Retatrutide (LY3437943) is a synthetic, fatty-acid-conjugated long-acting peptide developed by Eli Lilly as the first-in-class triple agonist of the GIP, GLP-1 and glucagon receptors. Where dual agonists such as Tirzepatide drive weight management primarily through appetite and insulin pathways, the addition of glucagon receptor agonism in Retatrutide is studied as a mechanism for increasing resting energy expenditure — making it the most metabolically active research compound in the incretin class to date.
Retatrutide is a single peptide molecule engineered to simultaneously bind and activate three structurally related class B G-protein-coupled receptors: the GLP-1 receptor (GLP-1R), the glucose-dependent insulinotropic polypeptide receptor (GIPR) and the glucagon receptor (GCGR). Each receptor sits at a different node of the energy-balance regulatory network, and the combined agonism produces a metabolic phenotype that is qualitatively distinct from any single- or dual-agonist research compound studied to date.
The GLP-1 arm reproduces the satiety, gastric emptying and glucose-dependent insulin secretion effects characterised in decades of incretin research. The GIP arm contributes complementary insulinotropic activity and is studied for direct effects on adipose tissue metabolism. The glucagon arm is the differentiating feature — glucagon receptor activation drives hepatic glycogenolysis, lipolysis and a documented increase in resting energy expenditure. In short, Retatrutide is studied as a research compound that reduces energy intake (GLP-1) while simultaneously increasing energy expenditure (glucagon) — a pharmacological combination not achievable with monotherapy.
The molecule itself is a fatty-acid-conjugated long-acting peptide with a half-life of approximately six days, supporting once-weekly dosing in published Phase 1 and Phase 2 research protocols. Receptor-binding affinity has been tuned across all three targets to minimise off-target effects while preserving the metabolic synergy.
The Phase 2 TRIUMPH-1 trial (Jastreboff et al., New England Journal of Medicine, 2023) studied Retatrutide in adults with obesity over 48 weeks across multiple dose cohorts. At the 12 mg dose, mean body-weight reduction reached approximately 24.2% — the largest weight-loss effect observed in any incretin-class compound to date. Dose-response was clean and continued through the 48-week observation period without evidence of plateau, suggesting the molecule's full effect window had not yet been reached.
A parallel Phase 2 trial in adults with metabolic dysfunction-associated steatohepatitis (MASH) reported substantial reductions in hepatic fat content alongside the weight-management effect. The MASH programme is now a major branch of Retatrutide's Phase 3 development. Cardiovascular outcome trials (TRIUMPH-Outcomes) are also active. As of 2025, Retatrutide remains an unapproved investigational compound — not authorised for any clinical use outside trial protocols.
The 48-week TRIUMPH-1 readout documented a continued downward trajectory in body weight at all doses, with the 12 mg cohort still actively losing weight at study end. Reported reductions in waist circumference, blood pressure, triglycerides and HbA1c tracked alongside the weight effect. DXA substudies have been used to characterise the fat-mass vs lean-mass change profile across the dose range.
Retatrutide's energy-expenditure arm is studied as the mechanistic basis for its superior weight-loss magnitude compared with selective GLP-1 (Semaglutide) and dual GIP/GLP-1 (Tirzepatide) agonists in cross-trial comparisons.
In adults with MASH (Phase 2, 2024), Retatrutide reduced liver fat content by clinically meaningful margins within 24 weeks across dose cohorts. The triple-agonist mechanism is hypothesised to act on hepatic steatosis via combined appetite reduction, improved insulin sensitivity, and direct glucagon-mediated hepatic lipid mobilisation.
Glycaemic-control endpoints in adults with type 2 diabetes are being studied in the parallel TRIUMPH-T2D programme. Early Phase 1/2 data showed substantial HbA1c reductions with a tolerability profile dominated by transient gastrointestinal effects typical of the incretin class.