How Retatrutide Works: The Science of GLP-1, GIP and Glucagon Receptor Activation

Important: investigational medicine notice. Retatrutide is an investigational medication. It has not received marketing authorisation from the Medicines and Healthcare products Regulatory Agency (MHRA) or any other regulator, and it cannot be legally prescribed, dispensed or sold in the UK outside of an authorised clinical trial. This article is for information only. It is not medical advice and is not a recommendation to seek, request or use retatrutide. If you are considering weight-management medication, speak to your GP or a regulated UK prescriber about MHRA-approved options.

Retatrutide is described in the published clinical literature as a "triple hormone receptor agonist". That description is technically accurate but, on its own, not very illuminating. This article unpacks what it actually means: which three hormones retatrutide mimics, what each one does in the body, and why combining all three in a single molecule produced the magnitude of weight reduction reported in the published trials.

This is one of the more clinical articles in this series, but it is written for a motivated reader rather than a specialist. If you are new to the topic, the previous article in this series — What is Retatrutide? — gives a higher-level overview and a useful starting point.

The three hormones retatrutide acts on

Retatrutide is a single molecule that activates three different hormone receptors in the body. Each of these hormones plays a role in how the body manages hunger, blood sugar and the storage or release of energy.

GLP-1 (glucagon-like peptide-1)

GLP-1 is a hormone released by cells in the small intestine after a meal. It does several things at once. It tells the pancreas to release more insulin, which helps move glucose out of the blood and into cells. It tells the pancreas to release less glucagon, which prevents the liver from pumping extra glucose into the blood. It slows the rate at which the stomach empties, so food stays in the stomach longer. And it acts on the brain to signal fullness, reducing the drive to eat.

GLP-1 is also the hormone mimicked by semaglutide (the active ingredient in Wegovy and Ozempic) and is one of the two hormones mimicked by tirzepatide (Mounjaro). It is the foundational hormone class behind the current generation of weight-management medicines.

GIP (glucose-dependent insulinotropic polypeptide)

GIP is also released by the intestine after a meal. Like GLP-1, it stimulates insulin release from the pancreas — but only when blood glucose is elevated, which is what "glucose-dependent" in the name refers to. GIP also has effects on fat tissue: the precise role here remains an area of active research, but it appears to influence how fat cells store and release energy, and how the body responds to insulin overall.

GIP is the second hormone targeted by tirzepatide. The combination of GLP-1 and GIP activation in tirzepatide is part of why it produces greater weight reduction than semaglutide in head-to-head and parallel trials.

Glucagon

Glucagon is the third hormone, and the one that makes retatrutide distinctive among the medicines being developed in this space. Glucagon is best known for the opposite of what we usually associate with weight management: it tells the liver to release glucose into the blood, which raises blood sugar. Historically it was thought of mainly as the "anti-insulin" hormone.

What more recent research has clarified is that glucagon also plays a role in energy expenditure. It is associated with increased resting metabolic rate, increased breakdown of stored fat (lipolysis), and increased burning of fat in the liver. The published Phase 2a trial of retatrutide in metabolic dysfunction-associated steatotic liver disease, published in Nature Medicine in 2024, reported reductions in liver fat of up to approximately 82% in the highest-dose group — a magnitude of effect that the authors attributed in part to glucagon receptor activation.

No medicine currently approved by the MHRA for weight management targets the glucagon receptor. Retatrutide is the first triple agonist of this class to reach Phase 3 clinical trials.

Why combine all three?

The hypothesis underpinning retatrutide, as set out in the published clinical literature, is that activating GLP-1, GIP and glucagon receptors together produces effects that none of them produces in isolation — and that the combination addresses obesity from several angles at once.

GLP-1 reduces appetite and slows gastric emptying, so less food is eaten and less is absorbed. GIP adds metabolic and insulin-related effects on top of that. Glucagon adds an increase in energy expenditure and fat oxidation. In theory, this means the body is simultaneously taking in less energy and burning more — a combination that single-receptor and dual-receptor medicines do not deliver to the same degree.

Whether this hypothesis fully holds up in long-term Phase 3 data, across larger and more diverse populations, is what the ongoing TRIUMPH programme is designed to answer. The published Phase 2 trial in the New England Journal of Medicine (Jastreboff et al., 2023) reported mean weight reductions of up to 24.2% at 48 weeks on the 12 mg dose, accompanied by improvements in blood pressure, lipids, glycated haemoglobin and fasting glucose. The Phase 3 TRIUMPH-4 topline reported by Eli Lilly in December 2025 showed mean reductions of 28.7% at 68 weeks on the 12 mg dose. These numbers exceed what has been reported for currently approved medicines, although direct head-to-head comparisons are limited.

