Retatrutide for NASH: Off-Label Use and Current Evidence

What Is Retatrutide and Why Is It Being Studied for NASH?

Retatrutide is a once-weekly injectable peptide developed by Eli Lilly that simultaneously activates three receptors: the glucose-dependent insulinotropic polypeptide receptor (GIPR), the glucagon-like peptide-1 receptor (GLP-1R), and the glucagon receptor (GCGR). This triple mechanism distinguishes it from semaglutide (GLP-1R only) and tirzepatide (GIP/GLP-1R only). The glucagon receptor arm adds a direct hepatic energy expenditure signal that may be particularly relevant to fatty liver disease.

NASH (nonalcoholic steatohepatitis), now increasingly termed MASH (metabolic dysfunction-associated steatohepatitis) per updated nomenclature from the American Association for the Study of Liver Diseases, is defined by hepatic steatosis, lobular inflammation, and hepatocellular ballooning, with or without fibrosis. The 2023 AASLD nomenclature guidance states that the term MASH "better reflects the metabolic drivers of this disease" and is now the preferred clinical and research terminology [1].

Why Triple Agonism May Matter for Liver Fat

The glucagon receptor, when activated, increases hepatic fatty acid oxidation and suppresses hepatic lipogenesis. GLP-1R activation reduces hepatic glucose production and promotes weight loss that secondarily reduces liver fat. GIPR activation modulates adipose tissue lipolysis and may augment the metabolic effects of GLP-1.

Together, these three pathways address the main metabolic drivers of hepatic steatosis: excess substrate delivery from adipose tissue, insufficient hepatic fat oxidation, and insulin resistance. No other single approved agent targets all three simultaneously.

The Off-Label Status Problem

Retatrutide has no FDA approval for obesity, diabetes, or MASH as of January 2025. Every use outside a registered clinical trial is off-label and, in most cases, not covered by insurance. Physicians prescribing it compounded or sourced outside trial settings do so without Phase 3 histologic data, without an approved label, and without post-market safety surveillance.

What Does the Phase 2 Trial Data Actually Show?

The most important published data comes from a Phase 2 randomized controlled trial (NCT04881760), reported in the New England Journal of Medicine in July 2023, enrolling 338 adults with obesity (BMI 30-75) without type 2 diabetes [2]. The primary endpoint was percent change in body weight at 24 weeks. Liver fat was a secondary endpoint measured by MRI-based proton density fat fraction (MRI-PDFF).

Liver Fat Reduction Results

At 24 weeks, participants receiving retatrutide 12 mg showed a mean relative reduction in liver fat of 81.7% by MRI-PDFF, compared with 3.3% in the placebo arm (P<0.001) [2]. Participants receiving 4 mg showed a 55.5% relative reduction, and those receiving 8 mg showed a 72.5% relative reduction.

The absolute liver fat percentages dropped substantially. Baseline liver fat in the highest-quartile participants (those with steatosis, defined as MRI-PDFF 5% or higher) fell from approximately 18% to below 5% in a large proportion of the 12 mg group at 24 weeks.

These numbers are meaningful. For context, the STEP-1 trial of semaglutide 2.4 mg (N=1,961) produced 14.9% mean body weight loss at 68 weeks [3], and separate liver-focused semaglutide studies showed MRI-PDFF reductions of roughly 30-40% [4]. The retatrutide liver fat signal at 24 weeks is numerically larger than what semaglutide typically produces, though direct head-to-head trials do not yet exist.

Weight Loss and Metabolic Improvements

Body weight reduction reached 24.2% from baseline at 48 weeks in the 12 mg group [2]. Reductions in ALT, AST, fasting insulin, and triglycerides were also observed. ALT normalized (fell to below 40 U/L) in a greater proportion of the 12 mg group than placebo, suggesting reduced hepatocellular stress.

These are surrogate markers, not histologic endpoints. Liver biopsy data showing NASH resolution or fibrosis improvement have not been published from this trial.

What the Phase 2 Trial Cannot Tell Us

The 2023 NEJM publication did not include liver biopsies. It cannot confirm:

• NASH histologic resolution (as defined by NAS score reduction)
• Fibrosis stage improvement
• Reduced risk of cirrhosis or hepatocellular carcinoma
• Durability beyond 48 weeks

These gaps are clinically significant. MRI-PDFF reduction correlates with histologic improvement in many cases, but the correlation is imperfect. A drug can reduce liver fat measured by MRI without resolving the inflammation or ballooning that defines NASH.

