Rezdiffra (Resmetirom) in Adults 65 and Older: What the Evidence Actually Shows

What FDA Approval Covers and Where "Off-Label" Begins for Older Patients

Resmetirom received FDA approval on March 14, 2024, making it the first drug approved specifically for metabolic dysfunction-associated steatohepatitis (MASH) with liver fibrosis [1]. The approval covers adults with noncirrhotic MASH and stage F2 or F3 fibrosis, based on the MAESTRO-NASH phase 3 trial [2].

The label does not restrict use by age. A clinician prescribing resmetirom to a 67-year-old with biopsy-confirmed F2 MASH is prescribing on-label. The "off-label" framing in geriatric discussions arises from two realities: the trial enrolled proportionally fewer older adults, and the prescribing information does not include age-stratified dosing guidance or specific geriatric warnings beyond general polypharmacy caution [1].

Why Age 65 Is a Meaningful Clinical Threshold

Physiologic changes accumulating after 65 directly affect how resmetirom behaves in the body. Hepatic blood flow decreases by roughly 40% between ages 25 and 65 [3]. Plasma albumin falls modestly, altering protein binding of highly bound drugs. Renal clearance declines at approximately 1% per year after age 40, affecting metabolite excretion [4].

Resmetirom is metabolized primarily via CYP2C8 and undergoes hepatic uptake through OATP1B1 and OATP1B3 transporters [1]. Reduced hepatic mass and transporter expression in older adults could theoretically increase systemic exposure, though the FDA label does not mandate dose reduction based on age alone.

What the MAESTRO-NASH Data Say About Older Subgroups

The MAESTRO-NASH trial (N=966 randomized, mean age approximately 56 years) did not pre-specify age ≥65 as a primary subgroup analysis [2]. The published New England Journal of Medicine report by Harrison et al. (2024) noted that baseline demographics included patients with type 2 diabetes (67%), hypertension (57%), and dyslipidemia (75%), a profile increasingly common in older adults [2].

Post-hoc subgroup analyses presented at liver disease conferences suggest the direction of effect in older participants was consistent with the overall trial result, though confidence intervals widen substantially when the subgroup is small. No statistically significant age-by-treatment interaction has been published as of mid-2025.

Pharmacokinetics in Older Adults: What Changes and What Probably Does Not

Resmetirom's pharmacokinetic profile is worth examining systematically in the context of aging. The drug reaches peak plasma concentration roughly 4 hours after an oral dose, has a half-life of approximately 5 hours for parent compound, and produces active metabolites with longer half-lives [1].

Hepatic Extraction and First-Pass Effects

Because resmetirom relies on OATP1B1-mediated hepatic uptake for its target-organ delivery, age-related reductions in transporter expression matter. A 2021 analysis in Clinical Pharmacology and Therapeutics found OATP1B1 protein expression declines by approximately 20-30% in livers older than 65 years compared with younger donors [5]. This decline could reduce hepatic drug concentrations while paradoxically increasing systemic (non-hepatic) exposure, potentially widening the gap between intended THR-β liver activation and off-target thyroid-receptor alpha effects elsewhere.

Muscle Mass and Sarcopenia Risk

THR-β agonism is largely liver-selective, but incomplete selectivity means some skeletal muscle THR signaling may occur. Thyroid hormone excess accelerates muscle protein catabolism [6]. In older adults already at risk for sarcopenia (estimated prevalence 10-27% in community-dwelling adults over 65 by EWGSOP2 criteria) [7], even modest increases in muscle protein turnover warrant monitoring.

The MAESTRO-NASH trial reported small decreases in lean body mass in the resmetirom arms compared with placebo over 52 weeks, though the absolute differences were modest and the clinical significance in younger trial participants was considered minimal [2]. Whether those differences compound meaningfully in a 72-year-old with pre-existing low muscle mass is not established by current trial data.

Renal Function and Metabolite Clearance

The FDA label states that no dose adjustment is required for mild-to-moderate renal impairment, and limited data exist for severe impairment [1]. Since approximately 38% of adults over 65 have an eGFR <60 mL/min/1.73 m² [4], this data gap is clinically relevant. The prescribing information advises caution and closer monitoring when eGFR falls below 30 mL/min/1.73 m².

Drug Interactions: A Heightened Concern After 65

Polypharmacy is the rule, not the exception, in patients over 65. A 2019 CDC analysis found that 42% of adults aged 65-79 took five or more prescription medications simultaneously [8]. Resmetirom carries several interaction risks that become more consequential in this context.

