Hormones are chemical substances 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.