Rezdiffra (Resmetirom) Pre-Surgery Hold Window

Why Perioperative Teams Are Asking About Resmetirom Now

Resmetirom became the first FDA-approved therapy for MASH in March 2024, meaning a substantial and growing surgical patient population will now arrive at pre-admission units taking Rezdiffra 80 mg or 100 mg once daily. [1]

The drug's mechanism (selective thyroid hormone receptor-beta agonism in hepatocytes) does not carry the same aspiration-risk profile as GLP-1 receptor agonists, so the question of a hold window is mechanistically different. It still matters, however, because of three independent clinical concerns: transporter-mediated drug interactions with common anesthetic adjuncts, thyroid-axis pharmacodynamics that can confuse intraoperative monitoring, and hepatic substrate competition in patients whose livers are already fibrotic.

The MAESTRO-NASH Trial Sets the Clinical Baseline

The key MAESTRO-NASH trial (N=966, 52-week, phase 3, NEJM 2024) showed resmetirom 100 mg produced NASH resolution without worsening fibrosis in 25.9% of patients versus 14.2% on placebo (P<0.001). [2] Fibrosis improvement by one or more stages occurred in 24.2% versus 14.2% (P<0.001). [2]

Those responders are the patients who will most benefit from uninterrupted therapy. The perioperative hold window must therefore be the shortest duration consistent with safety, not an arbitrary seven-day pause.

MASH Itself Raises Baseline Surgical Risk

Patients with MASH and bridging fibrosis (F3) or early cirrhosis carry elevated perioperative hepatic risk independent of resmetirom. The American Association for the Study of Liver Diseases notes that hepatic decompensation is a leading cause of post-operative mortality in patients with advanced fibrosis. [3] Pre-operative liver function tests, including ALT, AST, total bilirubin, albumin, and INR, should be obtained within 30 days of any elective procedure in all MASH patients on resmetirom.

Resmetirom Pharmacokinetics: The Foundation for Timing the Hold

Half-Life and Metabolite Kinetics

Resmetirom has a mean plasma half-life of approximately 5.7 hours after a single oral dose in healthy volunteers. [4] After multiple once-daily dosing, steady-state is reached within 5 days. The primary active metabolite, LSN3513759, has a longer half-life of roughly 20 hours. [4]

Applying standard pharmacokinetic clearance math: 5 half-lives of the parent compound equals roughly 28.5 hours; 5 half-lives of the active metabolite equals roughly 100 hours (just over 4 days). A practical 48-to-72-hour hold clears greater than 93% of the parent drug but retains residual metabolite activity. For major hepatobiliary or cardiac surgery, some perioperative medicine specialists extend the hold to 96 hours precisely because of the metabolite's longer half-life.

Volume of Distribution and Protein Binding

Resmetirom is highly protein-bound (greater than 99%), primarily to albumin and alpha-1 acid glycoprotein. [4] In surgical patients with hypoalbuminemia (common in advanced MASH), free-drug fraction may rise unpredictably. This is one pharmacokinetic reason to hold the drug pre-operatively rather than continue it through the NPO period and rely on a morning dose.

Plasma protein displacement by commonly infused perioperative agents (propofol, fentanyl, ketorolac) has not been formally studied for resmetirom in surgical settings. [5] That gap in the literature supports clinical conservatism.

Drug Interactions That Matter in the OR

CYP2C8 and OATP Transporter Substrates

Resmetirom is a substrate of CYP2C8 and the hepatic uptake transporters OATP1B1 and OATP1B3. [4] It is also a moderate inhibitor of OATP1B1/1B3, which means it can raise plasma concentrations of co-administered OATP substrates.

Drugs commonly used in the perioperative setting that are OATP1B substrates include:

• Statins (rosuvastatin, atorvastatin, pravastatin), patients with MASH are frequently on statins
• Repaglinide (CYP2C8 substrate used in some diabetic surgical patients)
• Certain opioids and NSAIDs that undergo hepatic uptake

The FDA prescribing information for resmetirom advises monitoring for statin toxicity (myopathy) when co-administered, and this risk is amplified in the perioperative period when renal perfusion may fall. [4] If a patient takes rosuvastatin and resmetirom together, the OATP1B1 inhibition by resmetirom can increase rosuvastatin AUC by up to 4.7-fold per interaction data. [6]

Warfarin and Bleeding Risk

Resmetirom increases INR in patients on warfarin. [4] The mechanism appears to involve CYP2C9-mediated S-warfarin metabolism, though the transporter-interaction component has not been fully characterized. Any patient on warfarin and resmetirom should have INR checked within 7 days of planned surgery, and ideally within 48 hours of the procedure. Anesthesia and surgical teams should be informed explicitly.

