A hormone is any of a number of 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.
The most important hormones in humans include:
- adrenaline - synonymous with epinephrine
- noradrenaline - synonymous with norepinephrine
- growth hormone
One special group of hormones are trophic hormones that act as stimulants of hormone production of other endocrine glands. For example thyroid stimulating hormone (TSH) causes growth and increased activity of another endocrine gland--the thyroid--hence increasing output of thyroid hormones.