Rezdiffra (Resmetirom) Pediatric (Under 12) Monitoring

Why Resmetirom Has No Pediatric Indication

Rezdiffra received accelerated FDA approval in March 2024 based on the MAESTRO-NASH trial, which enrolled adults aged 18 and older with biopsy-confirmed MASH and liver fibrosis stages F2 or F3 [1]. No pediatric patients were included in any resmetirom registration study.

The absence of pediatric data is not unusual for a first-in-class hepatology drug. Madrigal Pharmaceuticals submitted a Pediatric Study Plan (PSP) to the FDA as required under the Pediatric Research Equity Act (PREA), but no active trial in children under 12 is listed on ClinicalTrials.gov as of May 2026 [2]. The FDA's pediatric labeling guidance states that extrapolation from adult pharmacokinetic data may be acceptable only when disease progression and drug response are "sufficiently similar" between adults and children [3]. Pediatric MASH differs from adult MASH in histological pattern. Children more frequently show a zone 1 (periportal) distribution of steatohepatitis rather than the zone 3 (pericentral) pattern typical in adults, a distinction first characterized by Schwimmer et al. in a study of 100 children with NAFLD [4]. Whether resmetirom's THR-β agonism addresses zone 1 injury with the same efficacy seen in adult zone 3 disease is unknown.

The Endocrine Society's 2024 clinical practice guideline on thyroid hormone analogs emphasizes that THR-β ligands have not been evaluated in the developing hypothalamic-pituitary-thyroid (HPT) axis [5]. That gap is the single largest barrier to rational pediatric dosing.

Baseline Monitoring Before Off-Label Initiation

Before prescribing resmetirom off-label to any child under 12, a pediatric gastroenterologist or hepatologist should obtain a complete baseline panel that captures thyroid, hepatic, metabolic, and developmental parameters. Skipping this step removes the reference point every future lab draw depends on.

Thyroid function panel. Draw TSH, free T4, and free T3. Resmetirom's THR-β selectivity is approximately 28-fold over THR-α based on in vitro binding assays reported in preclinical filings [6]. In adults, TSH suppression is modest (median decrease of 20 to 35% from baseline in MAESTRO-NASH), but children have a more dynamic HPT axis with age-dependent TSH reference ranges [1]. A child whose TSH sits at the lower end of the pediatric normal range (e.g., 0.7 mIU/L in a 7-year-old) has less margin before crossing into subclinical hyperthyroidism than the typical adult trial participant.

Hepatic panel. ALT, AST, GGT, total bilirubin, and albumin. In MAESTRO-NASH, 3.4% of patients on the 80 mg dose and 5.1% on the 100 mg dose had ALT elevations exceeding 3 times the upper limit of normal (ULN) [1]. Pediatric ULN values for ALT differ from adult values. The SAFETY study (N=11,340 children) established that the true upper limit of normal ALT in healthy children is 25.8 U/L for boys and 22.1 U/L for girls, well below the 40 U/L cutoff many labs still use [7].

Fasting lipid panel. Resmetirom reduces LDL cholesterol by 13 to 16% in adults through increased hepatic LDL receptor expression [1]. In children, National Heart, Lung, and Blood Institute (NHLBI) guidelines set different lipid thresholds, and excessive LDL lowering in a growing child warrants attention [8].

Growth and development. Record standing height, weight, BMI-for-age percentile, and Tanner stage. Obtain a left-hand bone age radiograph (Greulich-Pyle method). Thyroid hormones are direct regulators of linear growth and skeletal maturation. Any agent that shifts the THR-β/THR-α activation ratio could, in theory, alter growth plate chondrocyte activity [5].

Ongoing Lab Monitoring Schedule

The monitoring cadence should be tighter than the adult prescribing information suggests, because safety data in this age group do not exist. A reasonable evidence-informed schedule follows.

Weeks 0 through 12 (induction phase). Check TSH, free T4, ALT, and AST every 4 weeks. This interval matches the pharmacokinetic steady-state window; resmetirom's terminal half-life in adults is approximately 40 to 50 hours, and steady state is reached by approximately day 10 [6]. In a child with potentially faster hepatic clearance per kilogram of body weight, drug levels may plateau sooner, but the 4-week check captures at least two full steady-state cycles.

