[6] Ghrelin activates cells in the anterior pituitary gland and hypothalamic arcuate nucleus,[6][9] including neuropeptide Y neurons that initiate appetite.

[6] The hormone name is based on its role as a growth hormone-releasing peptide, with reference to the Proto-Indo-European root gʰre-, meaning "to grow".

It does not activate the GHS-R receptor but does have other effects: cardiac,[17] anti-ghrelin,[18] appetite stimulation,[19] and inhibition of hepatic glucose output.

[23] Ghrelin cells are found mainly in the stomach[24] and duodenum, but also in the jejunum, lungs, pancreatic islets,[25] gonads, adrenal cortex, placenta, and kidney.

The net result of these processes is reflected in body weight, and is under continuous monitoring and adjustment based on metabolic signals and needs.

[50] Circulating ghrelin concentrations rise before eating and fall afterward,[46] more strongly in response to protein and carbohydrate than to lipids.

[51] The ghrelin receptor GHS-R1a (a splice-variant of the growth hormone secretagogue receptor, with the GHS-R1b splice being inactive) is involved in mediating a wide variety of biological effects of ghrelin, including: stimulation of growth hormone release, increase in hunger, modulation of glucose and lipid metabolism, regulation of gastrointestinal motility and secretion, protection of neuronal and cardiovascular cells, and regulation of immune function.

[6] A review reported finding strong evidence that sleep restriction affected ghrelin or leptin levels, or energy expenditure.

[50] Ghrelin functions as a cardio-protective peptide by being an anti-inflammatory agent, promoting angiogenesis, inhibiting arrhythmia, and improving heart failure.

[58] Additionally, Ghrelin and its endogenous receptor, GHSR1a, along with GOAT are expressed in primary immune tissues such as the spleen and thymus where it has a role in modulating interactions between metabolic state and inflammation, mediating energy balance homeostasis.

[61] Gastric bypass surgery not only reduces gut capacity for food, but also lowers ghrelin levels compared to both lean people and those who lost weight through dieting.

[62][63] Studies have not clarified whether ghrelin levels return to normal in people who had gastric bypass surgery after weight loss has stabilized.

[64] Gastric bypass surgery involving vertical-sleeve gastrectomy reduces plasma ghrelin levels by about 60% in the long term.

[70][71] The level of ghrelin increases during the time of day from midnight to dawn in thinner people, which suggests there is a flaw in the circadian rhythm of obese individuals.

Studies have shown that in mice models of myocardial infarction (MI) with knock-outs of ghrelin, subjects with no endogenous ghrelin production had a significantly increased mortality rate along with worse metrics in terms of cardiac sympathetic activity and systolic function when compared to wild-type subjects.