Hormones bind to receptors on a cell's surface or inside a cell. The binding of a hormone to a receptor speeds up, slows down, or in some other way alters the cell's function. Ultimately, hormones control the function of entire organs. They control growth and development, reproduction, and sexual characteristics. They influence the way the body uses and stores energy.
Hormones also control the volume of fluid and the levels of salt and sugar in the blood.
Some hormones affect only one or two organs, while others affect the whole body. For example, thyroid-stimulating hormone is produced in the pituitary gland and affects only the thyroid gland. In contrast, thyroid hormone is produced in the thyroid gland, but it affects cells throughout the body. Insulin, produced by the islet cells of the pancreas, affects the metabolism of glucose, protein, and fat throughout the body. When endocrine glands malfunction, hormone levels in the blood can become abnormally high or low, disrupting body functions.