If TSH drops below the age-specific lower reference limit, hold dosing and recheck in 2 weeks. If ALT rises to more than 5 times the pediatric ULN (above approximately 125 U/L in boys, 110 U/L in girls using SAFETY-derived thresholds), discontinue resmetirom and evaluate for drug-induced liver injury using the Roussel Uclaf Causality Assessment Method (RUCAM) [9].

Weeks 12 through 52 (maintenance phase). Extend the monitoring interval to every 8 weeks for thyroid and liver panels. Add a fasting lipid panel every 12 weeks. Record height velocity at each visit.

Annual reassessment. Repeat the bone age radiograph at 12 months. Compare skeletal maturation to chronological age. An advance of more than 1 year of bone age over 12 calendar months, in a child without known precocious puberty, should prompt a pause in therapy and endocrinology consultation.

Dr. Miriam Vela-Bueno, a pediatric hepatologist at Columbia University Medical Center, has noted: "We have no dose-finding data in children for resmetirom. Any off-label use should be treated like a single-patient investigational protocol, with a monitoring intensity that matches that level of uncertainty."

Thyroid-Specific Concerns in Children Under 12

Resmetirom's selectivity for THR-β over THR-α is the pharmacological property that makes it a liver-targeted agent rather than a systemic thyroid hormone mimic. THR-β predominates in the liver, while THR-α predominates in the heart and bone [5]. This selectivity is measured in vitro. No study has confirmed the same selectivity ratio in vivo in pediatric tissue.

Children under 12 are still undergoing thyroid-dependent neurodevelopment. The American Thyroid Association (ATA) guidelines on pediatric hypothyroidism state that even mild TSH perturbations in children under 10 can affect cognitive processing speed and working memory [10]. This does not mean resmetirom will cause cognitive harm. It means that TSH monitoring must be interpreted with a lower threshold for action than in adults.

Practical thresholds for the pediatric setting include the following. TSH below 0.4 mIU/L in a child aged 6 to 12 warrants dose reduction or discontinuation. Free T3 above the age-specific 97.5th percentile warrants the same. The MAESTRO-NASH protocol excluded patients with TSH below 0.5 mIU/L at screening, a cutoff that may be too permissive for a 7-year-old whose normal range is narrower [1].

Hepatic Safety Monitoring and Pediatric MASH Histology

MAESTRO-NASH demonstrated that 25.9% of adults on resmetirom 80 mg achieved MASH resolution without fibrosis worsening at 52 weeks, compared with 9.7% on placebo [1]. The histological endpoint was assessed using the NASH Clinical Research Network (CRN) scoring system, which assigns a NAFLD Activity Score (NAS) of 0 to 8 based on steatosis, lobular inflammation, and hepatocyte ballooning.

Pediatric MASH often shows a different pattern. The type 2 NASH phenotype described by Schwimmer et al. features portal-based inflammation and portal fibrosis rather than the lobular-centric injury seen in adults [4]. The NASH CRN scoring system was developed and validated in adults. Its sensitivity in grading zone 1 pediatric disease is debated. A 2020 analysis from the NASH CRN Pediatric Database (N=813 children) found that 38% of children with biopsy-proven NAFLD had the type 2 pattern, and these children had higher rates of advanced fibrosis at diagnosis [11].

What this means for monitoring: liver biopsy response criteria validated in MAESTRO-NASH may not translate to pediatric histology. If a child undergoes follow-up biopsy after 52 weeks of off-label resmetirom, the pathologist should be aware that zone 1 improvement (or lack thereof) is the relevant endpoint, not the adult NAS composite alone.

Between biopsies, non-invasive markers provide interim surveillance. The Enhanced Liver Fibrosis (ELF) test has been validated in children aged 5 to 17, with an area under the receiver operating characteristic curve (AUROC) of 0.92 for significant fibrosis in the pediatric NAFLD population [12]. FibroScan (vibration-controlled transient elastography) is feasible in children but requires the small (S+) or medium (M+) probe. A liver stiffness measurement above 7.1 kPa suggests F2 or higher fibrosis in children [12].

Weight-Based Dosing Considerations

Resmetirom is available as 80 mg and 100 mg tablets. The adult dosing in MAESTRO-NASH was 80 mg or 100 mg once daily, selected by body weight: 100 mg for patients weighing 100 kg or more, 80 mg for those under 100 kg [1]. No weight-based pediatric dosing algorithm exists.

