Hormones are chemicals produced by glands, tissues, and organs to control the function of a target organ or regulate the production of another hormone. The body produces a specific amount of a hormone in response to stimuli (signals) from inside and outside. The hormone balance helps keep the body functioning in ordinary or stressful situations--a process called homeostasis.

Dozens of human hormones play important roles in growth, sex and reproduction, digestion, blood composition, and stress control. Other animals and plants produce hormones as well. Several hormones may work as a team on the same organ or tissue. Their combined effect may be greater than the sum of their single effects (synergism).

Many hormones are produced by the endocrine system, a large group of ductless glands. They include the pituitary and pineal, at the base of the brain; the thyroid and parathyroid in the throat; and the adrenal glands...