Hormone

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)
aldosterone
angiotensinogen
antidiuretic hormone (ADH)
atrial-natriuretic peptide (ANP)
calciferol (vitamin D₃)
calcitonin
cholecystokinin (CCK)
corticotropin-releasing hormone (CRH)
cortisol
dopamine
erythropoietin (EPO)
follicle stimulating hormone (FSH)
gastrin
glucagon
gonadotropin-releasing hormone (GnRH)
Growth hormone-releasing hormone (GHRH)
human chorionic gonadotrophin (HCG)
Human growth hormone (hGH) (see also Growth hormone)
insulin
insulin-like growth factor
leptin
luteinizing hormone (LH)
melatonin
neuropeptide Y
noradrenaline - synonymous with norepinephrine
oestrogens
oxytocin
parathyroid hormone (PTH)
progesterone
prolactin (PRL)
renin
secretin
somatostatin
testosterone
thrombopoietin
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.

Hunger Hormones

Ghrelin
PYY-336

Ghrelin is a hormone that regulates hunger made by the stomach. If there is too much in the body, it wastes away eating (doesn't get signal). Low levels are most common in obese individuals. PYY-336 is a satiey signal from the large and small intestine. This is a sign of fullness and the body no longer wants to eat.