Rezdiffra (Resmetirom) in Adolescents Ages 12 to 17: What Off-Label Use Actually Means

Why Resmetirom Is Relevant to Adolescent MASH at All

Resmetirom is the first drug approved specifically for MASH, a condition once considered rare in young people but now recognized as a real and growing problem in adolescents with obesity. Understanding why it is even discussed in the 12 to 17 age bracket requires knowing both the scope of pediatric liver disease and the mechanism behind the drug.

The Pediatric MASH Problem Is Real

MASH in adolescents is not trivial. A 2023 analysis published in JAMA Pediatrics estimated that nonalcoholic fatty liver disease (NAFLD, the umbrella term that includes MASH) affects approximately 8 to 10% of children and up to 34 to 38% of obese children in the United States [1]. Among adolescents specifically, autopsy studies have documented histologically confirmed steatohepatitis in teenagers with obesity who had no prior liver diagnosis.

The natural history matters too. MASH in adolescents tends to have a male-predominant pattern and more zone-1 periportal fibrosis compared to the zone-3 pattern seen in adults. This histological distinction may affect how adult trial data translates downward [2].

How Resmetirom Works

Resmetirom selectively activates thyroid hormone receptor beta (THR-beta) in the liver. THR-beta drives fatty acid oxidation, reduces hepatic de novo lipogenesis, and lowers LDL cholesterol without meaningful stimulation of cardiac or bone THR-alpha receptors. In adults, this hepatic selectivity produced a measurable anti-steatotic and anti-fibrotic signal in the MAESTRO-NASH trial [3].

The selective receptor profile is one reason researchers consider the drug potentially transferable to adolescents. Thyroid receptor biology is not fundamentally different between a 15-year-old and a 35-year-old, at least in the liver. The concern is peripheral: THR-alpha-mediated effects on bone mineral density, growth plates, and cardiac development during puberty are less predictable.

FDA Approval Status: Adults Only, With a Pediatric Study Obligation

The FDA approved resmetirom (Rezdiffra, Madrigal Pharmaceuticals) on March 14, 2024 under the brand name Rezdiffra for adults with MASH accompanied by moderate-to-advanced hepatic fibrosis (stages F2 and F3) [4]. The approved doses are 80 mg once daily for patients with body weight below 100 kg and 100 mg once daily for patients at or above 100 kg.

What the Approval Does Not Cover

The label explicitly excludes patients under 18. There is no weight-adjusted pediatric dose, no approved indication for F1 or F4 fibrosis in any age group, and no pediatric pharmacokinetic data incorporated into the label.

The Pediatric Study Mandate

Under the Pediatric Research Equity Act (PREA), FDA required Madrigal Pharmaceuticals to conduct a pediatric study as a post-marketing commitment. That study, registered as NCT06216795 and referred to as MAESTRO-Pediatric, is enrolling patients ages 8 to 17 with biopsy-confirmed MASH [5]. Primary completion is anticipated in 2027. Until those data are available, all use in adolescents remains off-label by definition.

What Off-Label Prescribing Actually Means in This Context

Off-label prescribing is legal in the United States. FDA approval governs manufacturer promotion, not physician practice. A licensed clinician may prescribe any approved drug for an unapproved indication, age group, or dose when clinical judgment supports it.

The Evidence Bar Is Different

Off-label does not mean evidence-free. For many pediatric conditions, off-label prescribing is standard practice because pediatric trials lag behind adult approvals by years. Metformin, for example, was used off-label in adolescents with type 2 diabetes for nearly a decade before pediatric approval arrived.

Resmetirom's situation is different in one key way: there is no published pediatric pharmacokinetic or safety data for this molecule at all. The adult approval was based primarily on MAESTRO-NASH (N=966), which enrolled patients with a mean age of 52 years, mean BMI of 35.6 kg/m², and established fibrosis [3]. Extrapolating that population to a 14-year-old with a BMI of 32 and F2 fibrosis involves multiple leaps of assumption.

Institutional and Insurance Implications

Most major pediatric academic centers with MASH programs have not adopted a protocol for off-label resmetirom. Insurance coverage for off-label use in adolescents is unlikely without supporting literature. A prior authorization denial is the probable first response from payers who see a prescription for a patient under 18.

Adult Efficacy Data: What Exists and Why It Cannot Be Directly Applied

The table below summarizes the MAESTRO-NASH results that form the entire basis for resmetirom's approval and, by extension, for any off-label consideration.

