Hormone
A
hormone (from
Greek horman - "to set in motion") is a
chemical messenger from one
cell (or group of cells) to another. All
multicellular organisms produce hormones (including
plants -
see article phytohormone).
The best-known animal hormones are those produced by
endocrine glands of
vertebrate animals, but hormones are produced by nearly every
organ system and
tissue type in an animal body. Hormone
molecules are secreted (released) directly into the
bloodstream; some hormones, called ectohormones, aren't secreted into the blood stream, they move by circulation or
diffusion to their target cells, which may be nearby cells (paracrine action) in the same tissue or cells of a distant organ of the body. The function of hormones is to serve as a signal to the target cells; the action of hormones is determined by the pattern of secretion and the
signal transduction of the receiving tissue.
Hormone actions vary widely, but can include stimulation or inhibition of growth, induction or suppression of
apoptosis (programmed cell death), activation or inhibition of the
immune system, regulating
metabolism and preparation for a new activity (e.g., fighting, fleeing, mating) or phase of life (e.g., puberty, caring for offspring, menopause). In many cases, one hormone may regulate the production and release of other hormones. Many of the responses to hormone signals can be described as serving to
regulate metabolic activity of an organ or tissue. Hormones also control the
reproductive cycle of virtually all multicellular organisms.
The concept of internal secretion developed in the
19th century;
Claude Bernard described it in
1855, but did not specifically address the possibility of secretions of one organ acting as messengers to others. Still, various endocrine conditions were recognised and even treated adequately (e.g.,
hypothyroidism with extract of thyroid glands).
The major breakthrough was the identification of
secretin, the hormone secreted by the
duodenum that stimulates
pancreatic secretions, by
Ernest Starling and
William Bayliss in
1902. Previously, the process had been considered (e.g., by
Ivan Pavlov) to be regulated by the nervous system. Starling and Bayliss demonstrated that injecting duodenal extract into dogs rapidly increased pancreatic secretions, raising the possibility of a chemical messenger.
Starling is also credited with introducing the term
hormone, having coined it in a
1905 lecture. Later reports indicate it was suggested to him by the Cambridge physiologist
William B. Hardy (Henderson 2005).
The remainder of the
20th century saw all the major hormones discovered, as well as the cloning of the relevant
genes and the identification of the many interlocking feedback mechanisms that characterise the endocrine system.
Most cells are capable of producing one or more, sometimes many, molecules which signal other cells to alter their growth, function, or metabolism. The classical
endocrine glands and their hormone products are specialized to serve regulation on the overall organism level, but can often be used in other ways or only on the tissue level.
The rate of production of a hormone is often regulated by a
homeostatic control system, generally by
negative feedback. Homeostatic regulation of hormones depends, apart from production, on the
metabolism and
excretion of hormones.
Hormone secretion can be stimulated and inhibited by:
*Other hormones (
stimulating- or
releasing-hormones)
*Plasma concentrations of ions or nutrients, as well as binding
globulins
*
Neurons and mental activity
*Environmental changes, e.g., of light or temperature
One special group of hormones is
trophic hormones that stimulate the 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.
A recently-identified class of hormones is that of the "Hunger Hormones" -
ghrelin,
orexin and
PYY 3-36 - and "Satiety hormones" - e.g.,
leptin, obestatin.
Vertebrate hormones fall into four chemical classes:#
Amine-derived hormones are derivatives of the
amino acids
tyrosine and
tryptophan. Examples are
catecholamines and
thyroxine.#
Peptide hormones consist of chains of amino acids. Examples of small peptide hormones are
TRH and
vasopressin. Peptides composed of scores or hundreds of amino acids are referred to as
proteins. Examples of protein hormones include
insulin and growth hormone.#
Steroid hormones are derived from
cholesterol. The
adrenal cortex and the
gonads are primary sources. Examples of steroid hormones are
testosterone and
cortisol.