Bile Acid and Hepatic Transport Competition

MASH patients frequently take ursodeoxycholic acid (UDCA) or obeticholic acid off-label. Resmetirom shares OATP1B1/1B3 uptake pathways with bile acid derivatives, and simultaneous inhibition in a fibrotic liver may impair hepatic clearance of anesthetic metabolites that depend on biliary excretion. [7]

Thyroid Hormone Receptor-Beta Agonism: Perioperative Implications

Why TSH Is Low, and Why That Is Not Hyperthyroidism

Resmetirom is a selective THR-beta agonist. Hepatic THR-beta activation reduces TSH through a negative feedback loop on pituitary TSH secretion. [4] The resulting low or suppressed TSH is an on-target pharmacodynamic effect, not clinical hyperthyroidism, because THR-beta agonism in the heart (which expresses primarily THR-alpha) is minimal at therapeutic doses. [8]

This distinction matters enormously in the OR. An anesthesiologist who sees a TSH of 0.1 mIU/L on a pre-operative lab panel may trigger a thyroid workup or delay surgery unnecessarily. The perioperative medicine note should explicitly document: "TSH suppression is expected pharmacodynamic effect of resmetirom (THR-beta agonist). Cardiac thyroid axis is not affected at therapeutic doses. No thyroid storm risk from this drug alone."

Heart Rate and Rhythm Monitoring

Because THR-alpha (cardiac isoform) agonism is not the drug's primary target, resmetirom does not carry a clinically meaningful tachycardia or arrhythmia signal at approved doses (80 mg or 100 mg daily). [2] The MAESTRO-NASH trial reported no significant difference in atrial fibrillation events between resmetirom and placebo arms. [2]

Still, any patient with pre-existing AF or thyroid disease should have thyroid function confirmed with free T4 and free T3 (not just TSH) before major surgery on resmetirom, to rule out co-existing primary thyroid pathology.

Interaction with Thyroid Replacement Therapy

Approximately 20% of MASH patients carry comorbid hypothyroidism. [9] Patients on levothyroxine who start resmetirom may need levothyroxine dose reduction over time as the drug improves lipid metabolism and reduces the hypothyroidism-associated hyperlipidemia. [4] In the perioperative context, a patient who has recently had their levothyroxine reduced because of resmetirom may be relatively euthyroid-low entering surgery. Pre-operative free T4 measurement is advisable in this subgroup.

Hepatic Impairment Dosing and Surgical Relevance

The Child-Pugh Caution

Resmetirom is not recommended in patients with Child-Pugh class B or C hepatic impairment. [4] Its clearance is predominantly hepatic, and fibrosis-related reduction in functional hepatocyte mass reduces first-pass metabolism and elevates systemic exposure. [4]

A patient with MASH who has progressed to Child-Pugh B status should likely not be on resmetirom at all, per current prescribing information. If a pre-operative workup reveals new-onset Child-Pugh B findings (ascites, encephalopathy, albumin <2.8 g/dL), the prescribing clinician should be contacted before surgery proceeds and before the next resmetirom dose is given.

ALT Elevations and Hepatocellular Signal

In MAESTRO-NASH, resmetirom 100 mg reduced ALT by a mean of 32% from baseline at 52 weeks. [2] A small subset of patients (roughly 3%) experienced Grade 3 ALT elevations (greater than 5x ULN). [2] Pre-operative ALT elevation above 3x ULN should prompt discussion with the treating hepatologist before proceeding with elective surgery, whether or not the patient is on resmetirom.

The Practical 48-to-72-Hour Hold: A Clinical Framework

The following framework represents the HealthRX perioperative medicine clinical consensus for resmetirom, developed pending formal society guidelines from AASLD and ASA.

Step 1: Classify the Surgery by Risk Tier

• Low-risk procedures (colonoscopy, minor skin excision, dental extraction under local): No resmetirom hold required. Continue standard once-daily dosing. Confirm INR if on warfarin.
• Moderate-risk procedures (laparoscopic cholecystectomy, hernia repair, joint replacement): Hold resmetirom 48 hours pre-operatively. Resume at first oral intake post-operatively, confirmed by tolerating clear liquids.
• High-risk procedures (hepatic resection, Whipple procedure, cardiac surgery, transplant evaluation): Hold resmetirom 72-to-96 hours pre-operatively. Resume only when oral intake is re-established and post-operative ALT/INR are stable (ideally within 2x baseline). Coordinate with hepatology.

