Rezdiffra (Resmetirom) Pregnancy & Lactation Safety

What Is Resmetirom and How Does It Work?

Resmetirom is a first-in-class, orally active, liver-directed thyroid hormone receptor-beta (THR-beta) agonist approved by the FDA on March 14, 2024, for adults with non-cirrhotic or compensated cirrhotic metabolic dysfunction-associated steatohepatitis (MASH) with moderate-to-advanced liver fibrosis (stages F1 through F4). Understanding the mechanism matters for pregnancy risk assessment because the thyroid hormone axis has direct effects on fetal development.

Selective THR-Beta Agonism

Thyroid hormones signal through two receptor subtypes. THR-alpha predominates in the heart, brain, and skeletal muscle. THR-beta predominates in the liver and is the primary mediator of hepatic lipid metabolism. Resmetirom binds THR-beta with approximately 28-fold selectivity over THR-alpha, concentrating activity in hepatocytes while reducing cardiac and neurological off-target effects seen with nonselective thyromimetics. The FDA pharmacology review documents this selectivity profile in the Rezdiffra prescribing information published on accessdata.fda.gov [1].

Hepatic Lipid Reduction Pathway

Once bound to THR-beta, resmetirom upregulates genes governing mitochondrial fatty acid beta-oxidation, reduces expression of genes driving de novo lipogenesis, and lowers very-low-density lipoprotein (VLDL) secretion. The net effect is a reduction in intrahepatic triglyceride accumulation, which drives MASH pathology. This mechanism is distinct from GLP-1 receptor agonists and FXR agonists. Published pharmacodynamic data in NEJM 2024 confirm dose-dependent reductions in liver fat fraction by magnetic resonance imaging proton density fat fraction (MRI-PDFF) [2].

Why Mechanism Matters for Reproductive Safety

Thyroid hormone signaling is indispensable for fetal neurodevelopment, cardiac morphogenesis, and skeletal maturation. Even a drug with preferential THR-beta selectivity cannot guarantee complete exclusion of fetal THR-alpha activity if placental transfer occurs. Fetal THR-beta expression is also detectable in hepatic tissue by gestational week 8 to 10 in humans, according to data from developmental biology studies indexed at pubmed.ncbi.nlm.nih.gov [3]. The theoretical risk of disrupting fetal thyroid hormone signaling forms the biological rationale for the contraindication.

MAESTRO-NASH Trial: Efficacy Data That Established the Indication

The MAESTRO-NASH trial is the key registration study for resmetirom. Published in the New England Journal of Medicine in 2024, it enrolled 966 adults with biopsy-confirmed MASH and liver fibrosis stage F1 to F3 [2].

Primary Endpoints and Results

Two co-primary histological endpoints were assessed at 52 weeks. The first was NASH resolution (defined as NAS score of 0 to 1 with no worsening of fibrosis). The second was fibrosis improvement of at least one stage with no worsening of NASH. Resmetirom 80 mg met both endpoints, with 25.9% of patients achieving NASH resolution versus 9.7% on placebo (P<0.001). At the 100 mg dose, 29.9% achieved NASH resolution versus the same placebo rate [2].

Fibrosis improvement of at least one stage occurred in 24.2% of patients on 80 mg and 25.9% on 100 mg, compared with 14.2% on placebo (P<0.001 for both doses) [2]. These results represented the first time any pharmacological agent met both histological co-primary endpoints in a Phase 3 MASH trial.

Trial Population: Reproductive-Age Women

Approximately 52% of MAESTRO-NASH participants were female. Women of childbearing potential were required to use contraception and undergo pregnancy testing before enrollment. Any participant who became pregnant was withdrawn from the trial. This enrollment restriction means the trial generated no efficacy or safety data in pregnant women, which is a direct contributor to the FDA labeling contraindication.

