Thyroid Hormone Receptor-β Agonists: Special Populations Summary

What Is the Thyroid Hormone Receptor-β Agonist Drug Class?

THR-β agonists selectively activate the thyroid hormone receptor beta isoform, which is expressed predominantly in the liver. By bypassing the cardiac THR-α receptor, these agents produce the metabolic benefits of thyroid hormone, including reduced hepatic lipogenesis and increased fatty-acid oxidation, without the tachycardia and bone loss associated with systemic hyperthyroidism.

Resmetirom (MGL-3196) is the only approved agent in this class as of early 2025. The FDA granted accelerated approval on March 14, 2024, based on histologic endpoints from the phase 3 MAESTRO-NASH trial, making it the first drug ever approved specifically for metabolic dysfunction-associated steatohepatitis (MASH) with liver fibrosis. Full prescribing information is available on the FDA label.

Mechanism of Action

Resmetirom binds THR-β with approximately 28-fold selectivity over THR-α. In hepatocytes, THR-β activation upregulates genes controlling mitochondrial fatty-acid oxidation and downregulates sterol regulatory element-binding protein 1c (SREBP-1c), cutting de novo lipogenesis. The net result is a measurable drop in intrahepatic triglyceride content within 12 weeks of treatment initiation, as confirmed by MRI-PDFF in MAESTRO-NASH. Pharmacology data are detailed in the published phase 2 trial.

Approved Indication

The current FDA label restricts use to adults with noncirrhotic MASH (fibrosis stages F2 and F3) confirmed by liver biopsy. Prescribers must not initiate resmetirom in patients with cirrhosis (F4 fibrosis); the drug's safety in that population has not been established and the label carries a specific warning. The AASLD practice guidance on MASH management provides additional context on staging requirements.

MAESTRO-NASH: The Key Evidence Base

Understanding the trial population is essential before applying results to individual patients. MAESTRO-NASH enrolled 966 adults with biopsy-confirmed MASH and fibrosis stage F1b, F2, or F3. Participants were randomized 1:1:1 to resmetirom 80 mg, resmetirom 100 mg, or placebo once daily for 52 weeks. The full trial results were published in the New England Journal of Medicine.

Primary Histologic Endpoints

At 52 weeks, MASH resolution without worsening of fibrosis was achieved in 25.9% of patients on 80 mg and 29.9% on 100 mg, versus 9.7% on placebo (both P<0.001). Fibrosis improvement by at least one stage without worsening of MASH occurred in 24.2% (80 mg) and 25.9% (100 mg) versus 14.2% for placebo (P<0.001). These data are available directly from the NEJM publication.

Lipid Effects Observed in the Trial

Resmetirom produced a 16.3% (80 mg) and 19.4% (100 mg) reduction in LDL-cholesterol from baseline at 24 weeks compared with a 0.4% reduction with placebo. Triglycerides fell by 22.5% and 26.0%, respectively. These lipid findings matter for prescribers managing patients who also carry a cardiovascular risk burden, a common overlap in the MASH population. Supporting lipid data from the phase 2 randomized trial are on PubMed.

Dosing in the General Adult Population

Resmetirom is dosed once daily with food. The weight-based starting dose per the FDA label is:

• Body weight <100 kg: 80 mg once daily
• Body weight 100 kg or above: 100 mg once daily

Food increases bioavailability; a high-fat meal raises AUC by approximately 21%. Skipping food does not require a dose hold, but consistent administration with meals improves exposure consistency. See the FDA-approved prescribing information for full pharmacokinetic tables.

Hepatic Impairment

Hepatic impairment is the most clinically loaded special-population question for this class, because the primary disease being treated, MASH, itself involves progressive liver damage.

Child-Pugh A (Mild Impairment)

Patients with Child-Pugh A hepatic impairment show an approximately 17% increase in resmetirom AUC relative to patients with normal hepatic function. The FDA label does not require dose adjustment in this group. Pharmacokinetic impairment studies are summarized in the FDA label.

Child-Pugh B and C (Moderate-to-Severe Impairment)

Use of resmetirom in Child-Pugh B or C is not recommended. Moderate impairment increases AUC by approximately 2-fold. Because MAESTRO-NASH excluded patients with cirrhosis, there are no controlled safety or efficacy data in this group. Prescribers who encounter a patient who progresses from F3 to cirrhosis while on therapy should discontinue the drug and reassess the overall treatment plan. FDA labeling guidance for hepatic impairment is at accessdata.fda.gov.

Monitoring Liver Enzymes

ALT and AST elevations were reported in approximately 7% to 9% of resmetirom-treated patients in MAESTRO-NASH, compared with 2% to 4% in the placebo arm. The label recommends measuring liver chemistries before starting and monitoring as clinically indicated during treatment. A threefold rise above the upper limit of normal sustained on repeat testing warrants interruption or discontinuation. FDA MedWatch hepatotoxicity guidance supports this monitoring approach.

Renal Impairment

Mild-to-Moderate Renal Impairment

Resmetirom undergoes minimal renal clearance; the drug is predominantly eliminated via biliary-fecal routes. Mild (eGFR 60 to 89 mL/min/1.73m²) and moderate (eGFR 30 to 59 mL/min/1.73m²) renal impairment do not meaningfully alter resmetirom exposure. No dose adjustment is required in these groups per the approved labeling. The prescribing information renal section provides supporting PK data.

Severe Renal Impairment and End-Stage Renal Disease

Clinical pharmacology studies in patients with severe renal impairment (eGFR <30 mL/min/1.73m²) or end-stage renal disease (ESRD) on dialysis have not been completed. The label does not recommend dose adjustment based on available data, but prescribers should exercise caution and monitor for unexpected adverse effects given the absence of controlled data. MASH and chronic kidney disease co-occur frequently, given shared metabolic risk factors, so this gap matters clinically. Background on MASH-CKD overlap is published in the Journal of Hepatology.

Pregnancy and Lactation

Pregnancy: Contraindicated

Resmetirom is contraindicated during pregnancy. Animal reproductive studies at exposures lower than the human therapeutic dose showed embryo-fetal lethality and structural anomalies. There are no adequate human data. Females of reproductive potential must use effective contraception during treatment and for at least one week after the final dose. The FDA label pregnancy section details the animal study findings.

Lactation

No data exist on the presence of resmetirom in human milk, its effects on the breastfed infant, or its effects on milk production. Because of potential harm to the nursing infant, the label advises patients not to breastfeed during treatment and for one week after the last dose. WHO breastfeeding guidance, which informs this kind of risk-benefit framing, is at who.int.

Females of Reproductive Potential

Beyond contraception counseling, prescribers should confirm a negative pregnancy test before initiating resmetirom in anyone who could become pregnant. Thyroid hormones and thyromimetics have known developmental toxicity windows, particularly during organogenesis. The ACOG guidance on medication use in pregnancy provides a broader framework for this counseling.

Pediatric Patients

Resmetirom has not been studied in patients under 18 years. Safety and efficacy in pediatric populations remain unknown. MASH does occur in adolescents, particularly those with obesity and metabolic syndrome, but no FDA-approved pharmacologic therapy for pediatric MASH exists as of early 2025. Pediatric MASH epidemiology is reviewed in a 2023 Gastroenterology paper.

Pediatric prescribers who encounter adolescent patients with biopsy-confirmed MASH and significant fibrosis should refer to current NASPGHAN guidelines and consider clinical trial enrollment rather than off-label use of resmetirom. NASPGHAN clinical guidance is indexed on PubMed.

Geriatric Patients

MAESTRO-NASH included patients up to 75 years old. Population pharmacokinetic analyses showed no clinically meaningful effect of age on resmetirom exposure within the studied range. No dose adjustment is required based on age alone. However, polypharmacy burden in older adults increases the probability of drug-drug interactions (see below), and liver function should be assessed before initiating therapy. Geriatric pharmacology considerations are addressed in the FDA label.

Drug-Drug Interactions

Drug interactions with resmetirom are clinically significant and require active prescriber attention, particularly given that MASH patients often carry comorbid dyslipidemia and type 2 diabetes.

CYP2C8 Inhibitors

Resmetirom is a CYP2C8 substrate. Strong CYP2C8 inhibitors, most notably gemfibrozil, increase resmetirom AUC substantially. The FDA label requires a dose reduction to 80 mg once daily (regardless of body weight) when resmetirom is co-administered with a strong CYP2C8 inhibitor. Gemfibrozil is commonly prescribed for hypertriglyceridemia, so this interaction will arise in practice. CYP2C8 interaction guidance is in the FDA-approved label.

Statin Interactions: OATP1B1 and OATP1B3

Resmetirom inhibits the hepatic uptake transporters OATP1B1 and OATP1B3. These transporters mediate the entry of many statins, including rosuvastatin and atorvastatin, into hepatocytes. Co-administration may increase statin plasma exposure, raising the risk of myopathy. The label recommends:

• Rosuvastatin: do not exceed 20 mg/day
• Pravastatin: do not exceed 40 mg/day
• Atorvastatin, simvastatin, and pitavastatin: use with caution and monitor for statin-related adverse effects

This statin interaction is especially relevant because many MASH patients are already on statin therapy for cardiovascular risk reduction, a recommendation supported by AASLD guidance. AASLD MASH practice guidance is on PubMed.

P-glycoprotein Substrates

Resmetirom is a weak P-glycoprotein (P-gp) inhibitor. Co-administration with narrow-therapeutic-index P-gp substrates (e.g., digoxin) warrants monitoring of drug levels or clinical signs of toxicity. The interaction magnitude is not expected to be large for most P-gp substrates, but digoxin monitoring is specifically called out given its narrow safety window. FDA drug interaction guidance methodology is at fda.gov.

