone. Most often these tumors are benign, or non-cancerous, but symptoms such as visual changes, headaches, diabetes and growth abnormalities may result from the elevated growth hormone levels.
In the rare disease known as acromegaly, most often caused by a pituitary tumor producing growth hormone, blood tests typically reveal increased levels of IGF-1, which have been secreted in response to the high amounts of growth hormone. Along with other symptoms, abnormal growth may be seen to affect the forehead, lower jaw, hands and feet. Treatment generally involves surgical removal of the tumor, sometimes accompanied by medication, with the aim of returning growth hormone levels to normal.
Abnormal levels of IGF-2 are thought to be associated with a rare condition causing excessive growth in the fetus. Increased levels of both of the insulin-like growth factors are also thought to be involved in the development of certain cancers. Research is ongoing into the specific mechanisms involved.