How Does Retatrutide Compare to the Only FDA-Approved MASH Drug?

The FDA approved resmetirom (Rezdiffra, Madrigal Pharmaceuticals) in March 2024 specifically for MASH with moderate to advanced fibrosis (F2 or F3) in adults with a BMI of 27 or higher. This is the first and only drug with an FDA approval for MASH.

The MAESTRO-NASH Trial Benchmark

The MAESTRO-NASH trial (N=966) showed resmetirom 100 mg achieved NASH resolution without fibrosis worsening in 25.9% of participants vs. 9.7% on placebo at 52 weeks [5]. Fibrosis improvement of one or more stages occurred in 24.2% vs. 14.2% on placebo.

These are histologic endpoints from liver biopsies, the regulatory gold standard. Retatrutide has no comparable published data for NASH.

Why Some Clinicians Still Reach for Retatrutide Off-Label

Resmetirom costs approximately $47,400 per year at list price and requires a confirmed MASH diagnosis with fibrosis staging, typically requiring a liver biopsy or validated elastography result. Access is limited by payer criteria and specialist availability.

Retatrutide, sourced from compounding pharmacies, may cost less and is easier to obtain for some patients. The weight loss magnitude also exceeds resmetirom's profile, which matters because MASH is strongly linked to obesity and metabolic syndrome. A patient who loses 20-25% of body weight may see meaningful liver improvement regardless of the specific drug mechanism.

"may see improvement" is not the same as "has demonstrated improvement on biopsy." The off-label use of retatrutide for NASH substitutes a larger weight loss signal for the absence of histologic evidence, which is a pharmacologic argument, not a regulatory or safety-validated one.

The TRIUMPH Phase 3 Trial: What to Watch

Eli Lilly has initiated the TRIUMPH trial (NCT05929118), a Phase 3 program examining retatrutide in adults with MASH and liver fibrosis. This is the study that will generate histologic biopsy data needed for an FDA submission.

The trial uses two co-primary endpoints aligned with FDA guidance for MASH drug development:

1. NASH resolution (NAS score reduction with no worsening of fibrosis) at approximately week 48-52
2. Fibrosis improvement of one or more stages without worsening of NASH activity

If TRIUMPH results mirror the MRI-PDFF reductions seen in the Phase 2 obesity trial, retatrutide could emerge as a leading MASH therapy. But results are not expected before 2026-2027 based on standard Phase 3 timelines.

The clinical question for today is: should patients wait, or is the existing evidence sufficient to justify off-label use now?

Mechanism Deep Dive: Why Glucagon Receptor Activation Matters for the Liver

Most GLP-1 agents reduce liver fat primarily through weight loss, reduced caloric intake, and improved insulin sensitivity. The glucagon receptor adds a different and more direct mechanism.

Glucagon and Hepatic Fat Oxidation

Glucagon activates hepatic GCGR signaling, which upregulates AMP-activated protein kinase (AMPK) pathways and increases expression of genes involved in mitochondrial beta-oxidation. The net effect is greater hepatic fat burning rather than solely reduced fat delivery to the liver.

Animal model data and early human pharmacodynamic studies suggest that the GCGR component of retatrutide contributes roughly 20-30% of the total liver fat reduction independently of weight loss, though definitive human data parsing this contribution remain limited [6].

GLP-1R and Hepatic Insulin Sensitization

GLP-1 receptor activation in hepatocytes and indirectly via central and portal mechanisms reduces hepatic glucose production (hepatic gluconeogenesis) and improves insulin signaling. Insulin resistance at the liver level is a primary driver of de novo lipogenesis (DNL), meaning the liver produces fat from carbohydrates. Reducing hepatic insulin resistance lowers DNL and thereby reduces steatosis.

GIPR and Adipose-Liver Crosstalk

Visceral adipose tissue dysfunction drives free fatty acid flux to the liver via the portal circulation. GIPR agonism reduces visceral adiposity more effectively than GLP-1R agonism alone, based on tirzepatide vs. Semaglutide comparisons [7]. Less visceral fat means less portal free fatty acid delivery to the liver, which may reduce hepatic steatosis independently of central food intake reduction.

Safety Considerations for Off-Label NASH Use

The safety profile of retatrutide in NASH patients specifically has not been formally studied. The Phase 2 trial excluded patients with significant liver disease (ALT greater than three times the upper limit of normal in some cohorts), which means the safety data available does not directly apply to patients with active NASH and elevated liver enzymes.

