Human Insulin-like growth factor-I (IGF-I) is a 7.6 kD polypeptide endocrine hormone. IGF-I functions as the major mediator of growth hormone (GH)-stimulated somatic growth, as well as a mediator of GH-independent anabolic responses in many cells and tissues including muscle, bone, and cartilage tissue.
IGF-I is produced primarily by the liver as an endocrine
hormone as well as in target tissues in a paracrine/autocrine fashion. Production is stimulated by growth hormone (GH) and can be retarded by undernutrition, growth hormone insensitivity, lack of growth hormone receptors, or failures of the downstream signaling pathway post GH receptor. Approximately 98% of IGF-I is always bound to binding proteins (IGF-BP).