Secondary Lipid and Biomarker Outcomes

Resmetirom 100 mg reduced LDL-C by a mean of 16.3% and triglycerides by 22.6% at 24 weeks [2]. Liver stiffness measured by vibration-controlled transient elastography (VCTE) also decreased significantly. These lipid effects are relevant to pregnancy safety discussions because large shifts in maternal lipoprotein metabolism could compound the physiological hyperlipidemia of pregnancy if the drug were inadvertently used during gestation.

FDA Pregnancy Category and Official Labeling Language

The FDA approved Rezdiffra under the current Pregnancy and Lactation Labeling Rule (PLLR), which replaced the older A/B/C/D/X letter categories in 2015. The prescribing information carries explicit risk statements rather than a letter grade [1].

Animal Embryo-Fetal Toxicity Studies

The Rezdiffra prescribing information reports that oral administration of resmetirom to pregnant rats and rabbits at clinically relevant exposures caused embryo-fetal toxicity [1]. In rats, doses producing exposures 1.6 times the human area under the curve (AUC) at the 100 mg clinical dose caused reduced fetal body weight and skeletal ossification delays. In rabbits, doses producing exposures approximately 0.6 times the human AUC caused increased post-implantation loss. These findings at sub-to-near clinical exposures signal meaningful teratogenic potential that cannot be dismissed, even without confirmatory human data.

The FDA's reproductive toxicology review, accessible via accessdata.fda.gov [4], details the dose-finding methodology and the specific skeletal endpoints observed in rat fetuses. Delayed ossification of vertebral centra and sternebrae were the most consistently reported findings.

No Human Pregnancy Data

As of the 2025 labeling, zero published case reports, cohort studies, or registry data describe resmetirom use in human pregnancy. The drug was approved less than 24 months ago, and the mandatory contraception protocol during MAESTRO-NASH precluded incidental exposures in the trial. The prescribing information states: "There are no available data on the use of Rezdiffra in pregnant women to evaluate for a drug-associated risk of major birth defects, miscarriage, or other adverse maternal or fetal outcomes" [1].

This absence of human data does not mean the drug is safe in pregnancy. Combined with positive animal findings, the FDA default position is to label the drug as contraindicated in pregnancy [1].

Contraception Requirements

The prescribing information instructs clinicians to verify a negative pregnancy test before initiating resmetirom in females of reproductive potential. Effective contraception must be used during treatment. The label does not specify a washout period after discontinuation, though resmetirom has a terminal half-life of approximately 5.5 days, suggesting plasma clearance to below quantifiable limits within 4 to 5 half-lives (roughly 27 to 28 days) for most patients [1]. Clinicians commonly apply a 30-day washout before permitting attempted conception, though this has not been formally studied.

Lactation Safety: What the Data (and Lack of Data) Show

No studies have evaluated resmetirom transfer into human breast milk, its effects on milk production, or its effects on the breastfed infant [1]. Animal lactation data are similarly absent from the public label. Given the hepatic concentration mechanism and the drug's lipophilicity (which generally predicts higher milk-to-plasma ratios for lipophilic small molecules), passive diffusion into breast milk is plausible but unquantified.

Infant Risk Considerations

Neonatal and infant THR-beta is expressed in the liver from early postnatal life. If resmetirom were to reach an infant through breast milk, even at low concentrations, the potential for disrupting normal neonatal thyroid hormone receptor signaling in a rapidly developing liver cannot be excluded. Published pharmacokinetic modeling of lipophilic small molecules and lactation, detailed in a review indexed at pubmed.ncbi.nlm.nih.gov [5], suggests oral bioavailability in the infant and the drug's protein binding characteristics (resmetirom is greater than 99% protein-bound) both affect relative infant dose calculations. At greater than 99% protein binding, free drug concentrations in plasma are very low, which may limit milk transfer somewhat. However, this theoretical mitigation is insufficient to recommend breastfeeding during resmetirom treatment.

Clinical Recommendation

The prescribing information advises that breastfeeding should be discontinued during resmetirom treatment [1]. Clinicians counseling patients should frame this as a standard precaution applied across all drugs lacking lactation safety data when the drug carries animal developmental toxicity signals.