Sterol hormones such as
calcitriol are a
homologous system.#
Lipid and
phospholipid hormones are derived from lipids such as
linoleic acid and phospholipids such as
arachidonic acid. The main class is the
eicosanoids, which includes the widely-studied
prostaglandins.
Many hormones are used as
medication. The most commonly-prescribed hormones are
estrogens and
progestagens (in the
contraceptive pill and as
HRT),
thyroxine (as
levothyroxine, for
hypothyroidism) and
steroids (for
autoimmune diseases and several
respiratory disorders).
Insulin is used by many
diabetics. Local preparations for use in
otolaryngology often contain
pharmacologic equivalents of
adrenaline, while
steroid and
vitamin D creams are used extensively in
dermatological practice.
A "pharmacologic dose" of a hormone is a medical usage referring to an amount of a hormone far greater than naturally occurs in a healthy body. The effects of pharmacologic doses of hormones may be different from responses to naturally-occurring amounts and may be therapeutically useful. An example is the ability of pharmacologic doses of
glucocorticoid to suppress inflammation.
Spelling is not uniform for many hormones. Current North American and international usage is estrogen, gonadotropin, while British usage retains the Greek
diphthong in oestrogen and the unvoiced aspirant h in gonadotrophin.
Amine hormones
Amine-derived hormones:
*
catecholamines
**
adrenaline (or epinephrine)
**
dopamine **
noradrenaline (or norepinephrine)
*
tryptophan derivatives
**
melatonin (N-acetyl-5-methoxytryptamine)
**
serotonin (5-HT)
*
tyrosine derivatives
**
thyroxine (T4)
**
triiodothyronine (T3)
Peptide hormones
Peptide hormones:
*
antimullerian hormone (AMH, also mullerian inhibiting factor or hormone)
*
adiponectin (also Acrp30)
*
adrenocorticotropic hormone (ACTH, also corticotropin)
*
angiotensinogen and
angiotensin*
antidiuretic hormone (ADH, also vasopressin, arginine vasopressin, AVP)
*
atrial-natriuretic peptide (ANP, also atriopeptin)
*
calcitonin*
cholecystokinin (CCK)
*
corticotropin-releasing hormone (CRH)
*
erythropoietin (EPO)
*
follicle-stimulating hormone (FSH)
*
gastrin*
ghrelin*
glucagon*
gonadotropin-releasing hormone (GnRH)
*
growth hormone-releasing hormone (GHRH)
*
human chorionic gonadotropin (hCG)
*
growth hormone (GH or hGH)
*
inhibin*
insulin*
insulin-like growth factor (IGF, also somatomedin)
*
leptin*
luteinizing hormone (LH)
*
melanocyte stimulating hormone (MSH or α-MSH)
*
neuropeptide Y*
oxytocin*
parathyroid hormone (PTH)
*
prolactin (PRL)
*
relaxin*
secretin*
somatostatin*
thrombopoietin*
thyroid-stimulating hormone (TSH)
*
thyrotropin-releasing hormone (TRH)
Steroid and sterol hormones
Steroid hormones:
*
Glucocorticoids
**
cortisol*
Mineralocorticoids
**
aldosterone*
Sex steroids
**
Androgens
***
testosterone***
dehydroepiandrosterone (DHEA)
***dehydroepiandrosterone sulfate (DHEAS)
***
androstenedione***
dihydrotestosterone (DHT)
**
Estrogens
***
estradiol**
Progestagens
***
progesterone***
ProgestinsSterol hormones:
*
Vitamin D derivatives
**
calcitriolLipid hormones
Lipid and
phospholipid hormones (
eicosanoids):
*
prostaglandins
*
leukotrienes
*
prostacyclin*
thromboxane*
endocrinology*
endocrine system*
neuroendocrinology*
plant hormones or
plant growth regulators*
autocrine signalling*
paracrine signalling*
cytokine*
growth factor*
hormone disruptor* Henderson J. "Ernest Starling and 'Hormones': an historical commentary."
J Endocrinol 2005;184:5â€"10. PMID 15642778.
*
Hormones and endocrine system
