What is a peptide hormone?
Hormones are chemical messengers released by glands and tissues into the bloodstream. They fall into three broad chemical classes, and the class determines how the hormone behaves:
- Peptide (and protein) hormones — chains of amino acids. Water-soluble; they cannot cross the fatty cell membrane, so they act on receptors on the cell surface. Insulin, glucagon, oxytocin, ACTH.
- Steroid hormones — built from cholesterol. Fat-soluble; they pass through the membrane and act on receptors inside the cell. Testosterone, estrogen, cortisol.
- Amino-acid-derived hormones — made from a single amino acid. Thyroid hormone, adrenaline.
The terms peptide hormone, polypeptide hormone, and protein hormone all describe the same amino-acid class; the only difference is chain length, and the boundaries are informal. Insulin (51 amino acids) is called both a peptide and a protein hormone.
How peptide hormones work
Because they are water-soluble, peptide hormones cannot enter a cell. Instead they bind to a receptor on the cell surface — very often a G-protein-coupled receptor — which triggers a cascade of second messengers inside the cell.[1]
This has three practical consequences worth knowing:
- They act fast. Surface signaling produces effects in seconds to minutes, versus hours for steroid hormones that must alter gene transcription.
- They are short-lived. Many are broken down within minutes. Natural GLP-1, for instance, has a half-life of about two minutes — which is precisely why drugmakers re-engineered it to last a week.[1]
- They can't be taken as pills. Digestion breaks them into amino acids, which is why insulin is injected rather than swallowed.
Release is usually controlled by feedback loops: rising blood glucose triggers insulin, and rising insulin lowers glucose, which shuts the signal off.[2] Some are also released in pulses on a daily rhythm, as with ACTH and cortisol.[4]
List of peptide hormones (with examples)
Peptide hormones are found throughout the body. Here are the major ones grouped by where they come from — entries linked in blue have a full guide on this site.
| Peptide hormone | Source | Main role |
|---|---|---|
| Insulin | Pancreas (beta cells) | Lowers blood glucose; stores nutrients[2] |
| Glucagon | Pancreas (alpha cells) | Raises blood glucose |
| GLP-1 & GIP (incretins) | Gut | Amplify insulin release after eating; curb appetite[1] |
| Oxytocin | Posterior pituitary | Labour contractions, milk let-down, social bonding |
| Vasopressin (ADH) | Posterior pituitary | Water retention by the kidney |
| Growth hormone (GH) | Anterior pituitary | Growth and metabolism |
| ACTH | Anterior pituitary | Drives cortisol release from the adrenals[4] |
| GnRH (gonadorelin) | Hypothalamus | Controls LH and FSH, and therefore sex hormones |
| Ghrelin | Stomach | Stimulates hunger and GH release[3] |
| Leptin | Fat tissue | Signals energy stores; suppresses appetite[3] |
| Calcitonin | Thyroid (C cells) | Lowers blood calcium |
| Parathyroid hormone (PTH) | Parathyroid glands | Raises blood calcium — see teriparatide |
| Secretin | Small intestine | The first hormone ever discovered; controls pancreatic bicarbonate |
| α-MSH | Pituitary | Pigmentation and appetite via melanocortin receptors |
Peptide hormones as medicines
Because peptide hormones are so specific, copying or blocking them has produced some of medicine's most important drugs. Insulin, isolated in 1921, was the first — and the model for everything since.
- Replacing a hormone. Insulin for diabetes; teriparatide (a PTH fragment) for osteoporosis.
- Mimicking one, but longer-lasting. Semaglutide and dulaglutide are engineered GLP-1 analogues; tirzepatide mimics both GIP and GLP-1 at once. See is Ozempic a peptide?
- Blocking or shutting one down. Leuprolide and degarelix act on the GnRH system to suppress sex hormones in prostate cancer and endometriosis.
These are all FDA-approved, prescription-only medicines — a very different category from the unapproved "research peptides" sold online. See what are peptides? for that distinction, or browse the full A–Z list of peptides.
Frequently asked questions
What is a peptide hormone?
A peptide hormone is a hormone made of a chain of amino acids, such as insulin, glucagon, or oxytocin. Because they are water-soluble they cannot enter cells, so they bind to receptors on the cell surface to deliver their signal.
Which of the following are peptide hormones?
Insulin, glucagon, oxytocin, vasopressin, growth hormone, ACTH, GnRH, ghrelin, leptin, calcitonin, PTH, and the incretins GLP-1 and GIP are all peptide hormones. Testosterone, estrogen, and cortisol are not — they are steroid hormones.
What is the difference between peptide and steroid hormones?
Peptide hormones are chains of amino acids that are water-soluble and act on receptors on the cell surface, producing fast effects. Steroid hormones are made from cholesterol, are fat-soluble, pass through the cell membrane, and act more slowly by changing gene expression.
Is insulin a peptide hormone?
Yes. Insulin is a peptide hormone of 51 amino acids released by the beta cells of the pancreas to lower blood glucose. It sits at the informal border between peptide and protein, so it is described as both.
Why can't peptide hormones be taken as pills?
Digestive enzymes break amino-acid chains down before they reach the bloodstream, so a swallowed peptide hormone is destroyed. That is why insulin is injected. A few peptide drugs use special absorption enhancers to work orally.
Are polypeptide hormones the same as peptide hormones?
Yes — the terms describe the same class of amino-acid-based hormones. 'Polypeptide' or 'protein hormone' generally implies a longer chain, but there is no strict cut-off and the terms are used interchangeably.
Further reading
Selected peer-reviewed sources on this topic, labelled by type. A citation is a reference, not an endorsement.
- Baggio LL, Drucker DJ. Biology of incretins: GLP-1 and GIP. Gastroenterology. 2007. Peer-reviewed study
- Fu Z, Gilbert ER, Liu D. Regulation of insulin synthesis and secretion and pancreatic Beta-cell dysfunction in diabetes. Curr Diabetes Rev. 2013. Peer-reviewed study
- Klok MD, Jakobsdottir S, Drent ML. The role of leptin and ghrelin in the regulation of food intake and body weight in humans: a review. Obes Rev. 2007. Peer-reviewed study
- Lightman SL, Birnie MT, Conway-Campbell BL. Dynamics of ACTH and Cortisol Secretion and Implications for Disease. Endocr Rev. 2020. Peer-reviewed study