Resmetirom

Resmetirom

Resmetirom (brand name Rezdiffra) is a selective thyroid hormone receptor beta (THR-β) agonist developed by Madrigal Pharmaceuticals. The FDA granted accelerated approval in March 2024 for the treatment of adults with non-cirrhotic MASH (metabolic dysfunction-associated steatohepatitis, formerly NASH) with moderate to advanced liver fibrosis (F2-F3), in conjunction with diet and exercise. It was the first drug approved in the United States for MASH and established a regulatory precedent for the indication.

Mechanism: THR-β is expressed predominantly in the liver, where it regulates lipid metabolism, mitochondrial function, and cholesterol clearance. Resmetirom selectively activates the beta isoform (avoiding cardiac and bone effects mediated by THR-α), increasing hepatic fatty acid oxidation, reducing de novo lipogenesis, and improving mitochondrial efficiency. The result is reduced liver fat content and, in clinical trials, histological resolution of steatohepatitis and improvement in fibrosis.

Resmetirom’s existence is relevant to retatrutide for two reasons. First, it establishes the regulatory pathway for MASH approval — paired liver biopsy endpoints, accelerated approval based on histological resolution and fibrosis improvement, with confirmatory long-term outcomes data required. This is the pathway Eli Lilly’s SYNERGY program is pursuing for retatrutide in MASH.

Second, resmetirom and retatrutide work through complementary but distinct hepatic mechanisms. Resmetirom drives hepatic lipid clearance via THR-β. Retatrutide’s glucagon receptor component drives hepatic fatty acid oxidation through a different pathway, alongside its GLP-1/GIP-mediated weight loss (which independently reduces liver fat). Whether these mechanisms could be additive or whether the GLP-1-class superiority in producing weight loss makes them effectively redundant is an open question that future trials may address.