The short answer
There is no hormone called GLP-3. When people write “GLP-3” or “GLP-3 peptide” they almost always mean retatrutide (Eli Lilly's LY3437943), an investigational peptide that activates three metabolic receptors — GIP, GLP-1 and glucagon. The nickname is shorthand for “three receptors”, on the pattern of GLP-1 being the drug class everyone knows. It does not appear in any scientific paper, regulatory document or product name, and searching for it in PubMed returns nothing useful.
Where the name came from
The reasoning runs: semaglutide is a “GLP-1”, tirzepatide activates two receptors so it must be a “GLP-2”, and retatrutide activates three so it is a “GLP-3”. Each step is wrong. Tirzepatide is a GIP/GLP-1 dual agonist, not a “GLP-2”, and GLP-2 is a real, unrelated hormone. Retatrutide's backbone is not even derived from GLP-1 — like tirzepatide, it is built on GIP, the other incretin, with an exendin-4 tail. Its correct description is a GIP, GLP-1 and glucagon receptor triple agonist, sometimes written GGG tri-agonist.
The real proglucagon family
GLP-1 does have relatives, and the naming is where the confusion starts. A single precursor protein, proglucagon, is cut differently in different tissues to release several peptides. In the pancreas it yields glucagon; in the gut it yields GLP-1, GLP-2, oxyntomodulin and glicentin. GLP-2 is a 33-residue hormone that maintains the intestinal lining; its analogue teduglutide is an approved drug for short bowel syndrome and has nothing to do with obesity. GIP, the other incretin, comes from a different gene entirely. Nobody has ever isolated a “GLP-3” because proglucagon does not contain one.
| Peptide | Origin | Length | Role | Drug examples |
|---|---|---|---|---|
| Glucagon | Proglucagon (pancreas) | 29 aa | Raises glucose; energy expenditure | Glucagon rescue kits; GCGR component of retatrutide, survodutide, mazdutide |
| GLP-1 | Proglucagon (gut L-cells) | 30–31 aa | Incretin: insulin, satiety, gastric emptying | Liraglutide, semaglutide, dulaglutide |
| GLP-2 | Proglucagon (gut L-cells) | 33 aa | Intestinal growth and repair | Teduglutide (short bowel syndrome) |
| Oxyntomodulin | Proglucagon (gut) | 37 aa | Weak GLP-1R + GCGR agonist | Scaffold for mazdutide |
| GIP | Separate gene (gut K-cells) | 42 aa | Incretin: insulin, adipose signalling | Scaffold for tirzepatide and retatrutide |
| “GLP-3” | — | — | Does not exist | Nickname for retatrutide |
What retatrutide actually is
Retatrutide is a synthetic 39-amino-acid peptide (C221H342N46O68, about 4,731 Da) engineered from GIP with an exendin-4-derived C-terminal tail, three non-canonical residues and a C20 fatty diacid on lysine 17 that binds albumin for a roughly 6-day half-life. Its three receptor activities are balanced so the incretin effects dominate and glucose falls despite the glucagon component. In phase 2 it produced a mean weight change of −24.2% at 48 weeks; phase 3 trials are running and it is not approved anywhere as of September 2026. The full profile, structure block and trial tables are on the retatrutide page.
Why the nickname matters
Names shape searches, and searches shape what people find. “GLP-3” returns forums and sellers, not literature, because the literature never uses it. It also implies a lineage — a next-generation GLP-1 — that misdescribes the molecule's origin and mechanism, and it collides with GLP-2, a real hormone whose drug treats a completely different condition. Anyone reading a claim about “GLP-3” should mentally substitute “retatrutide, a triple agonist” and then check whether the claim survives contact with the phase 2 and phase 3 reports.
How to search the literature instead
Use “retatrutide” or “LY3437943”; the key papers are Jastreboff et al. (NEJM 2023, phase 2 obesity), Rosenstock et al. (Lancet 2023, phase 2 type 2 diabetes) and Bajaj et al. (Lancet 2026, phase 3 TRANSCEND-T2D-1). For the mechanism class, search “triple agonist”, “GIP GLP-1 glucagon receptor agonist” or “tri-agonist”. For the receptor biology, the GLP-1 vs GIP vs glucagon guide on this site walks through what each pathway contributes.
Sources and what they cover
These primary studies, official records and standards cover the topics linked below. The scope notes distinguish laboratory findings, human trials and regulatory records. This is a selected bibliography, not a systematic review or verification of every sentence; absence of a study here does not prove that no study exists.
- Li et al. (2024) — Structural basis of retatrutide triple-receptor agonism
Structural receptor research; biological plausibility is separate from proven clinical benefit.
Relevant sections: What retatrutide actually is · Why the nickname matters
- Jastreboff et al. (2023) — Retatrutide for obesity, phase 2 trial
Phase 2 obesity trial: studied regimens, weight and adverse events. Trial doses are not instructions for personal use.
Relevant sections: What retatrutide actually is · How to search the literature instead
- IUPAC Gold Book — peptides
Chemical terminology; no evidence of treatment effectiveness.
Relevant sections: The short answer
FAQ
What is GLP-3?+
Nothing — no hormone by that name exists. It is an internet nickname for retatrutide, an investigational peptide that activates the GIP, GLP-1 and glucagon receptors (a triple agonist).
Is retatrutide a GLP-3 peptide?+
No. Retatrutide is a GIP-based peptide that activates three receptors. The scientific term is GIP/GLP-1/glucagon receptor triple agonist; “GLP-3” appears in no paper or regulatory document.
Is tirzepatide GLP-2?+
No. Tirzepatide is a dual GIP/GLP-1 receptor agonist. GLP-2 is a separate, real gut hormone that maintains the intestinal lining; its drug analogue teduglutide treats short bowel syndrome.
What is the difference between GLP-1 and GLP-2?+
Both come from the same precursor, proglucagon, but GLP-1 is an incretin that boosts insulin and reduces appetite, while GLP-2 promotes intestinal growth and repair. They act on different receptors.
Is GLP-3 approved?+
Since it means retatrutide: no. Retatrutide is in phase 3 trials and not approved anywhere as of September 2026.