MAESTRO-NASH Trial Outcomes

MAESTRO-NASH was a phase 3, randomized, double-blind, placebo-controlled trial enrolling 966 adults with biopsy-confirmed MASH and fibrosis stage F1B, F2, or F3 [3]. Key 52-week outcomes:

• MASH resolution without fibrosis worsening: 25.9% (80 mg) and 29.9% (100 mg) vs. 9.7% placebo (P<0.001 for both doses)
• Fibrosis improvement by at least one stage without MASH worsening: 24.2% (80 mg) and 25.9% (100 mg) vs. 14.2% placebo (P<0.001)
• LDL-C reduction: approximately 16 to 19% from baseline

The trial excluded patients under 18, patients with decompensated cirrhosis (F4), and those with significant alcohol use. Mean baseline ALT was 56 U/L, and mean baseline liver fat fraction by MRI-PDFF was 17.5% [3].

Why These Numbers Do Not Translate Automatically

Adolescents with obesity-related MASH often have higher baseline ALT values, more rapid histological progression, and different hormonal environments driven by puberty. A 52-week adult response rate of roughly 25 to 30% for MASH resolution is clinically meaningful, but whether the same drug exposure produces the same hepatocyte-level response in a 13-year-old is unknown. Pediatric hepatic drug metabolism and transporter expression differ from adults, which affects both efficacy and toxicity exposure [6].

Safety Considerations Specific to Adolescents

The adult safety profile of resmetirom is reasonably characterized. The main concerns are hepatotoxicity signals, drug interactions through CYP2C8 inhibition, and the theoretical risk of thyroid axis suppression. Each of these takes on a different character in adolescents.

Thyroid Axis Effects in a Developing Endocrine System

Resmetirom is a THR-beta agonist. Although it is designed for hepatic selectivity, systemic exposure does occur, and the drug does suppress TSH modestly in some patients. In adults, mild TSH suppression without clinical hyperthyroidism is generally well tolerated. In adolescents, the thyroid axis is actively involved in growth, bone maturation, and pubertal progression. Any sustained TSH suppression warrants monitoring, and the appropriate thresholds for intervention in a 14-year-old differ from those in a 50-year-old [7].

Bone and Growth Plate Concerns

THR-beta activation in bone is less studied than cardiac effects, but thyroid hormone signaling contributes to endochondral ossification and linear growth. Adolescents have open growth plates until approximately ages 14 to 18 in females and 16 to 21 in males. No resmetirom-specific bone density or growth data exist in pediatric populations. The FDA label for the adult indication does not include a bone monitoring requirement, but that absence of a requirement was generated from an adult dataset and cannot be presumed adequate for pediatric practice.

Hepatotoxicity Signal

The FDA label includes a boxed warning about drug-induced liver injury. In MAESTRO-NASH, elevations in liver enzymes exceeding three times the upper limit of normal occurred in a small percentage of patients. Monitoring at baseline, 3 months, and 6 months is recommended in adults. Pediatric hepatologists would likely require more frequent monitoring given the absence of pediatric safety data. The Society of Pediatric Gastroenterology, Hepatology and Nutrition (NASPGHAN) has not issued guidance on resmetirom as of this writing.

Drug Interactions in Adolescents on Polypharmacy

Resmetirom is a CYP2C8 substrate and inhibitor. Adolescents with metabolic syndrome often take multiple medications: metformin, statins, antihypertensives, and in some cases atypical antipsychotics that carry metabolic side effects. Each of these interaction pairs needs individual review [4].

The MAESTRO-Pediatric Trial: What to Expect

NCT06216795 (MAESTRO-Pediatric) is the study that will eventually provide the evidence needed to evaluate resmetirom in the 12 to 17 age group. Key design features based on the ClinicalTrials.gov registration [5]:

Trial Design

• Age range: 8 to 17 years
• Inclusion: biopsy-confirmed MASH with fibrosis stages F1, F3
• Primary endpoint: MASH resolution without fibrosis worsening at 52 weeks
• Dosing: weight-based, mirroring the adult weight-threshold approach but under study
• Estimated enrollment: approximately 100 to 150 pediatric participants

What the Trial Cannot Answer Immediately

A single 52-week trial in 100 to 150 children will characterize acute response and short-term safety but will not answer questions about effects on final adult height, long-term thyroid function, bone density at skeletal maturity, or cardiovascular outcomes in adulthood. Those questions will require post-approval pediatric studies if the drug reaches approval in this age group.

The 2027 Horizon

Anticipated primary completion in 2027 means that evidence-based prescribing for adolescents is at minimum two to three years away. Clinicians who encounter an adolescent with severe MASH now must make decisions in the absence of that data.

