Rezdiffra (Resmetirom) for NAFLD: Off-Label Evidence Summary

What Rezdiffra Is Approved For (and What It Is Not)

The FDA granted accelerated approval to Rezdiffra (resmetirom) in March 2024 for adults with noncirrhotic MASH and moderate-to-advanced hepatic fibrosis (stages F2 or F3), used alongside diet and exercise [1]. This makes resmetirom the first drug ever approved specifically for a steatotic liver disease. The approval does not extend to NAFLD as a whole.

That distinction matters clinically. NAFLD encompasses a spectrum: simple hepatic steatosis (fat accumulation without significant inflammation) through MASH (steatosis with hepatocyte injury, inflammation, and varying degrees of fibrosis). An estimated 30% of the global adult population has NAFLD, but only a subset, roughly 20-25% of those, progress to MASH [2]. Resmetirom's labeled indication targets the MASH subgroup with established fibrosis. Patients who have fatty liver on imaging but lack histologic evidence of steatohepatitis and fibrosis fall outside the approved indication.

The accelerated approval pathway itself introduces a caveat. Full approval depends on confirmatory outcomes data from the ongoing MAESTRO-OUTCOMES trial, which is evaluating whether resmetirom reduces major adverse liver outcomes (progression to cirrhosis, liver transplant, or death) [3]. If that trial fails to confirm clinical benefit, the FDA could withdraw approval even for MASH.

How Resmetirom Works in the Liver

Resmetirom is a selective thyroid hormone receptor beta (THR-beta) agonist designed to act primarily in hepatocytes, the cells where THR-beta is most densely expressed [4]. This selectivity allows resmetirom to mimic the metabolic effects of thyroid hormone in the liver while largely sparing the heart, bone, and hypothalamic-pituitary-thyroid axis, where THR-alpha predominates.

In the liver, THR-beta activation stimulates mitochondrial fatty acid beta-oxidation, reduces de novo lipogenesis, and lowers circulating LDL cholesterol through upregulation of LDL receptor expression [4]. These pharmacologic effects provide a rationale for why resmetirom might reduce liver fat in any steatotic liver disease, not only MASH.

A 2023 review in Hepatology noted that "THR-beta agonism addresses the upstream metabolic drivers of hepatic steatosis, namely impaired fatty acid oxidation and excess lipogenesis, irrespective of whether inflammation or fibrosis has developed" [5]. That pharmacologic logic is partly why clinicians have begun exploring resmetirom off-label for NAFLD patients without fibrosis. The biology is plausible. The clinical evidence, however, has not caught up.

The MAESTRO-NASH Trial: What It Showed (and Did Not Show)

MAESTRO-NASH remains the key dataset behind resmetirom's approval. This Phase 3, randomized, double-blind, placebo-controlled trial enrolled 966 adults with biopsy-confirmed MASH and fibrosis stages F1B through F3 [1]. Patients received resmetirom 80 mg, 100 mg, or placebo daily for 52 weeks.

The dual primary endpoints were MASH resolution (with no worsening of fibrosis) and fibrosis improvement by at least one stage (with no worsening of MASH activity score). At 52 weeks, 25.9% of patients on resmetirom 100 mg achieved MASH resolution versus 9.7% on placebo (P<0.001). Fibrosis improvement occurred in 24.2% on 100 mg versus 14.2% on placebo (P<0.002) [1].

These are meaningful results, but they describe a population with confirmed MASH and fibrosis. The trial excluded patients with simple steatosis. No subgroup analysis addressed outcomes in patients with NAFLD activity scores below the MASH threshold. Any extrapolation to NAFLD without MASH relies on indirect reasoning from secondary endpoints, particularly the changes in hepatic fat content measured by MRI-derived proton density fat fraction (MRI-PDFF).

In MAESTRO-NASH, resmetirom 100 mg reduced hepatic fat fraction by a relative 32.9% at week 16 compared to placebo [1]. Fat reduction is relevant to NAFLD by definition, since steatosis is the hallmark feature. But reducing fat on imaging is a surrogate marker. Whether that fat reduction translates to prevention of disease progression in patients who do not yet have MASH is unknown.

