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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:
* adrenaline - synonymous with epinephrine
* adrenocorticotropic hormone (ACTH)
* antidiuretic hormone (ADH)
* atrial-natriuretic peptide (ANP)
* calciferol (vitamin D3)
* cholecystokinin (CCK)
* corticotropin-releasing hormone (CRH)
* erythropoietin (EPO)
* follicle-stimulating hormone (FSH)
* gonadotropin-releasing hormone (GnRH)
* Growth hormone-releasing hormone (GHRH)
* human chorionic gonadotrophin (HCG)
* Human growth hormone (hGH) (see also Growth hormone)
* insulin-like growth factor
* luteinizing hormone (LH)
* neuropeptide Y
* noradrenaline - synonymous with norepinephrine
* parathyroid hormone (PTH)
* prolactin (PRL)
* thyroid-stimulating hormone (TSH)
* thyrotropin-releasing hormone (TRH)
* thyroxine (T4)
* triiodothyronine (T3)
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.