A peptide hormone made by most tissues of the body and believed to be secreted by the liver and released into the bloodstream. Insulinlike growth factor I (IGF-I) is involved in various biological activities in the cells. These activities include mitosis (normal cell division) and may be similar to the action of insulin. IGF-I is important to human survival because it regulates normal growth. It is mainly active in adults; the main growth factor in fetuses is insulinlike growth factor II.