Resmetirom, sold as Rezdiffra, is the first drug the FDA approved for MASH (metabolic-associated steatohepatitis, the inflammatory, scarring form of fatty liver). It is a once-daily pill that switches on a thyroid receptor found mainly in the liver, which raises how the liver burns fat and lowers the inflammation and scarring, without the heart effects of thyroid hormone. In its main trial, about 1 in 4 to 1 in 3 people had their steatohepatitis resolve and about 1 in 4 had less scarring at one year. It is for people with moderate-to-advanced liver fibrosis, used alongside diet and exercise, and it also lowers LDL cholesterol. Its approval was based on liver biopsy results; the proof that it prevents liver failure and death is still being gathered.
TL;DR: Resmetirom, sold as Rezdiffra, is the first drug the FDA approved for MASH, the inflammatory, scarring form of fatty liver disease (once called NASH). It is a once-daily pill that activates a thyroid hormone receptor found mainly in the liver, which pushes the liver to burn its excess fat and eases the inflammation and scarring, without the heart or bone effects of thyroid hormone. In its pivotal trial, about 1 in 4 to 1 in 3 people had their steatohepatitis resolve and about 1 in 4 saw their liver scarring improve at one year, more than on placebo. It is approved for people with moderate-to-advanced liver fibrosis (stages F2 to F3), used alongside diet and exercise, and it lowers LDL cholesterol as a bonus. The approval was based on liver biopsy improvement; the proof that it prevents liver failure, transplant, and death is still being gathered in ongoing trials, and it is expensive.
What is resmetirom, and how does it work?
Resmetirom, sold as Rezdiffra, is a once-daily pill for a disease that until 2024 had no approved drug at all: MASH, short for metabolic dysfunction-associated steatohepatitis. MASH is the stage of fatty liver where fat has driven inflammation and scarring, and left unchecked it can progress to cirrhosis. Our fatty liver guide covers the disease itself; this piece is about the drug.
The mechanism is clever. Thyroid hormone speeds up metabolism, including how the liver handles fat, but taking thyroid hormone itself would strain the heart and bones. Resmetirom gets around that by targeting only one of the two thyroid receptors, the beta type, which sits mainly in the liver, while largely sparing the alpha type that drives the heart and bone effects. Switching on the liver's beta receptor tells the liver to burn its stored fat and lowers the fat-driven damage, and in trials it did this without meaningfully affecting the thyroid gland, heart rate, or bone. In effect, it borrows thyroid hormone's fat-burning power for the liver alone.
What did the trial show?
The evidence comes from MAESTRO-NASH, a phase 3 trial of nearly 1,000 people with biopsy-confirmed MASH and fibrosis, who took resmetirom or placebo on top of lifestyle advice for a year.1 It measured two things by liver biopsy: whether the steatohepatitis resolved, and whether the scarring improved.
Both goals were met. Steatohepatitis resolved in roughly 26 to 30% of people on resmetirom, compared with about 10% on placebo, and liver scarring improved by at least one stage in roughly 24 to 26%, compared with about 14% on placebo.1 Those are meaningful gains, and they made resmetirom the first drug to show this kind of benefit. Read them plainly, though: the majority of people on the drug did not hit either target at one year, and a notable share of the placebo group improved on lifestyle changes alone. So resmetirom improves the odds of reversing the liver damage; it does not reverse it in most people, and it works with lifestyle change rather than instead of it.
Who is resmetirom for?
The approval is specific, and the specificity matters. Resmetirom is for adults with MASH and moderate-to-advanced fibrosis, the stages doctors label F2 and F3, who do not yet have cirrhosis. It is meant to be used alongside diet and exercise, as an accelerant on the metabolic work rather than a replacement for it.
That leaves two groups outside its approval. People with only simple fatty liver or mild MASH, without significant scarring, were not the ones it was studied in, so it is not for early, low-risk disease. And people who already have cirrhosis, the F4 stage, are also outside the approval; the drug is being tested there separately. Identifying the F2-to-F3 middle band used to require a liver biopsy, and increasingly relies on noninvasive tests instead: a FibroScan, an MRI-based elastography, or blood-marker scores like FIB-4. Getting the staging right is what separates the people likely to benefit from those unlikely to.
