Peptide hormones are hormones built from short chains of amino acids. Well-known examples include insulin, glucagon, oxytocin and ghrelin. Because they dissolve in water but cannot pass through cell membranes, they work by binding to receptors on the outside of their target cells, which then pass the message inside. This guide explains what peptide hormones are, how the body makes them, how they work, how they differ from steroid hormones, and why so many research peptides are based on them.
What are peptide hormones?
A hormone is a chemical messenger. One tissue releases it, it travels (usually in the blood) and it changes what another tissue is doing. Biologists group hormones by their chemistry, and peptide hormones are the largest group.
A peptide is simply a chain of amino acids joined by peptide bonds. Peptide hormones range from just three amino acids (thyrotropin-releasing hormone, or TRH) to around 50. Longer chains, such as growth hormone with 191 amino acids, are normally called protein hormones, although the line between the two is blurry. Our guide to the different types of peptides explains how chain length is used to name them.
Most peptide hormones share a few traits:
- They are water-soluble. They dissolve in blood plasma and travel freely.
- They are made in advance and stored. Cells pack them into small sacs, ready for quick release.
- They act from outside the cell. They bind to receptors on the cell surface.
- They act fast and briefly. Enzymes in the blood and tissues break them down, so many last only minutes in circulation.
How the body makes peptide hormones
Peptide hormones are not usually made in their final form. A cell first builds a longer precursor protein, often called a prohormone, using the instructions in a gene.
That precursor then moves through the cell’s packaging system. Along the way, enzymes called prohormone convertases (such as PC1/3 and PC2) cut it at specific points to release the active hormone. A review from Brown University describes how this cutting, sorting and chemical modification is a key control point in how much active hormone a cell produces (Perello and Nillni, Frontiers in Bioscience, 2007).
The finished hormone is stored in secretory granules until a signal, such as a rise in blood glucose or a nerve impulse, tells the cell to release it.
Finishing touches that matter
Many peptide hormones get small chemical changes that are essential for their activity. A common one is C-terminal amidation, where the end of the chain is capped with an amide group. Oxytocin, vasopressin and TRH are all amidated.
Ghrelin is a striking example. When researchers in Osaka first isolated it from rat stomach, they found a 28-amino-acid peptide carrying a fatty octanoyl group on its third amino acid, a serine. Without that fatty group, the peptide did not release growth hormone (Kojima and colleagues, Nature, 1999).
How peptide hormones work on cells
Because peptide hormones cannot cross the fatty cell membrane, they bind to receptor proteins that sit in the membrane with one end facing outwards. Two receptor families do most of the work:
- G protein-coupled receptors (GPCRs). Most peptide hormones, including oxytocin, glucagon, GHRH, ghrelin and kisspeptin, use these. When the hormone binds, the receptor switches on a G protein inside the cell.
- Receptor tyrosine kinases. The insulin receptor is the classic example. Binding switches on an enzyme built into the receptor itself.
Either way, the signal is passed on by “second messengers” inside the cell, such as cyclic AMP (cAMP) or calcium ions. One hormone molecule can set off a large cascade, which is why such tiny amounts have a big effect. For a wider look at receptors and signalling, see our guide on how peptides work.
Peptide hormones vs steroid hormones
Steroid hormones, such as testosterone, oestrogen and cortisol, are the other big family. They are made from cholesterol rather than amino acids, and that changes almost everything about how they behave.
| Peptide hormones | Steroid hormones | |
|---|---|---|
| Built from | Amino acids | Cholesterol |
| Dissolves in | Water | Fats (lipid-soluble) |
| Storage | Made ahead and stored in granules | Made when needed, not stored |
| Travel in blood | Mostly free | Mostly bound to carrier proteins |
| Receptor | On the cell surface | Inside the cell |
| Typical speed | Seconds to minutes | Hours, as gene activity changes |
This is why peptides and steroids should not be lumped together. We cover the comparison in more depth in are peptides like steroids?
Examples of peptide hormones
Here are some of the best-known peptide hormones, with their length and where they are made.
| Hormone | Length | Made in | Main job |
|---|---|---|---|
| TRH | 3 amino acids | Hypothalamus | Signals the pituitary to release thyroid-stimulating hormone |
| Oxytocin | 9 | Hypothalamus (released from the pituitary) | Uterine contraction, milk let-down and social bonding |
| Vasopressin | 9 | Hypothalamus (released from the pituitary) | Helps control the body’s water balance |
| Ghrelin | 28 | Stomach | Growth hormone release and hunger signalling |
| Glucagon | 29 | Pancreas (alpha cells) | Raises blood glucose |
| GHRH | 44 | Hypothalamus | Signals the pituitary to release growth hormone |
| Kisspeptin | Several forms (10 to 54) | Hypothalamus | Triggers GnRH release, which drives the reproductive axis |
| Insulin | 51 (two chains) | Pancreas (beta cells) | Lowers blood glucose |
A short history of peptide hormone discoveries
Peptide hormones have been at the centre of some landmark science. Vincent du Vigneaud won the 1955 Nobel Prize in Chemistry “for the first synthesis of a polypeptide hormone”, namely oxytocin (Nobel Prize summary, 1955). Ghrelin was only identified in 1999. In 2003, genetic studies in people and mice, with co-authors at the University of Cambridge, showed that the receptor GPR54 (now called KISS1R), which kisspeptin acts on, is needed for normal puberty (Seminara and colleagues, New England Journal of Medicine, 2003).
Why research peptides are often based on peptide hormones
Natural peptide hormones are broken down quickly, which makes them hard to study for long. So scientists design shorter fragments or modified versions, called analogues, to probe a single receptor or pathway in the lab.
- Sermorelin is the first 29 amino acids of GHRH, the shortest part of the chain that keeps its activity at the GHRH receptor. See our Sermorelin Acetate research peptide.
- Kisspeptin-10 is the short active end of the longer kisspeptin chain.
- KPV is just the last three amino acids of alpha-MSH, a melanocortin peptide hormone. Our guide explains what KPV peptide is and what lab studies have reported.
- Ipamorelin is a small synthetic peptide that acts on the same receptor as ghrelin.
At Peptides UK, our research peptides are Third-Party HPLC Verified and each batch comes with a certificate of analysis. A paper about a natural hormone does not prove that any research peptide has the same effect, and these products are not medicines. All of our products are for laboratory research use only and are not intended for human or veterinary use.
Peptide hormones: frequently asked questions
Is insulin a peptide hormone?
Yes. Insulin is a peptide hormone made of 51 amino acids in two chains (A and B) joined by disulfide bridges. Because it sits right on the boundary between a peptide and a small protein, you will also see it called a protein hormone.
Are peptide hormones water-soluble?
Yes, most are. Peptide hormones dissolve readily in blood plasma and travel freely in the bloodstream. Because they are water-soluble, they cannot slip through the fatty cell membrane, so they act through receptors on the cell surface.
What is the difference between a peptide hormone and a protein hormone?
It is mainly a question of size. Short chains, roughly under 50 amino acids, are usually called peptide hormones, while longer ones such as growth hormone (191 amino acids) are called protein hormones. Both are made and act in much the same way.
Are peptide hormones the same as steroid hormones?
No. Peptide hormones are chains of amino acids that act on cell-surface receptors. Steroid hormones, such as testosterone and cortisol, are made from cholesterol, pass through the cell membrane and act on receptors inside the cell.
Where are peptide hormones made?
In many places. The hypothalamus and pituitary gland make several, including oxytocin and GHRH. The pancreas makes insulin and glucagon, the stomach makes ghrelin, and the gut, heart and fat tissue release others.
