A hormone is a substance that is produced in tiny amounts in one part of an organism and is transported to other parts where it has a definite physiological effect. Hormones also often affect the functioning of other hormone-producing tissues so that the effects can be complex and integrated. They influence the rate at which reactions proceed. They are important in both animals and plants, affecting growth, differentiation, metabolism, digestive function, mineral and fluid balance.

 

The rate of secretion, efficacy on target organs, and rate of removal are all affected by numerous factors include feedback from their metabolic effects, mineral or sugar concentration in the blood, and the action of controlling hormones. The latter usually originate in the pituitary gland and those controlling the pituitary in the hypothalamus.

 

Hormones do not fall into any one particular chemical class. They may be steroids, peptides, or relatively simple compounds such as adrenaline.

 

In animals, hormones are usually secreted by endocrine glands directly from gland cells into the bloodstream, where they often circulate in combination with serum proteins. They occur copiously in vertebrates but are also known in some invertebrates.

 

Plant hormones, called phytohormones (e.g., auxins and gibberllins), play a prominent role in plant growth.

 

Principal hormones in the human body
hormone	chemical type	secreting gland	target
adrenaline	modified amino acid	adrenal medulla	heart
adrenocorticotropic hormone	polypeptide	anterior pituitary	adrenal cortex
androgen	steroid	testes	many tissues
antidiuretic hormone	peptide	hypothalamus	kidneys
atrial natriuretic peptide	peptide	heart	kidneys
calcitonin	peptide	thyroid	peptide
cholecystokinin	peptide	lining of small intestine	pancreas
corticotropin-releasing hormone	peptide	hypothalamus	anterior pituitary
dopamine	peptide	hypothalamus	anterior pituitary
enterogastrone	polypeptide	lining of small intestine	stomach
estrogen	steroid	ovaries	uterus
follicle-stimulating hormone	glycoprotein	anterior pituitary	gonads
gastrin	peptide	stomach lining	stomach
glucagon	protein	pancreas	liver
glucocorticoid	steroid	adrenal cortex	muscles
gonaotrophin-releasing hormone	peptide	hypothalamus	anterior pituitary
growth hormone	protein	anterior pituitary	bones
growth hormone-releasing hormone	peptide	hypothalamus	anterior pituitary
insulin	protein	pancreas	muscles
luteinizing hormone	glycoprotein	anterior pituitary	gonads
melanocyte-stimulating hormone	peptide	anterior pituitary	melanocytes
melatonin	modified amino acid	pineal gland	hypothalamus
mineralocorticoid	steroid	adrenal cortex	kidneys
noradrenaline	modified amino acid	adrenal medulla	heart
oxytocin	peptide	hypothalamus	uterus
parathyroid hormone	protein	parathyroid	bones
progesterone	steroid	ovaries	uterus
prolactin	protein	anterior pituitary	mammary glands
prostaglandin	modified fatty acid	many tissues	many tissues
secretin	peptide	lining of small intestine	pancreas
somatostain	peptide	pancreas	gastrointestinal tract
thymosin	peptide	thymus	white blood cells
thyroid-stimulating hormone	glycoprotein	anterior pituitary	thyroid
thyrotropin-releasing hormone	peptide	hypothalamus	anterior pituitary
thyroxine	iodinated amino acid derivative	thyroid	many tissues
vitamin D	sterol	skin	bones