The catch: approved on biopsy, not yet on outcomes
Resmetirom's approval, granted by the FDA in March 2024 and by European regulators in 2025, was an accelerated one. That is an important distinction. The trial proved that the drug improves the liver on biopsy, the inflammation and the scarring. It did not, and could not in one year, prove that this translates into fewer people progressing to cirrhosis, needing a transplant, or dying of liver disease.
Those hard outcomes are what the ongoing extension and follow-up trials are built to measure over several more years. The expectation, grounded in decades of evidence that reducing liver scarring improves long-term liver health, is that the biopsy benefit will carry through to clinical outcomes. But until those trials report, the fair statement is that resmetirom is proven to improve the liver's appearance and biology, with its effect on liver failure and survival still being confirmed. That is the normal trade-off of accelerated approval: earlier access to a needed drug, with the final proof arriving later.
Side effects, lipids, weight, and cost
Resmetirom is taken as a daily pill and is reasonably well tolerated. Its most common side effects are digestive, mainly diarrhea and nausea, usually mild and most noticeable early on, and they are the main reasons people stop. It carries a caution about gallbladder problems, including gallstones, and it interacts with statins: because it raises statin levels, the doses of certain statins are capped when the two are combined, which matters because many people with fatty liver are on a statin. It does not cause the racing heart or other symptoms of too much thyroid hormone at its approved dose.
Two features stand out. First, resmetirom lowers LDL cholesterol, by roughly 15%, along with other harmful lipids, a welcome bonus in a group at high cardiovascular risk, though this is a lipid improvement rather than proven protection against heart attacks. Second, it is weight-neutral: unlike the GLP-1 drugs, it does not cause weight loss, and its liver benefit happened independent of weight. That makes it a different kind of tool from semaglutide or tirzepatide, and the two approaches can be combined, treating the liver directly while a GLP-1 addresses weight and metabolism. The main practical barrier is cost: the drug runs around $47,000 a year, and access depends on insurance and correct fibrosis staging.
Guidance from the Clinic
Key Takeaways
- Resmetirom (Rezdiffra) is the first FDA-approved drug for MASH, approved in 2024, a once-daily pill that activates the liver's beta thyroid receptor to burn fat and reduce inflammation and scarring without the heart effects of thyroid hormone.
- In its pivotal trial, it resolved steatohepatitis in about 26 to 30% of people and improved scarring in about 24 to 26% at one year, better than placebo but a minority of those treated.
- It is approved specifically for moderate-to-advanced fibrosis (F2 to F3), used with diet and exercise, and is not for mild fatty liver or for cirrhosis.
- The approval was accelerated, based on biopsy improvement; proof that it prevents liver failure, transplant, and death is still being gathered in ongoing trials.
- It lowers LDL cholesterol as a bonus and is weight-neutral, so it complements GLP-1 drugs; the main drawbacks are digestive side effects, a statin interaction, and a cost near $47,000 a year.
Related at Fishtown Medicine
- Fatty Liver (MASLD/MASH) - the disease resmetirom treats, and the metabolic work underneath it
- Tirzepatide (Zepbound, Mounjaro) - the GLP-1-class drugs that improve MASH through weight loss
- Semaglutide (Wegovy) for MASH - the other approved MASH drug, working through weight and metabolism
- Survodutide (GLP-1/Glucagon) - the investigational glucagon drug with a dedicated MASH program
- Metabolic Health and Insulin Resistance - the root driver of fatty liver
- ApoB and Heart Health - the cardiovascular risk that travels with a fatty liver
- Ozempic vs Metformin - where the metabolic drugs fit
Scientific References
- Harrison SA, Bedossa P, Guy CD, et al. "A Phase 3, Randomized, Controlled Trial of Resmetirom in NASH with Liver Fibrosis." New England Journal of Medicine. 2024;390(6):497-509.
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