Hormones (from Greek horman "to set in motion") are a kind of biochemical messengers. Most hormones are chemical substances produced by specialized tissue formations called endocrine glands. The substances are secreted directly into the bloodstream, other body fluids, or into adjacent tissues. The purpose of hormones is to regulate metabolic activity of some other organs or tissues of the body.
The rate of production of a given hormone is most commonly regulated by a mechanism called a negative feedback loop.
The notion "endocrine" is contrasted with "exocrine", which means a specialized gland with a distinct secretory duct. Examples of exocrine glands are salivary or sweat glands; examples of endocrine glands are thyroid and adrenal glands. With our growing understanding of intricacies of homeostasis regulation the term "endocrine" is becoming rather of historical value. Every cell is capable of producing a vast number of regulatory molecules - tissue hormones, neurotransmitters, cytokines and many more. The classical endocrine glands and their hormone products are only better specialized to serve regulation on the overall organism level. Nevertheless they can in many instances be used in other ways or only on the tissue level.
The most important hormones in humans include: