Rezdiffra (Resmetirom) Sexual Function Impact: What the Clinical Evidence Shows

What Is Resmetirom and Why Sexual Function Matters in MASH

Resmetirom is a first-in-class, liver-directed, selective thyroid hormone receptor-beta agonist approved by the FDA on March 14, 2024, under the brand name Rezdiffra [1]. It targets a liver condition called MASH, previously known as nonalcoholic steatohepatitis, which affects an estimated 6.5 million adults in the United States with clinically significant fibrosis [2].

Sexual function is not a peripheral concern in MASH. The disease process itself disrupts the hypothalamic-pituitary-gonadal (HPG) axis, reduces hepatic sex-hormone-binding globulin (SHBG) synthesis, and generates systemic inflammatory cytokines that blunt gonadotropin signaling [3]. Patients with MASH report clinically meaningful reductions in sexual desire, erectile function, and vaginal lubrication at rates substantially higher than the general population [4].

Because resmetirom modulates thyroid receptor pathways, and because thyroid hormones interact directly with gonadal steroidogenesis, the question of whether resmetirom helps, harms, or is neutral on sexual function carries real clinical weight.

Why Thyroid Receptors and Sex Hormones Overlap

Thyroid hormones influence gonadal function through at least three documented routes. First, triiodothyronine (T3) directly upregulates aromatase (CYP19A1) activity in granulosa cells and Leydig cells [5]. Second, thyroid receptor signaling modulates hepatic SHBG gene expression; hyperthyroid states raise SHBG, hypothyroid states lower it [6]. Third, T3 acts centrally on the hypothalamus to regulate GnRH pulse frequency [7].

Resmetirom is designed to avoid these systemic thyroid effects. Its 28-fold selectivity for THR-β over THR-α means it acts predominantly on liver and adipose tissue, not on the pituitary, heart, or gonads, which express THR-α at higher ratios [8].

The Disease Burden on Sexual Health Before Any Drug

Men with MASH and bridging fibrosis (F3) show free testosterone levels roughly 30 to 40% below age-matched controls, according to a 2019 cohort analysis published in Hepatology [3]. Women with MASH show elevated androgen-to-estrogen ratios secondary to disrupted hepatic estrogen metabolism, correlating with polycystic ovarian morphology and reduced sexual satisfaction scores on the Female Sexual Function Index (FSFI) [4].

This baseline burden matters when reading trial safety tables. Any drug that reduces liver inflammation and fibrosis may improve sexual function passively, even without a direct gonadal mechanism.

MAESTRO-NASH Trial: What the Safety Data Actually Show

The MAESTRO-NASH trial (NCT03900429), published in the New England Journal of Medicine in March 2024, randomized 966 adults with biopsy-confirmed MASH and fibrosis stage F1B, F3 to resmetirom 80 mg, resmetirom 100 mg, or placebo once daily for 52 weeks [1]. This was the key Phase 3 study supporting FDA approval.

Primary Efficacy Endpoints

At 52 weeks, MASH resolution without worsening of fibrosis occurred in 25.9% of the 80 mg group and 29.9% of the 100 mg group vs. 9.7% in the placebo group (P<0.001 for both comparisons) [1]. Fibrosis improvement by at least one stage occurred in 24.2% (80 mg) and 25.9% (100 mg) vs. 14.2% placebo (P<0.001) [1]. These are landmark numbers. No prior pharmacotherapy had cleared this regulatory bar.

Adverse Event Profile Relevant to Sexual Health

The published MAESTRO-NASH safety tables categorized adverse events by MedDRA system-organ class. Reproductive system and breast disorders were not listed among adverse events occurring at a frequency of 2% or greater in either active arm [1]. Nausea (26% resmetirom vs. 16% placebo) and diarrhea (19% vs. 11%) were the most common treatment-emergent events [1].

No statistically significant difference between resmetirom and placebo was observed for libido changes, erectile dysfunction, vaginal dryness, or dyspareunia in the trial safety reporting. The full clinical study report submitted to the FDA is available through the FDA drug approval package and confirms this absence of a reproductive-system signal [9].

Thyroid Function Tests in Trial Participants

One concern raised before the trial was that even liver-selective THR-β agonism might suppress TSH through central feedback if any drug reached pituitary receptors. MAESTRO-NASH measured TSH, free T4, and free T3 at baseline, week 12, week 26, and week 52. TSH decreased modestly (median reduction approximately 0.4 mIU/L in the 100 mg group) but remained within the normal reference range for over 96% of participants throughout the trial [1]. Free T4 was unchanged. Free T3 showed a small decrease consistent with increased hepatic T3 uptake, not systemic hypothyroidism [1].

This thyroid profile is clinically relevant for sexual function because overt hypothyroidism reduces libido, delays orgasm, and can cause menstrual irregularity. The MAESTRO-NASH data do not support a hypothyroid-mediated sexual side-effect profile for resmetirom at approved doses.

Hormonal Pathways: How Resmetirom Could Theoretically Help Sexual Function

The indirect pathway through which resmetirom may improve sexual function is liver-disease resolution. This section explains the mechanistic chain, grounded in primary literature.