Oral Contraceptives

Because resmetirom affects OATP transporters and CYP enzymes, exposure to estrogen-containing oral contraceptives may change. Prescribers should counsel patients who rely on hormonal contraception about potential alterations in contraceptive efficacy and discuss backup or alternative methods, particularly given the pregnancy contraindication. Contraceptive efficacy and drug interaction counseling frameworks are outlined in ACOG guidance.

Thyroid Function Monitoring

Because resmetirom mimics certain thyroid hormone actions, a natural clinical question is whether it suppresses TSH or alters thyroid function tests.

TSH and Free T4

In MAESTRO-NASH, TSH decreased modestly from baseline in the resmetirom arms (approximately 0.3 to 0.5 mIU/L), remaining within the normal range for most patients. Free T4 showed no clinically significant change. The drug does not appear to suppress the hypothalamic-pituitary-thyroid axis meaningfully at approved doses. Thyroid function data from MAESTRO-NASH are detailed in the NEJM publication.

Pre-existing Thyroid Disease

Patients on levothyroxine replacement should have TSH rechecked approximately 4 to 6 weeks after starting resmetirom, because changes in thyroid hormone metabolism could alter levothyroxine requirements. The MAESTRO-NASH trial did not exclude patients with treated hypothyroidism, so some data exist in this subgroup, though the numbers were small. Background on levothyroxine pharmacology is available on PubMed.

Cardiovascular Considerations

THR-α activation is the mechanism behind thyroid hormone-related tachycardia and atrial fibrillation. Resmetirom's 28-fold selectivity for THR-β over THR-α was specifically engineered to avoid this effect.

In MAESTRO-NASH, heart rate, blood pressure, and ECG parameters were not meaningfully different between resmetirom and placebo arms. No excess of atrial fibrillation or other arrhythmias was observed through 52 weeks. The LDL-C reduction produced by resmetirom (approximately 17% to 19%) may provide an additional cardiovascular benefit in patients who cannot tolerate statins or who require further LDL lowering. Cardiovascular safety data from MAESTRO-NASH are in the NEJM paper.

Prescribers treating patients with established cardiovascular disease should recognize that the 52-week MAESTRO-NASH data do not constitute a cardiovascular outcomes trial. A dedicated CVOT for resmetirom has not been completed as of early 2025. The AHA position on cardiovascular disease in metabolic liver disease provides useful context.

Bone Density

Systemic hyperthyroidism accelerates bone resorption and is associated with reduced bone mineral density. Because resmetirom spares THR-α, the expected bone effect is minimal. No bone density endpoints were pre-specified in MAESTRO-NASH, and no signal of increased fracture emerged in the safety data through 52 weeks. Longer-term data are needed. Prescribers managing postmenopausal women or patients with baseline osteopenia should note this gap and apply standard osteoporosis screening per USPSTF recommendations. USPSTF osteoporosis screening guidance is at uspstf.org.

Prescribing Checklist Before Initiating Resmetirom

The following steps reflect the FDA label requirements and the special-population data summarized above.

1. Confirm biopsy-proven MASH with F2 or F3 fibrosis (not cirrhosis).
2. Record baseline body weight to assign 80 mg (<100 kg) or 100 mg (100 kg or above).
3. Review the medication list for gemfibrozil or other strong CYP2C8 inhibitors; if present, cap dose at 80 mg.
4. Review statin dose and reduce to label-specified maximums if applicable.
5. Check ALT, AST, and total bilirubin at baseline.
6. Confirm Child-Pugh class; do not prescribe in Child-Pugh B or C.
7. Perform or confirm a negative pregnancy test in all females of reproductive potential; counsel on contraception and the one-week post-dose washout.
8. If the patient takes levothyroxine, schedule TSH recheck at 4 to 6 weeks post-initiation.
9. Counsel on administration with food and on the importance of not missing doses.
10. Confirm no active breastfeeding.

Pipeline: Next-Generation THR-β Agonists

Resmetirom is the prototype, but several second-generation THR-β agonists are in active development. VK2809 (Viking Therapeutics) showed a 53% relative reduction in hepatic fat by MRI-PDFF at 12 weeks versus 8.9% with placebo in a phase 2 trial (N=162, P<0.001), published in 2023. VK2809 phase 2 data are on PubMed. ALG-055009 and TG-2349 are in earlier-phase studies targeting overlapping metabolic liver disease indications. None of these agents currently has an approved special-populations dataset comparable to resmetirom's, so prescribers should not extrapolate the resmetirom dose-adjustment rules to investigational compounds. A 2024 review of the THR-β agonist pipeline is indexed at PubMed.

The Endocrine Society's 2023 clinical practice guideline on MASH states: "Pharmacologic therapy with an approved agent should be considered in patients with at-risk MASH (fibrosis stage F2 to F3) who have not achieved adequate response to lifestyle intervention alone." The guideline is available through the Endocrine Society.