Statin Interactions via OATP1B1

Resmetirom inhibits OATP1B1 and OATP1B3, the same hepatic transporters responsible for statin uptake [1]. Co-administration with rosuvastatin increases rosuvastatin AUC by approximately 100%, and simvastatin AUC increases substantially as well [1]. The FDA label recommends limiting rosuvastatin to 20 mg daily and simvastatin to 20 mg daily when given with resmetirom [1].

Given that statin use in adults over 65 with MASH-associated dyslipidemia is common, this interaction requires proactive dose adjustment rather than a post-hoc response to adverse effects. The American Heart Association 2019 guidelines already recommend moderate-intensity statins as first-line in most adults over 75, which partially mitigates this concern, but the interaction is pharmacokinetic and dose-dependent regardless of age [9].

Cyclosporine and Strong CYP2C8 Inhibitors

Cyclosporine, an OATP inhibitor, is contraindicated with resmetirom [1]. Strong CYP2C8 inhibitors such as gemfibrozil can increase resmetirom AUC markedly; the label contraindicates co-administration [1]. Older patients with mixed hyperlipidemia sometimes receive fibrates; a medication reconciliation step is non-negotiable before initiating resmetirom in any patient over 65.

Anticoagulants and Thyroid Monitoring

Thyroid hormone affects warfarin metabolism. Although resmetirom's systemic thyroid effects are attenuated by its beta-selectivity, the prescribing information notes that INR should be monitored more frequently when resmetirom is initiated or discontinued in patients on warfarin [1]. Anticoagulant use is prevalent in older adults with atrial fibrillation, estimated at 33.5% of adults over 65 with diagnosed AF [10].

Dosing in Patients Over 65: The 80 mg vs. 100 mg Question

The approved doses are 80 mg and 100 mg once daily with food. The label does not specify which starting dose is preferred in older adults. In the MAESTRO-NASH trial, both doses produced nearly identical MASH resolution rates (35.8% vs. 36.0%), while the 100 mg dose produced marginally higher fibrosis improvement (25.9% vs. 24.2%) [2].

A clinically reasonable approach in patients over 65, based on available pharmacokinetic data and the conservative prescribing principles outlined in the American Geriatrics Society Beers Criteria update [11], would begin with 80 mg and reassess hepatic biomarkers (ALT, AST, GGT) and lean body mass at 12 weeks before any consideration of escalation. This is not a label-endorsed protocol but reflects the "start low, go slow" principle applied to a drug with transporter-mediated interactions and modest muscle-metabolism signals.

Liver Biomarker Targets in Older Adults

The MAESTRO-NASH trial demonstrated significant LDL-C reductions: approximately 15.7% with 80 mg and 19.4% with 100 mg at 52 weeks [2]. In older adults already on statins with dose-adjusted regimens per the OATP interaction, lipid monitoring takes on added complexity. Baseline and 12-week lipid panels help distinguish resmetirom's pharmacodynamic LDL effect from statin-dose-reduction effects operating simultaneously.

ALT normalization occurred in 39.4% of 80 mg patients and 40.6% of 100 mg patients versus 16.4% placebo at 52 weeks [2]. These thresholds remain valid monitoring targets in older patients with no known differential response.

Safety Profile: Adverse Events Most Relevant to Geriatric Patients

The most common adverse events in MAESTRO-NASH were nausea (26.2% with 100 mg vs. 10.5% placebo) and diarrhea (28.6% with 100 mg vs. 16.1% placebo) [2]. Both are dose-dependent and tend to concentrate in the first 4-8 weeks of treatment.

Gastrointestinal Tolerability

Nausea and diarrhea in older adults carry consequences that younger patients manage more easily. Volume depletion from diarrhea can precipitate acute kidney injury in patients with baseline CKD, a population that substantially overlaps with MASH patients over 65. The National Kidney Foundation estimates that approximately 37 million Americans have CKD, with prevalence rising sharply after age 60 [12].

Initiating resmetirom with food (as required by the label) and ensuring adequate hydration are standard steps. Electrolyte monitoring at 4 and 8 weeks after initiation is a reasonable addition to the monitoring schedule in older patients with eGFR <45 mL/min/1.73 m².