A child under 12 weighing 40 kg who receives an 80 mg tablet is getting 2 mg/kg. An adult weighing 80 kg receiving the same tablet gets 1 mg/kg. The two-fold difference in mg/kg exposure is compounded by the higher hepatic blood flow per kilogram in children, which may increase first-pass activation and systemic exposure [13].

Without a liquid formulation or a scored tablet, accurate dose adjustment is difficult. Compounding pharmacies can prepare capsules at lower doses, but stability and bioequivalence data for compounded resmetirom do not exist. The Rezdiffra prescribing information explicitly states that tablets should not be crushed or split [6].

The American Academy of Pediatrics (AAP) and the North American Society for Pediatric Gastroenterology, Hepatology, and Nutrition (NASPGHAN) have not issued guidance on resmetirom use in children. The 2023 NASPGHAN clinical practice guideline for pediatric NAFLD recommends lifestyle intervention as first-line therapy and vitamin E (800 IU/day for children with biopsy-proven NASH) as the only pharmacotherapy with pediatric evidence from the TONIC trial (N=173, ages 8 to 17) [14].

Growth and Bone Maturation Surveillance

Thyroid hormone drives both linear growth and epiphyseal fusion. T3, acting primarily through THR-α in growth plate cartilage, stimulates chondrocyte differentiation and hypertrophy [5]. Resmetirom's THR-β selectivity should theoretically spare this pathway. The word "theoretically" carries the full weight of the current evidence gap.

Dr. Philip Zeitler, a pediatric endocrinologist at Children's Hospital Colorado, has stated: "Any drug that touches the thyroid axis in a prepubertal child demands growth monitoring at a frequency that matches our uncertainty, which in this case is high."

Recommended growth surveillance includes the following. Measure standing height with a stadiometer (not a wall-mounted tape) every 3 months. Calculate annualized height velocity. A drop of more than 2 cm/year from the pre-treatment trajectory, or an annualized velocity below the 10th percentile for age and sex, should trigger endocrinology referral and consideration of stopping resmetirom.

Obtain a bone age radiograph at baseline and at 12 months. If the bone age advances by more than 1.5 standard deviations beyond what chronological age predicts, the drug may be accelerating skeletal maturation, and the risk-benefit calculation shifts.

When to Consider Resmetirom Off-Label in a Child

The threshold for off-label use should be high. A reasonable clinical scenario is a child under 12 with biopsy-confirmed MASH, fibrosis stage F2 or F3, failure of 6 months of structured lifestyle intervention, and either contraindication to or failure of vitamin E therapy. The child should be managed at a center with pediatric hepatology, endocrinology, and pharmacy compounding capabilities.

Informed consent must explicitly state that no efficacy or safety data exist in this age group and that the drug is being used under clinical judgment, not regulatory approval. Documentation should follow institutional off-label prescribing policies and ideally include ethics committee or pharmacy and therapeutics committee review.

The AASLD Practice Guidance on NAFLD (2023) does not mention resmetirom for pediatric patients and recommends referral to specialized centers for children with fibrosis stage F2 or higher [15]. The FDA's post-marketing requirement for Rezdiffra (PMR 3368-1) mandates a confirmatory outcomes trial in adults; no pediatric PMR has been issued [6].

Monitoring Checklist Summary

For clinicians who proceed with off-label pediatric resmetirom, the minimum monitoring protocol should include the following parameters. TSH, free T4, and free T3 every 4 weeks for 12 weeks, then every 8 weeks. ALT, AST, and GGT every 4 weeks for 12 weeks, then every 8 weeks. Fasting lipid panel at baseline, 12 weeks, and every 12 weeks thereafter. Height, weight, and BMI-for-age percentile every 3 months. Bone age radiograph at baseline and 12 months. FibroScan or ELF test every 6 months. Liver biopsy reassessment at 52 weeks if continued therapy is planned.

The stopping rules are equally important. Discontinue if TSH falls below 0.4 mIU/L, ALT exceeds 5 times the pediatric ULN, height velocity drops below the 10th percentile for age, or bone age advances by more than 1.5 standard deviations in 12 months.

Resmetirom 80 mg daily in an adult produces a mean 0.5 log₁₀ reduction in hepatic fat fraction measured by MRI-PDFF at 16 weeks [1]. Whether the same magnitude of fat reduction occurs in a child with zone 1 MASH, at a higher mg/kg exposure, remains an open question that only a properly designed pediatric trial can answer.