How retatrutide is administered

In all published and reported clinical trials to date, retatrutide is given as a once-weekly subcutaneous injection — that is, an injection under the skin, similar in administration route to existing GLP-1 medicines like Wegovy and Mounjaro. Trial participants started at a low dose (2 mg once weekly) and were titrated upwards over several weeks. The TRIUMPH programme has tested five dose levels: 2 mg, 4 mg, 6 mg, 9 mg and 12 mg.

This titration approach — starting low and increasing over time — is standard practice with incretin-based medicines because the gastrointestinal side effects (nausea, diarrhoea, constipation, vomiting) tend to be worst when these medicines are introduced too quickly. Dose escalation gives the body time to adapt.

There is no UK Summary of Product Characteristics (SmPC) for retatrutide because no UK marketing authorisation exists. The dose and titration schedule that would apply if and when the medicine is approved in the UK is not yet determined; the doses listed above are trial protocols only.

What happens after the injection

Once injected, retatrutide circulates in the bloodstream and binds to its three target receptors wherever they are expressed — in the pancreas, in the gut, in the brain, in fat tissue, and in the liver. The molecule is designed to have a long half-life, which is what allows for once-weekly rather than daily dosing. As receptor activation occurs over the days following each injection, the cumulative effects on appetite, blood sugar and energy expenditure build up.

The clinical effects participants experience reflect this multi-system action. Reduced appetite and earlier feelings of fullness are typically the first noticeable effects, often within the first few weeks. Weight reduction is gradual and tends to accumulate over months rather than appearing all at once. Improvements in cardiometabolic measures — blood pressure, cholesterol, HbA1c — typically follow as overall weight reduces.

What this tells us about future medicines in this class

Retatrutide sits at the leading edge of a clear progression in obesity pharmacotherapy. Single-agonist GLP-1 medicines (semaglutide, liraglutide) target one receptor and produce 10–15% mean weight reduction in their Phase 3 trials. Dual-agonist GLP-1/GIP medicines (tirzepatide) target two receptors and produce around 20–22%. Retatrutide targets three receptors and has, in its published Phase 2 and Phase 3 data, reported still-larger reductions.

Whether further combinations or different receptor targeting will push outcomes further is an open scientific question. What is clear is that the era of single-receptor weight-management medicines has been succeeded, in pharmaceutical development pipelines, by an era of multi-receptor agonists. Retatrutide is the most advanced medicine in this newer category to reach Phase 3 trials in obesity.

The bottom line

Retatrutide activates three hormone receptors simultaneously: GLP-1 (appetite and fullness), GIP (insulin response and fat-tissue effects), and glucagon (energy expenditure and fat oxidation). This triple action is what differentiates it mechanistically from MHRA-approved medicines like Wegovy (GLP-1 alone) and Mounjaro (GLP-1 + GIP). The combination has, in published trials, produced larger mean weight reductions than any single-receptor or dual-receptor medicine — but the medicine remains investigational, is not approved in the UK, and the full long-term picture across all of Phase 3 is still emerging.

If you are considering weight-management medication, the place to begin is a conversation with your GP or a regulated UK prescriber about the medicines they can legally prescribe in the UK today.

Sources and further reading

Jastreboff AM, Kaplan LM, Frías JP, et al. Triple-Hormone-Receptor Agonist Retatrutide for Obesity — A Phase 2 Trial. New England Journal of Medicine. 2023;389(6):514–526. DOI: 10.1056/NEJMoa2301972.
Sanyal AJ, Kaplan LM, Frias JP, et al. Triple hormone receptor agonist retatrutide for metabolic dysfunction-associated steatotic liver disease: a randomized phase 2a trial. Nature Medicine. 2024.
Eli Lilly investor release, 11 December 2025: Lilly's triple agonist, retatrutide, delivered weight loss of up to an average of 71.2 lbs along with substantial relief from osteoarthritis pain in first successful Phase 3 trial (TRIUMPH-4 topline).
UK Government / MHRA: GLP-1 medicines for weight loss and diabetes: what you need to know (GOV.UK publication).
NICE technology appraisal TA875 (semaglutide 2.4 mg for managing overweight and obesity).
NICE technology appraisal TA1026 (tirzepatide for managing overweight and obesity).