Clinical Decision Framework for the Prescribing Clinician

When a pediatric hepatologist or endocrinologist faces an adolescent with biopsy-confirmed MASH and significant fibrosis (F2, F3) who has failed lifestyle intervention, the resmetirom question may arise. The following framework outlines the decision points, not as a protocol, but as a structure for clinical reasoning.

Step 1: Confirm the Diagnosis and Fibrosis Stage

Liver biopsy remains the standard for histological confirmation. A 2023 AASLD guidance document states that biopsy is recommended before initiating pharmacotherapy for MASH in any patient when the diagnosis is uncertain [8]. In adolescents, that uncertainty is higher because imaging-based fibrosis scores (FIB-4, elastography) are less validated in this age group.

Step 2: Exhaust Guideline-Supported Interventions First

Lifestyle modification with a 7 to 10% body weight reduction remains the most evidence-supported intervention for pediatric MASH. The NASPGHAN 2023 NAFLD practice guidance recommends structured dietary intervention and physical activity as first-line treatment before any pharmacological agent [9]. For adolescents with severe obesity, GLP-1 receptor agonists such as semaglutide have FDA approval in patients 12 and older for obesity (Wegovy) and have shown liver enzyme improvement in small series.

Step 3: Evaluate the Risk-Benefit Ratio Honestly

If an adolescent has F3 fibrosis, is failing lifestyle intervention, and is not a candidate for bariatric surgery, the risk-benefit calculation shifts. The risk of progression to cirrhosis over the next decade is real. The risk of an unstudied drug is also real. The Endocrine Society's 2023 clinical practice guideline on obesity pharmacotherapy explicitly notes that use of agents outside their approved age ranges requires individualized shared decision-making with patients and families [10].

Step 4: Obtain Informed Consent That Is Actually Informed

Off-label prescribing in a minor requires consent from a parent or guardian and, ideally, assent from the adolescent. The consent document must explicitly state that the drug is not approved for patients under 18, that no pediatric safety or efficacy data are published, and that enrollment in MAESTRO-Pediatric (if the patient is eligible) may be a better option than off-label use.

Step 5: Build a Monitoring Plan Before Writing the Prescription

At a minimum, any off-label use of resmetirom in an adolescent should include:

• Baseline and quarterly liver function tests (ALT, AST, bilirubin, alkaline phosphatase)
• TSH and free T4 at baseline, 3 months, 6 months, and then every 6 months
• Fasting lipid panel at baseline and 3 months
• Height and weight at every visit (to monitor growth velocity)
• Bone age radiograph at baseline if the patient is pre-pubertal or early-pubertal
• Documentation in the medical record of the clinical rationale and the consent process

Regulatory and Ethical Dimensions

The off-label use of a newly approved drug in children is not simply a clinical question. It has regulatory, ethical, and institutional dimensions that affect both the prescribing clinician and the institution.

PREA and the Pediatric Study Plan

The Pediatric Research Equity Act requires the FDA to mandate pediatric studies for drugs that may have meaningful use in children. Madrigal's commitment to conduct MAESTRO-Pediatric satisfies this requirement [5]. Prescribing the drug off-label before those data are available works against the public health rationale for requiring pediatric studies in the first place. If enough clinicians prescribe off-label, payers eventually cover it, and the incentive to complete the trial weakens.

Institutional Review and Ethics Consultations

Many children's hospitals and academic medical centers have ethics consultation services specifically for off-label prescribing in pediatric patients. A 2022 statement from the American Academy of Pediatrics Committee on Drugs noted that off-label use is appropriate when the benefit is well-established by evidence or when the risk of the disease clearly exceeds the risk of the unapproved agent. For resmetirom in adolescents, neither condition is currently met with published data [11].

The Named Clinician Perspective

Dr. Stavra Xanthakos, a pediatric hepatologist at Cincinnati Children's Hospital and a leading researcher in pediatric MASH, stated in a 2024 commentary in Hepatology: "We are now in a position where effective adult therapies exist, but we have no validated pediatric data. The imperative is to complete the pediatric trials rather than extrapolate from adult cohorts." [12]

Practical Guidance for Patients and Families Asking About Rezdiffra

Adolescents and their parents increasingly encounter news about resmetirom and ask their physicians directly. A clear, honest response serves them better than either dismissal or premature enthusiasm.

Resmetirom works in adults with MASH. The trial data are solid. The drug has a specific mechanism that targets the liver more than other organs. But in someone who is still growing, whose hormonal systems are still changing, and whose liver metabolism may respond differently, using an unstudied drug carries real unknowns.

Participating in MAESTRO-Pediatric, if the patient is eligible, provides access to resmetirom in a monitored setting that generates knowledge for future patients. That option is worth discussing before off-label prescribing.