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This article waslast modified on July 10, 2017.
A chemical substance produced and secreted by endocrine (ductless) glands that travels through the bloodstream and controls or regulates the activity of another organ or group of cells - its target organ. (For example, growth hormone released by the pituitary gland controls the growth of long bones of the body.) There are two main types of hormones - steroids (e.g., estrogen, testosterone, aldosterone, cortisol) and nonsteroidal. Secretion of hormones is regulated by feedback mechanisms and neurotransmitters.