GI Adverse Effects

Nausea, vomiting, and diarrhea were the most common adverse events in the Phase 2 trial, affecting 40-70% of participants in a dose-dependent manner during dose escalation [2]. These effects are typically manageable with slow titration but can be significant in patients already experiencing nausea from hepatic disease.

Gallbladder Risk

GLP-1 receptor agonists as a class carry a known signal for gallbladder disease. Cholelithiasis and cholecystitis rates were elevated in GLP-1 trial participants. Rapid weight loss with any agent, including retatrutide, increases the rate of gallstone formation. MASH patients may already have elevated gallstone risk due to metabolic syndrome and altered bile acid composition.

Cardiac Safety

No dedicated cardiovascular outcomes trial (CVOT) for retatrutide has been completed. The drug lacks the cardiovascular risk reduction evidence that semaglutide (SUSTAIN-6, SELECT trial) and liraglutide (LEADER trial) have accumulated [8].

Muscle Mass Preservation

All potent weight loss agents carry risk of lean mass loss. Whether retatrutide's glucagon component worsens or modulates this risk compared to GLP-1 monotherapy is an open question. Patients with advanced liver disease and sarcopenia risk warrant extra caution.

Current Evidence Grade and Clinical Positioning

Using the GRADE framework, the evidence for retatrutide in NASH is rated low:

• One Phase 2 RCT with liver fat as a secondary endpoint (not the primary outcome)
• No published Phase 3 histologic data
• No dedicated NASH safety cohort
• Surrogate endpoint (MRI-PDFF) rather than histologic resolution
• Short duration (24-48 weeks) relative to NASH natural history

The Grading of Recommendations Assessment, Development and Evaluation (GRADE) working group defines low-quality evidence as that where "further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate" [9].

This grading places retatrutide well below resmetirom for MASH treatment decisions based on current published evidence.

When Off-Label Use May Be Considered

Some hepatologists and obesity medicine specialists accept off-label retatrutide under specific clinical circumstances:

• The patient has MASH and significant obesity (BMI greater than 35) and requires both weight loss and liver disease management
• Resmetirom is inaccessible due to cost, insurance denial, or inability to obtain biopsy-based staging
• The patient has failed or not tolerated semaglutide or tirzepatide
• The prescriber has thoroughly informed the patient of the off-label and investigational nature of the treatment and documented this in the medical record
• Baseline liver function tests, imaging, and metabolic labs are obtained before starting and monitored at 12-week intervals

This is not an endorsement of off-label prescribing. The conditions above describe the narrowest reasonable clinical window, not a general recommendation.

What Guidelines Currently Say About MASH Pharmacotherapy

The 2023 AASLD Practice Guidance on MASH states that pharmacotherapy should be considered in patients with biopsy-proven MASH with fibrosis stage F2 or higher, or in those with F1 fibrosis plus additional risk factors for disease progression (diabetes, metabolic syndrome, or rapidly rising transaminases) [1].

The guidance recommends weight loss of 10% or more of body weight as the most effective lifestyle intervention for MASH resolution, citing evidence that 10% weight loss produces histologic improvement in approximately 90% of patients when achieved [1].

No triple agonist or retatrutide-specific recommendation exists in any current guideline from AASLD, EASL, or the American Gastroenterological Association, because the evidence base has not matured to that level.

The American Association of Clinical Endocrinology (AACE) 2023 Obesity Guidelines do not specifically address retatrutide for MASH, as the drug remains unapproved [10].

What Prescribers and Patients Should Document Before Starting

If a clinician decides to use retatrutide off-label for NASH after a thorough risk-benefit discussion, baseline documentation should include:

• Liver enzyme panel (ALT, AST, ALP, GGT, bilirubin, albumin, INR)
• Fibroscan or MRI-PDFF to quantify steatosis and fibrosis
• Liver biopsy if fibrosis stage is uncertain and the result would change management
• Fasting lipid panel, HbA1c, fasting glucose
• Abdominal ultrasound to evaluate gallbladder
• Documented informed consent noting investigational status, absence of Phase 3 histologic data, and the availability of the FDA-approved alternative (resmetirom)

Monitoring should occur at 12 weeks (labs), 24 weeks (labs plus repeat imaging if feasible), and 48 weeks minimum.