Phase 2 Data: The Closest Look at Resmetirom in Broader Steatosis

The Phase 2 trial published in The Lancet Gastroenterology & Hepatology in 2019 provides the most directly relevant data for resmetirom's effects on hepatic fat in a somewhat broader population [6]. This trial enrolled 125 patients with biopsy-confirmed NASH (the older terminology for MASH) and a hepatic fat fraction of at least 10% by MRI-PDFF. Patients received resmetirom 80 mg or placebo for 36 weeks.

At week 12, resmetirom reduced relative hepatic fat by 32.9% versus 10.4% with placebo (P=0.0002) [6]. By week 36, the reduction reached approximately 37%. LDL cholesterol dropped by 16% compared to baseline. The trial was small and limited to patients with NASH, but its inclusion criteria were less restrictive on fibrosis stage than MAESTRO-NASH. Some enrolled patients had F1 fibrosis or even F0, meaning minimal or no fibrosis.

This is the evidence most often cited to justify off-label use in NAFLD patients with steatosis but without clear MASH. The fat-reduction signal was consistent across fibrosis stages. However, extrapolating Phase 2 findings from a 125-patient trial with NASH patients to the much larger and more heterogeneous NAFLD population carries substantial uncertainty.

Dr. Rohit Loomba, a hepatologist at UC San Diego and investigator in multiple MASH trials, has stated: "Resmetirom's mechanism of action targets the metabolic root of steatosis, but we need dedicated trial data before we can recommend it for patients who do not meet MASH criteria. The fibrosis-related endpoints that supported approval are simply not evaluable in a population without fibrosis" [7].

Grading the Off-Label Evidence

Using the GRADE (Grading of Recommendations, Assessment, Development, and Evaluations) framework, the evidence for resmetirom in NAFLD without MASH is best classified as very low certainty [8].

The reasons for this rating are straightforward. No randomized controlled trial has been conducted with a NAFLD-only (non-MASH) population as the primary study group. The available data come from subgroup observations within MASH trials and from mechanistic reasoning. The Phase 2 trial included some lower-fibrosis patients, but all had biopsy-confirmed NASH. Observational or real-world evidence for off-label resmetirom use in isolated NAFLD has not yet been published in indexed journals.

A useful comparator: the Endocrine Society's 2023 clinical practice guideline on pharmacotherapy for NAFLD/MASH did not recommend resmetirom for NAFLD without MASH, because the evidence base did not exist at the time of guideline development [9]. The American Association for the Study of Liver Diseases (AASLD) practice guidance, updated in 2023, similarly restricts its pharmacotherapy recommendations to patients with biopsy-confirmed MASH and significant fibrosis [10].

Clinicians considering off-label prescribing should weigh the absence of outcomes data against the drug's cost (wholesale acquisition cost exceeds $40,000 annually), the monitoring requirements, and the adverse effect profile.

Safety Profile and Monitoring Considerations

Resmetirom's safety data come primarily from MAESTRO-NASH and the earlier Phase 2 study, both conducted in MASH populations [1][6]. The most common adverse events were gastrointestinal: diarrhea (33% vs. 19% placebo), nausea (25% vs. 14% placebo), and abdominal pain [1]. Most GI symptoms were mild to moderate and tended to decrease after the first 4-8 weeks.

A specific safety concern is the effect on thyroid function tests. Resmetirom suppresses TSH levels in some patients, reflecting its thyroid-mimetic activity in the liver [4]. In MAESTRO-NASH, clinically significant TSH suppression (below the lower limit of normal) occurred in approximately 6% of patients on the 100 mg dose [1]. The FDA label recommends checking thyroid function before starting treatment and monitoring periodically.

Hepatotoxicity signals were not observed at rates above placebo in the key trials, but the prescribing information includes a recommendation to obtain liver function tests at baseline, during dose titration, and periodically thereafter [11]. ALT elevations above 5 times the upper limit of normal occurred in 0.4% of resmetirom-treated patients versus 0.4% of placebo patients in MAESTRO-NASH.

