Rezdiffra Situation: FDA Safety Signals Explained

Which safety issues does the Rezdiffra label actually flag?

Rezdiffra is an oral, liver-directed partial agonist of thyroid hormone receptor-beta, taken once daily at 80 mg for patients under 100 kg and 100 mg for patients at 100 kg or more [1]. The current label is effective July 29, 2026 [1]. The safety content sits in three places: warnings and precautions covering hepatotoxicity, gallbladder-related reactions and statin interactions; the adverse reactions tables from the clinical trials; and the drug interaction section covering statins and CYP2C8 inhibitors [1].

The FDA approved Rezdiffra on March 14, 2024 under the accelerated pathway, describing it as the first treatment for patients with liver scarring due to fatty liver disease [3].

Hepatotoxicity: what the label records

The hepatotoxicity warning rests on one patient in the resmetirom program who had normal ALT, AST and total bilirubin at baseline and received 80 mg daily. Liver biochemistries rose substantially and resolved when treatment was interrupted; after the drug was restarted, ALT peaked at 58 times the upper limit of normal, AST at 66 times and total bilirubin at 15 times, with no elevation of alkaline phosphatase. Immunoglobulin G was also elevated, suggesting drug-induced autoimmune-like hepatitis. The liver tests returned to baseline after hospitalization and discontinuation without any other intervention [1].

The label directs monitoring for liver test elevations and for symptoms and signs of hepatotoxicity, such as fatigue, nausea, vomiting, right upper quadrant pain or tenderness, jaundice, fever, rash and/or eosinophilia above 5%. If hepatotoxicity is suspected, the label says to discontinue Rezdiffra and continue monitoring the patient; if laboratory values return to baseline, the potential risks are weighed against the benefits of restarting [1].

In the 888-patient F2/F3 trial, liver test elevations appeared in every arm. ALT above 3 times the upper limit of normal occurred in 10% on placebo, 11% on 80 mg and 13% on 100 mg; ALT above 5 times the upper limit in 2% of all three arms; AST above 3 times the upper limit in 10%, 9% and 12% [1]. Mean ALT and AST rose by less than 1.5 times baseline during the first 4 weeks and returned to baseline around week 8 [1].

Gallbladder-related adverse reactions

Cholelithiasis, acute cholecystitis and obstructive pancreatitis (gallstone) were observed more often in Rezdiffra-treated patients than in placebo-treated patients, although the exposure-adjusted incidence rates for these events were below 1 per 100 person-years in all treatment arms [1]. The label states that if cholelithiasis is suspected, gallbladder diagnostic studies and appropriate clinical follow-up are indicated, and that if an acute gallbladder event such as acute cholecystitis is suspected, Rezdiffra treatment is interrupted until the event resolves [1].

Diarrhea, nausea and the other common reactions

Diarrhea, nausea, pruritus, vomiting, constipation, abdominal pain and dizziness are the reactions the label lists as common, meaning at least 5% of patients and more frequent than placebo [1]. The trial reported exposure-adjusted incidence rates per 100 person-years for placebo, 80 mg and 100 mg respectively: diarrhea 14, 23 and 33; nausea 9, 18 and 15; pruritus 4, 6 and 10; vomiting 4, 7 and 8; constipation 4, 5 and 8; abdominal pain 4, 5 and 7; dizziness 1, 4 and 4 [1].

Diarrhea started early. Median time to a diarrheal event was 39 days on placebo, 17 days on 80 mg and 6 days on 100 mg, and severity was mild to moderate, with a median duration of 9 days on placebo compared with 20 days on both resmetirom arms. Among patients with nausea, median time to the event was 85, 28 and 5 days, with a median duration of 17, 26 and 28 days [1]. Discontinuation for any adverse reaction occurred at 4, 5 and 8 per 100 person-years across the three arms, most often for diarrhea and nausea [1].

Urticaria and rash, which may reflect drug hypersensitivity, were reported at 0.2, 0.7 and 1.5 per 100 person-years for urticaria and 3, 3 and 5 per 100 person-years for rash across placebo, 80 mg and 100 mg [1]. Less common reactions reported more frequently than placebo, in under 5% of patients, included decreased appetite, flatulence, abnormal feces, dysgeusia, vertigo, arrhythmia, palpitations, depression, erythema, hypoglycemia, tendinopathy and abnormal uterine bleeding [1].

Statins: the interaction the label limits

Rezdiffra raised plasma concentrations of atorvastatin, pravastatin, rosuvastatin and simvastatin, and the label limits the daily statin dose rather than prohibiting the combination: rosuvastatin and simvastatin to 20 mg, pravastatin and atorvastatin to 40 mg [1]. It also says to monitor for statin-related adverse reactions including liver test elevation, myopathy and rhabdomyolysis [1].

Measured in healthy subjects at steady state on resmetirom 100 mg daily: simvastatin 20 mg peak concentration rose 1.4-fold and total exposure 1.7-fold; rosuvastatin 20 mg 1.8-fold and 1.4-fold; pravastatin 40 mg 1.3-fold and 1.4-fold; atorvastatin 20 mg peak concentration was unchanged and total exposure rose 1.4-fold [1]. In the 888-patient trial, 49% of patients were taking a statin at baseline [1]. This is not a contraindication [1].

CYP2C8 inhibitors, liver impairment and kidney impairment

Resmetirom is a CYP2C8 substrate. Concomitant use with a strong CYP2C8 inhibitor such as gemfibrozil is not recommended; with a moderate inhibitor such as clopidogrel, the resmetirom dose is reduced to 60 mg daily for patients under 100 kg and 80 mg daily for patients at 100 kg or more [1]. Clopidogrel raised resmetirom peak concentration 1.3-fold and total exposure 1.7-fold [1].

The label says to avoid Rezdiffra in decompensated cirrhosis, and its use-in-specific-populations section states that use should be avoided in moderate to severe hepatic impairment (Child-Pugh B or C) [1]. Exposure was 2.7-fold higher in Child-Pugh B and 19-fold higher in Child-Pugh C; no dose adjustment is specified for Child-Pugh A [1]. For kidney function, the recommended dose is the same in mild, moderate or severe impairment, though severe impairment (eGFR under 30) raised total exposure about 1.5-fold and peak concentration 1.3-fold [1].

Thyroid hormone changes on treatment

Free T4 fell by a mean of 2%, 13% and 17% at 12 months on placebo, 80 mg and 100 mg, with minimal change in active hormone T3 or in TSH and no clinical findings associated with the decrease [1]. Resmetirom is not thyroid hormone replacement [1].

What is not known

The label says there are no available human data on use in pregnancy; in pregnant rabbits at 3.5 times the maximum recommended dose, embryo-fetal effects occurred with maternal toxicity, and in rats at 37 times the maximum recommended dose, lower birthweight, more stillbirths and early postnatal deaths occurred alongside marked suppression of maternal T4, T3 and TSH [1]. There is no information on the presence of Rezdiffra in human or animal milk [1]. Safety and effectiveness have not been established in MASH cirrhosis or in children [1].