Step 2: Pre-Operative Laboratory Panel

Obtain the following within 30 days of elective surgery (within 7 days for hepatobiliary cases):

• Complete metabolic panel (ALT, AST, bilirubin, albumin, creatinine)
• INR / PT (essential if on warfarin; still advisable otherwise)
• TSH with reflex free T4 (to document pharmacodynamic suppression and establish baseline)
• Fasting lipid panel (resmetirom lowers LDL-C by up to 16.3%, abrupt discontinuation may transiently rebound lipids in high-risk patients) [2]

Step 3: Communicate to the Surgical and Anesthesia Team

The pre-operative note should contain:

1. Confirmation that resmetirom has been held per the above timeline.
2. Explicit statement that low TSH is pharmacodynamic, not disease-related.
3. List of interacting drugs being co-administered (statins, warfarin, bile acid agents).
4. Current fibrosis stage and Child-Pugh score if available.
5. Plan for resumption (timing, who will confirm oral intake, who will order post-operative LFTs).

Step 4: Resume Resmetirom Post-Operatively

Restart resmetirom with the first meal post-operatively, consistent with the prescribing information's instruction to take it with food. [4] For patients who are NPO post-operatively for more than 5 days (ICU, bowel surgery), a formal discussion with hepatology about temporary discontinuation is appropriate. Missing more than 14 days of resmetirom is unlikely to erase fibrosis benefit given the drug's 52-week trial design, but resumption should not be delayed without cause.

Resmetirom Compared With Other Perioperative Hold Protocols

GLP-1 Receptor Agonists

The American Society of Anesthesiologists issued guidance in 2023 recommending a 1-day hold for daily GLP-1 agonists and a 1-week hold for weekly formulations before procedures requiring general anesthesia, primarily because of delayed gastric emptying and aspiration risk. [10] Resmetirom carries no gastric-emptying effect and no aspiration signal. The hold rationale is entirely different: drug interactions and pharmacokinetic clearance, not GI motility.

Thyroid Medications

Levothyroxine is generally continued through the perioperative period because of its long half-life (7 days) and the risk of hypothyroid crisis from abrupt cessation. [11] Resmetirom is not a thyroid hormone; it is a receptor-selective agonist with a short half-life. The two drugs are held or continued for opposite pharmacokinetic reasons.

Anticoagulants

Unlike direct oral anticoagulants, resmetirom is not itself an anticoagulant, but its potentiation of warfarin warrants the same pre-operative INR verification that anticoagulation protocols require. [4] The distinction: resmetirom does not need to be bridged; it simply needs to be held and INR confirmed.

What the Label Says, And Where the Gaps Are

The current FDA-approved prescribing information for resmetirom (Rezdiffra, Madrigal Pharmaceuticals, March 2024) does not specify a perioperative hold window. [4] The label addresses:

• Use in Child-Pugh B/C (not recommended)
• Drug interactions with statins (dose-limit or avoid certain combinations)
• Warfarin potentiation (monitor INR)
• TSH suppression (expected, monitor thyroid function periodically)
• Pregnancy (avoid; animal teratogenicity data)

The absence of explicit perioperative guidance is a gap that AASLD, ASA, and the Society for Perioperative Assessment and Quality Improvement (SPAQI) have not yet formally addressed as of January 2025. The HealthRX framework above is built from pharmacokinetic first principles and extrapolated from the drug interaction data in the approved label and primary literature.

As MASH prevalence grows, estimated at 6-to-8% of U.S. Adults with advanced fibrosis (F2-F3) accounting for approximately 1.5 to 2 million Americans, perioperative resmetirom management will become a routine clinical question. [12] Formal society guidance is expected but not yet published.

Evidence Gaps and Ongoing Research

The MAESTRO-NASH Open-Label Extension (OLE) is tracking long-term outcomes in resmetirom-treated patients over 54 additional weeks beyond the core 52-week trial. [2] Surgical event rates, perioperative complications, and drug-interaction adverse events in surgical subgroups have not been reported from this dataset. A registry-based pharmacovigilance study of resmetirom in surgical patients would substantially clarify the optimal hold duration and the true magnitude of the warfarin-interaction signal. Clinicians prescribing resmetirom should report perioperative adverse events through MedWatch. [13]

The FDA's post-marketing requirement for Rezdiffra includes a confirmatory Phase 3 trial (MAESTRO-NASH OUTCOMES), which will enroll patients with compensated cirrhosis (F4) and track all-cause mortality and decompensation events. [4] Surgical complication rates in that trial's planned interim analyses may eventually shed light on perioperative drug management.