Endocrinology is the study of hormones and the disorders related to them. At its simplest a hormone is a chemical messenger from one cell or group of cells to another. Hormones are released (secreted) in the body and have an effect on other parts of the body. The effect is to communicate with possibly distant parts of the body. So for example, the adrenal glands may secrete adrenaline, which has an effect on several different organs.