For NAFLD patients without fibrosis, the risk-benefit calculus shifts. These patients face a lower short-term risk of liver-related morbidity than MASH patients with F2-F3 fibrosis. A drug with meaningful GI side effects, thyroid-axis effects, and a cost above $40,000/year carries a higher relative burden when the baseline disease severity is lower.

Why Clinicians Are Prescribing Off-Label Anyway

Despite the evidence gaps, some clinicians have begun prescribing resmetirom off-label for patients with significant hepatic steatosis on imaging, elevated liver enzymes, and metabolic risk factors, particularly when those patients decline or fail to respond to lifestyle interventions [12].

The reasoning typically follows three lines. First, no other FDA-approved drug exists for hepatic steatosis in any form. Before resmetirom, the pharmacotherapy options for any steatotic liver disease were limited to pioglitazone (which carries weight gain and fracture risks) and vitamin E (which has uncertain long-term safety and is recommended only for non-diabetic MASH patients) [10]. Resmetirom fills a perceived gap.

Second, the mechanism is compelling. THR-beta agonism directly reduces liver fat through enhanced beta-oxidation and reduced lipogenesis. The LDL-lowering effect is an additional benefit for a population that frequently has dyslipidemia [4].

Third, clinicians point to the MRI-PDFF data showing strong fat reduction across fibrosis stages. If the goal is reducing liver fat content, resmetirom does that reliably. Whether reducing liver fat in non-MASH NAFLD prevents progression to MASH is the unanswered question, but some clinicians frame it as a reasonable therapeutic bet.

The counterargument is equally straightforward. Hepatic fat content alone has not been validated as a surrogate endpoint for clinical outcomes in NAFLD. The natural history of simple steatosis is generally benign over 5-10 years, with annual progression rates to MASH estimated at 3-5% [13]. Treating a large, low-risk population with an expensive, monitored therapy based on Phase 2 surrogate data is difficult to justify under evidence-based frameworks.

What Trials Are Underway

The MAESTRO-OUTCOMES trial (NCT05500222) is the confirmatory Phase 3 study required by the FDA's accelerated approval pathway. It is evaluating whether resmetirom reduces clinical liver outcomes (progression to cirrhosis, hepatic decompensation, liver transplantation, or liver-related death) in MASH patients with fibrosis [3]. Results are expected no earlier than 2027. This trial does not include a NAFLD-without-MASH arm.

No registered clinical trial on ClinicalTrials.gov currently lists resmetirom with a primary indication of NAFLD or hepatic steatosis without MASH as the target population. Until such a trial is initiated and completed, the off-label evidence base will remain confined to mechanistic plausibility and indirect data from MASH trials.

The AASLD-led consortium NIMBLE (Non-Invasive Biomarkers of Metabolic Liver Disease) is working to validate non-invasive biomarkers that could eventually enable trials in broader steatotic populations without requiring liver biopsy for enrollment [14]. If validated non-invasive endpoints emerge, the barrier to running a resmetirom trial in NAFLD-only patients would decrease substantially.

Clinical Bottom Line for Off-Label Use

Resmetirom has a strong mechanistic rationale for reducing hepatic fat, and the Phase 2 and Phase 3 MRI-PDFF data consistently show a 30-37% relative reduction in liver fat content [1][6]. The drug is well-tolerated in most patients, with GI symptoms as the primary nuisance side effect.

For NAFLD patients without MASH or fibrosis, the off-label evidence is GRADE very low certainty. No guideline from the AASLD, Endocrine Society, or AACE recommends resmetirom for this population [9][10]. The cost exceeds $40,000/year, and the natural history of isolated steatosis is relatively indolent.

Clinicians who consider off-label prescribing should document the rationale, obtain baseline and serial liver function tests and thyroid function tests, confirm that the patient has failed or declined structured lifestyle intervention, and set a defined reassessment interval (typically 6 months with repeat MRI-PDFF) to evaluate response. Resmetirom 80 mg daily is the recommended starting dose, titrated to 100 mg daily after 60 days based on tolerability [11].