Resmetirom (Rezdiffra) One-Year Outcomes: 53% Liver Fat Reduction and 30% Fibrosis Improvement in MASH

What Is Resmetirom and How Does It Treat Fatty Liver Disease?

Resmetirom (brand name Rezdiffra), developed by Madrigal Pharmaceuticals, is a liver-directed, orally administered selective agonist of thyroid hormone receptor beta (THR-beta). THR-beta is the predominant thyroid hormone receptor isoform in the liver and mediates the metabolic effects of thyroid hormone on hepatic lipid metabolism, including stimulation of fatty acid beta-oxidation, reduction of de novo lipogenesis, increased VLDL secretion and LDL receptor expression, and enhanced mitochondrial biogenesis. In MASH patients, hepatic THR-beta signaling is impaired due to intracellular lipotoxicity and inflammatory signaling, contributing to progressive fat accumulation and hepatocellular injury.

Resmetirom's selective agonism of THR-beta (with approximately 28-fold selectivity over THR-alpha, the isoform responsible for cardiac and skeletal effects of thyroid hormone) allows it to restore hepatic lipid metabolism without causing the tachycardia, bone loss, muscle wasting, and anxiety associated with systemic thyroid hormone excess. In MASH patients, resmetirom reduces intrahepatic triglyceride content by enhancing mitochondrial fatty acid oxidation, suppresses hepatic de novo lipogenesis through downregulation of SREBP-1c, and reduces liver inflammation by modulating hepatic stellate cell activation and macrophage-mediated fibrogenic pathways. This multi-targeted hepatic effect addresses the interconnected pathophysiology of steatosis, inflammation, and fibrosis that characterizes progressive MASH.

What Did the MAESTRO-NASH Trial Show at One Year?

The MAESTRO-NASH trial was a randomized, double-blind, placebo-controlled Phase 3 study enrolling approximately 960 adults with biopsy-confirmed MASH and liver fibrosis at stages F1B through F3 (clinically significant fibrosis but not yet cirrhosis). Patients were randomized to resmetirom 80mg, resmetirom 100mg, or placebo once daily. The trial used dual primary endpoints at 52 weeks: MASH resolution (defined as steatohepatitis resolution on liver biopsy with a NAFLD Activity Score [NAS] of 0–1 for inflammation and 0 for ballooning, without worsening fibrosis) and fibrosis improvement (defined as improvement by at least one stage without worsening of NAS).

At 52 weeks, resmetirom 100mg achieved MASH resolution in approximately 30% of patients versus about 10% on placebo (p<0.001), and fibrosis improvement by at least one stage in approximately 26% versus about 14% on placebo (p<0.001). The 80mg dose showed approximately 26% MASH resolution and 24% fibrosis improvement. MRI-proton density fat fraction (MRI-PDFF) assessments demonstrated a substantial relative reduction in liver fat content at 52 weeks with 100mg — approximately 50% — compared to a modest reduction with placebo. Serum ALT levels normalized in a significantly greater proportion of resmetirom-treated patients compared to placebo, indicating reduced hepatocellular injury. Non-invasive fibrosis biomarkers, including the Enhanced Liver Fibrosis (ELF) score and FibroScan-measured liver stiffness, showed significant improvements correlating with histologic findings.

Subgroup analyses revealed consistent benefit across baseline fibrosis stage (F2 and F3 both responded), diabetes status (approximately two-thirds of the cohort had type 2 diabetes), BMI categories, and age groups. Patients with F3 fibrosis (bridging fibrosis) also showed meaningful fibrosis improvement, a clinically critical finding as this population is at highest risk for progression to cirrhosis, liver failure, and hepatocellular carcinoma.

What Are the Side Effects of Resmetirom?

The most frequently reported adverse events with resmetirom 100mg were gastrointestinal in nature: diarrhea occurred in approximately 27% of patients (vs. about 13% on placebo) and nausea in approximately 19% (vs. about 8% on placebo). These GI effects were predominantly mild to moderate in severity, occurred primarily during the first 4–8 weeks of treatment, and attenuated substantially with continued dosing. Other reported adverse events included abdominal pain, modest weight decrease, and vomiting, all at relatively low rates.

Critically, resmetirom's THR-beta selectivity was supported by the absence of clinically significant THR-alpha-mediated adverse effects. Heart rate did not increase significantly compared to placebo, and no meaningful increase in atrial fibrillation was attributed to the drug. Bone mineral density remained stable, with no increase in fracture risk — a key concern with non-selective thyroid hormone analogs. Thyroid function tests (TSH, free T3, free T4) remained within normal ranges, confirming the absence of systemic thyroid hormone excess. LDL cholesterol decreased on resmetirom, a beneficial metabolic effect consistent with THR-beta-mediated upregulation of hepatic LDL receptors. Serious adverse events occurred at similar or numerically lower rates in the resmetirom group compared to placebo. The FDA granted accelerated approval with postmarketing requirements including continued evaluation of clinical outcomes.

Why Is Resmetirom Considered a Breakthrough for Liver Disease?

The accelerated approval of resmetirom (Rezdiffra) by the FDA on March 14, 2024 represented a watershed moment in hepatology. MASH (formerly known as NASH) is a progressive form of metabolic dysfunction-associated steatotic liver disease (MASLD, formerly NAFLD) characterized by hepatocyte ballooning injury, lobular inflammation, and steatosis, which can progress to fibrosis, cirrhosis, and hepatocellular carcinoma. Tens of millions of people worldwide are estimated to have MASH, with a significant proportion having clinically significant fibrosis. Despite decades of research and multiple failed clinical trials of various drug candidates (including obeticholic acid, selonsertib, cenicriviroc, and elafibranor), no pharmacotherapy had previously achieved FDA approval for MASH.

Resmetirom's success where others failed is attributed to its mechanism targeting a fundamental metabolic driver (impaired thyroid hormone signaling in the liver) rather than downstream inflammatory or fibrotic pathways. The dual efficacy on both MASH resolution and fibrosis improvement is particularly significant, as fibrosis stage is the strongest histologic predictor of liver-related morbidity and mortality. The MAESTRO-NASH outcomes trial is continuing to evaluate whether resmetirom reduces hard clinical endpoints (progression to cirrhosis, liver transplantation, hepatocellular carcinoma, and liver-related death), with longer-term results expected in the coming years. The 52-week data showing substantial liver fat reduction and meaningful fibrosis improvement have reinforced clinical confidence. Prescription uptake has grown since launch, though access remains limited for some patients due to the requirement for diagnostic confirmation and insurance coverage challenges.