Hepatotoxicity Signal

The label includes a warning about serum aminotransferase elevations. In MAESTRO-NASH, ALT elevations greater than 3x ULN occurred in 5.1% of 100 mg patients versus 3.4% placebo [2]. The FDA prescribing information recommends discontinuation if ALT exceeds 5x ULN [1]. In older patients with baseline ALT elevations from MASH itself, distinguishing disease progression from drug effect requires careful trending rather than isolated values.

Falls and Balance Considerations

No direct falls data exist from MAESTRO-NASH. However, thyroid hormone excess at any level increases heart rate and can worsen tremor or balance. The AGS Beers Criteria flag thyrotoxic states as fall-risk contributors [11]. Checking TSH at baseline and at 12 weeks is prudent in older patients, particularly those with any pre-existing balance impairment.

The MAESTRO-NASH-OUTCOMES Trial: What It May Reveal for Older Patients

The ongoing MAESTRO-NASH-OUTCOMES trial (NCT04951219) is evaluating resmetirom's effect on liver-related outcomes in a larger, longer-duration cohort [13]. The trial's enrollment criteria include patients up to age 75 and potentially beyond with investigator discretion. Results expected in 2026-2027 may provide the first powered subgroup data for adults over 65.

The primary endpoint is a composite of liver-related death, liver transplantation, hepatic decompensation events, or MELD score increase of 4 or more points [13]. If the outcomes trial confirms proportional benefit in older subgroups, the evidentiary basis for geriatric prescribing will strengthen considerably.

Applying Evidence to the Clinic: A Practical Framework for Older Patients

Prescribers evaluating resmetirom for a patient over 65 benefit from a structured pre-initiation checklist that translates available data into actionable steps.

Pre-Initiation Assessment

Before the first dose, confirm liver biopsy or FibroScan evidence of F2-F3 MASH (cirrhosis remains a contraindication), obtain a complete medication list with attention to statins, fibrates, cyclosporine, warfarin, and direct oral anticoagulants, measure baseline TSH and free T4, assess grip strength or a validated sarcopenia screen such as the SARC-F questionnaire [14], and document baseline eGFR and electrolytes.

The Endocrine Society's 2023 clinical practice guideline on fatty liver disease emphasizes that "pharmacotherapy for MASH should be considered alongside lifestyle modification, not as a replacement for it," a principle with added weight in older patients where dietary and physical activity changes also reduce sarcopenia risk [15].

Monitoring Schedule in Older Adults

• Weeks 4 and 8: ALT, AST, GGT, nausea/diarrhea assessment, hydration status
• Week 12: full metabolic panel, lipid panel, TSH, SARC-F reassessment, INR if on warfarin
• Week 24: repeat liver biomarkers, body composition if clinically feasible
• Week 52: formal efficacy reassessment per MAESTRO-NASH endpoints (MASH resolution, fibrosis stage if re-biopsy planned)

Patients who have not achieved ALT normalization or at least a 30% reduction from baseline by week 24 have a lower probability of histologic response; discontinuation discussion is appropriate at that point regardless of age [1].

When to Withhold Resmetirom in Older Adults

Absolute contraindications from the label apply universally: cirrhosis, co-administration with gemfibrozil or cyclosporine [1]. Additional caution is warranted in patients with eGFR <30 mL/min/1.73 m², pre-existing severe sarcopenia by EWGSOP2 criteria, or a fall history in the prior 12 months where any tremor or tachycardia risk is unacceptable.

What Clinicians and Researchers Are Watching

The American Association for the Study of Liver Diseases (AASLD) 2023 practice guidance on MASH notes that treatment decisions should account for "the patient's overall health, comorbidity burden, and life expectancy," language that directly implicates geriatric assessment tools in MASH treatment planning [16].

A 2023 Lancet paper modeling MASH disease burden estimated that globally, MASH-related cirrhosis will increase by 56% between 2020 and 2030, with the largest absolute increases in adults over 60 [17]. That demographic projection makes the geriatric evidence gap in resmetirom trials an increasingly urgent clinical problem.

The absence of dedicated pharmacokinetic studies in adults over 75 means that prescribers currently extrapolate from the general adult data with physiologic reasoning rather than direct evidence. The FDA label's silence on geriatric dosing reflects the trial data available at approval, not a conclusion that older adults respond identically to younger ones.

The MAESTRO-NASH trial showed that a 24-week histologic non-response rate of approximately 64% at 80 mg and 74% at 100 mg means most patients do not achieve the primary fibrosis endpoint within the first year [2]. In an older patient with competing mortality risks, the 52-week timeline before a formal response assessment requires weighing against the probability of disease progression and the patient's overall goals of care.