MASH During Pregnancy: The Underlying Disease Context

MASH does not disappear during pregnancy. Women with pre-existing MASH face potential worsening of hepatic steatosis during gestation due to physiological insulin resistance, hyperlipidemia, and weight gain. A review published at pubmed.ncbi.nlm.nih.gov [6] describes how NAFLD/MASH in pregnancy associates with increased risk of gestational diabetes, preeclampsia, and preterm birth.

No Approved Pharmacological MASH Treatment in Pregnancy

As of 2025, no pharmacological treatment for MASH carries a favorable safety profile in pregnancy. Obeticholic acid (Ocaliva), an FXR agonist approved for primary biliary cholangitis, carries a similar embryo-fetal toxicity signal in animals. Vitamin E (800 IU/day), recommended by the American Association for the Study of Liver Diseases (AASLD) as an option in non-diabetic MASH, has a more established safety record in pregnancy but no formal MASH-specific trial data in pregnant populations. The AASLD Practice Guidance on NAFLD/MASH is available at pubmed.ncbi.nlm.nih.gov [7].

Non-Pharmacological Management During Pregnancy

Weight management during pregnancy should follow obstetric guidelines rather than MASH-specific protocols. The Institute of Medicine gestational weight gain guidelines (endorsed by ACOG, accessible at acog.org) [8] recommend 5 to 9 kg total gain for obese women (BMI <30 or greater), which directly benefits hepatic steatosis through caloric restriction relative to non-pregnant recommendations. Resistance to carbohydrates, avoidance of fructose-sweetened beverages, and Mediterranean dietary patterns remain safe and evidence-supported throughout pregnancy, as reviewed at pubmed.ncbi.nlm.nih.gov [9].

Pharmacokinetics Relevant to Reproductive Planning

Understanding resmetirom's pharmacokinetics helps clinicians advise patients on timing of conception attempts and management of accidental exposure.

Absorption and Distribution

Resmetirom reaches peak plasma concentration (Tmax) approximately 4 hours after oral dosing. The volume of distribution at steady state is approximately 164 liters, indicating extensive tissue distribution [1]. High tissue distribution increases the theoretical time to complete drug elimination from body compartments beyond what plasma half-life alone predicts. Published pharmacokinetic data are summarized in the FDA clinical pharmacology review at accessdata.fda.gov [10].

Metabolism and Elimination

Resmetirom is primarily metabolized by CYP2C8 and, to a lesser extent, CYP3A4. Its major circulating metabolite, RC-1, is pharmacologically active and has a longer half-life than the parent compound. The prescribing information does not specify the half-life of RC-1 in detail, but its presence means total pharmacological exposure persists beyond what parent drug plasma levels indicate [1]. This is clinically relevant: a 30-day contraception continuation after the last dose is a conservative but reasonable approach, given that RC-1 clearance lags behind resmetirom.

Drug Interactions Relevant to Pregnancy Planning

Resmetirom is a moderate inhibitor of OATP1B1 and OATP1B3 hepatic uptake transporters. Co-administration with statins (substatin: rosuvastatin, pravastatin) increases statin plasma concentrations [1]. Women with MASH are frequently on statins, and statins carry their own FDA pregnancy category X (old labeling) or equivalent PLLR warning based on fetal cholesterol synthesis concerns. Statin discontinuation is typically recommended at pregnancy confirmation, which adds another management consideration for women transitioning off resmetirom before conception.

Thyroid Function Monitoring: Implications for Pregnancy Planning

THR-beta agonism by resmetirom produces measurable changes in thyroid function tests. Published data show resmetirom reduces thyroid-stimulating hormone (TSH) modestly (mean reduction of approximately 0.4 mIU/L in MAESTRO-NASH) and reduces total T3 and free T4 via feedback suppression, consistent with a thyromimetic effect [2]. These changes are generally within normal reference ranges at clinical doses.

