Resmetirom
Definition
A thyroid hormone receptor beta agonist from Madrigal Pharmaceuticals. The first FDA-approved drug for MASH, given accelerated approval in March 2024.
Resmetirom
Resmetirom (brand name Rezdiffra) is a selective thyroid hormone receptor beta (THR-β) agonist developed by Madrigal Pharmaceuticals. The FDA granted it accelerated approval in March 2024 for adults with non-cirrhotic MASH (metabolic dysfunction-associated steatohepatitis, formerly NASH) with moderate to advanced liver fibrosis (F2-F3), used with diet and exercise. It was the first drug approved in the United States for MASH and set a regulatory precedent for the condition.
Mechanism: THR-β is found mostly in the liver, where it controls fat metabolism, mitochondrial function and cholesterol clearance. Resmetirom selectively activates the beta form (avoiding the heart and bone effects that run through THR-α). It increases fat burning in the liver, reduces new fat production and makes mitochondria work more efficiently. The result is less liver fat and, in clinical trials, resolution of steatohepatitis on biopsy and improvement in fibrosis.
Resmetirom matters for retatrutide for two reasons. First, it set the regulatory path for MASH approval: paired liver biopsy endpoints, accelerated approval based on resolution of steatohepatitis and improvement in fibrosis, and confirmatory long-term outcome data required afterward. Eli Lilly’s SYNERGY program is following this path for retatrutide in MASH.
Second, the two drugs act on the liver in related but different ways. Resmetirom clears liver fat through THR-β. Retatrutide’s glucagon component drives fat burning in the liver through a different pathway, alongside its GLP-1/GIP-driven weight loss (which lowers liver fat on its own). Whether the two mechanisms could add up, or whether the GLP-1 class’s greater weight loss makes them effectively redundant, is an open question for future trials.