Insulin. Computer artwork of a molecule of the hormone insulin, essential in maintaining the balance of glucose metabolism in the body. Atoms are depicted as spheres and are colour-coded: carbon (purple), hydrogen (light blue), oxygen (pink), nitrogen (yellow). Insulin is a protein comprising 51 amino acids. It is a steroid hormone secreted by the pancreas gland of all vertebrates, including humans. Insulin is secreted in response to high blood glucose levels (such as after a meal). It promotes the uptake of blood glucose by body cells. Lack of insulin leads to the disease diabetes mellitus.