Preconception Thyroid Assessment

Women planning conception after resmetirom discontinuation should have TSH reassessed 4 to 6 weeks after the last dose, because thyroid axis suppression during treatment may mask underlying subclinical hypothyroidism. Maternal hypothyroidism during the first trimester, before fetal thyroid function is established (approximately gestational week 12), is a known risk factor for impaired fetal neurodevelopment. The American Thyroid Association recommends TSH below 2.5 mIU/L in women planning pregnancy, as detailed in guidelines available at pubmed.ncbi.nlm.nih.gov [11].

A Preconception Transition Protocol for Women on Resmetirom

No formal guideline exists for transitioning women off resmetirom before conception. Based on the pharmacokinetic data above and standard reproductive toxicology practice, a reasonable clinical framework includes:

1. Confirm MASH stability (ALT, AST, FIB-4) 3 months before planned discontinuation.
2. Stop resmetirom at least 30 days (approximately 5 to 6 half-lives of parent drug plus RC-1) before attempting conception.
3. Check TSH 4 weeks after last dose. Treat subclinical hypothyroidism (TSH greater than 2.5 mIU/L) before conception.
4. Discuss non-pharmacological MASH management for the duration of pregnancy and lactation.
5. Resume resmetirom after weaning, once breastfeeding has been fully discontinued and contraception is reinstated if the patient is not planning another pregnancy.

This framework has not been validated in a prospective study and should be individualized with obstetrics and hepatology co-management.

Accidental Exposure in Pregnancy: What to Do

Given that MASH affects women of reproductive age and resmetirom prescribing is expanding rapidly after FDA approval, accidental first-trimester exposure will occur in clinical practice.

Immediate Steps

If a patient becomes pregnant while on resmetirom, the drug should be discontinued immediately. Pregnancy should be confirmed by serum beta-hCG and dated by ultrasound. The patient should be referred to maternal-fetal medicine for counseling and to establish a surveillance plan for fetal growth and anatomy.

Reporting Requirements

Madrigal Pharmaceuticals has not established a formal pregnancy exposure registry as of early 2025. Clinicians should report accidental exposures via the FDA MedWatch program at fda.gov [12] and encourage patients to enroll in the MotherToBaby program (a North American registry for pregnancy exposures to medications), accessible at pubmed.ncbi.nlm.nih.gov [13], which collects prospective outcome data. These reports contribute to post-marketing safety databases and may eventually inform an updated label.

Prognosis After Accidental Exposure

No human outcome data exist. Animal data showed toxicity at exposures at or below clinical AUC, but the relevance to human teratogenicity is uncertain. Spontaneous resolution of early MASH inflammation during pregnancy (driven by immunological tolerance adaptations) has been described in case series indexed at pubmed.ncbi.nlm.nih.gov [6], meaning the underlying disease may pose less acute risk during gestation than it would in a non-pregnant state. The fetal risk from resmetirom exposure, by contrast, remains uncharacterized.

Guideline Positions on MASH Management in Reproductive-Age Women

The AASLD 2023 Practice Guidance on NAFLD and MASH does not specifically address resmetirom in pregnancy because the guidance predates FDA approval. The general statement in the AASLD guidance reads: "Weight loss through lifestyle intervention remains the most effective strategy for NAFLD/MASH in all populations, including those planning pregnancy" [7]. This implicitly applies to resmetirom candidates as well.

The European Association for the Study of the Liver (EASL) 2024 Clinical Practice Guidelines on MASH (available at pubmed.ncbi.nlm.nih.gov [14]) state that no pharmacological MASH treatment has established safety in pregnancy and that all approved agents should be withheld during gestation and lactation pending specific safety data. This aligns with the FDA labeling position for resmetirom.

The Endocrine Society's 2019 clinical practice guideline on thyroid disease in pregnancy (available at pubmed.ncbi.nlm.nih.gov [15]) does not address thyromimetic drugs specifically but affirms that any agent altering thyroid hormone receptor signaling carries theoretical reproductive risk and warrants precautionary use restriction until safety data are available.