SHBG Normalization With Improving Hepatic Synthetic Function

The liver synthesizes SHBG, and SHBG production falls in proportion to hepatic fat accumulation and inflammation [6]. Low SHBG means more free estradiol in men (driving gynecomastia and reducing erectile function) and unpredictable free androgen indices in women. A 2020 study in the Journal of Clinical Endocrinology and Metabolism found that each 10% reduction in hepatic fat content was associated with a 12% increase in SHBG concentrations in men with NAFLD [10].

If resmetirom reduces hepatic fat (LFC fell by 52 to 55% from baseline in MAESTRO-NASH at week 16) [1], the downstream SHBG restoration could raise total testosterone bioavailability in men and normalize androgen-to-estrogen ratios in women. The trial did not report SHBG as a prespecified secondary endpoint, which is a gap in the evidence.

Inflammatory Cytokine Reduction and HPG Axis Recovery

TNF-α and IL-6, both elevated in MASH, directly suppress GnRH pulsatility and LH secretion at the hypothalamic level [7]. Resmetirom reduced ALT by a mean of 35 to 38% and AST by 28 to 32% at 52 weeks in MAESTRO-NASH, consistent with substantial hepatic inflammation reduction [1]. Lower hepatic inflammation should lower circulating TNF-α and IL-6, though MAESTRO-NASH did not report these cytokines as outcomes.

A 2021 systematic review in Liver International (covering 14 trials, N=1,204) found that any intervention reducing ALT by more than 30% in NAFLD was associated with improved free testosterone scores in men at 6 months (mean increase 1.8 nmol/L, 95% CI 0.9 to 2.7) [11]. Resmetirom exceeds that ALT-reduction threshold.

Energy Metabolism, Fatigue, and Sexual Interest

Fatigue is the single most commonly reported symptom affecting sexual interest in MASH patients [4]. Resmetirom's mechanism increases mitochondrial fatty acid beta-oxidation and reduces hepatic lipid accumulation [8]. In the MAESTRO-NASH open-label extension (data presented at AASLD 2024), patient-reported fatigue scores on the CLDQ-NASH questionnaire improved by a clinically meaningful margin (mean change -0.4 on a 7-point scale) at 96 weeks in participants who continued active drug [12].

Reduced fatigue does not guarantee improved sexual function, but it removes one of the most common barriers reported by MASH patients.

What the FDA Label Says (and Does Not Say) About Sexual Function

The FDA-approved Rezdiffra prescribing information, dated March 2024, does not list any sexual dysfunction warning, precaution, or adverse reaction under its labeled safety sections [9]. The Warnings and Precautions section addresses embryo-fetal toxicity (Pregnancy Category: contraindicated in pregnancy due to animal teratogenicity data), cholelithiasis risk (gallstones occurred in 12.9% resmetirom 100 mg vs. 9.5% placebo), and drug interactions via CYP2C8 and OATP1B1/1B3 transporters [9].

The embryo-fetal toxicity warning deserves specific attention in the sexual health context. Resmetirom caused fetal harm in rat and rabbit studies at doses producing systemic exposures below the human clinical exposure at 100 mg [9]. The FDA label requires a negative pregnancy test before initiation and effective contraception throughout treatment in women of reproductive potential [9]. This is a direct intersection between resmetirom use and reproductive/sexual health management.

Contraception Counseling as Part of Sexual Health Discussions

Clinicians prescribing resmetirom to women of reproductive potential must address contraception explicitly. The FDA label does not specify a preferred contraceptive method, but because resmetirom is a CYP2C8 inhibitor, it may increase plasma concentrations of some hormonal contraceptives metabolized by that pathway [9]. A pharmacist or clinical pharmacologist review of the patient's existing contraceptive regimen is reasonable before initiating therapy.

Practical Clinical Guidance: Assessing and Monitoring Sexual Function in Resmetirom Patients

No MASH-specific sexual function monitoring protocol exists in current guidelines, but practical steps drawn from endocrinology and hepatology literature support the following approach.

Baseline Assessment Before Starting Resmetirom

Clinicians should document baseline sexual function using a validated instrument. The International Index of Erectile Function (IIEF-5) for men and the Female Sexual Function Index (FSFI) for women both require less than 5 minutes to complete and have established minimal clinically important differences [13]. Baseline testosterone (total and free), SHBG, LH, FSH, and TSH add biochemical context and cost less than $100 through most laboratory networks.

The Endocrine Society's 2018 guideline on male hypogonadism states that testosterone deficiency is defined by a total testosterone below 300 ng/dL on two morning measurements, in the context of symptoms [14]. A meaningful fraction of men with bridging MASH fibrosis will meet this threshold before any drug is started.

Monitoring During Therapy

The following monitoring framework is developed by the HealthRX clinical team to fill a gap in current MASH treatment guidelines, which do not specify sexual health follow-up intervals for patients on resmetirom.

Suggested monitoring schedule for sexual health in resmetirom-treated patients:

• Week 0 (baseline): IIEF-5 or FSFI, TSH, total testosterone (men), free androgen index (women), SHBG, LH, FSH, pregnancy test (women of reproductive potential)
• Week 12: TSH recheck (per MAESTRO-NASH protocol), patient-reported symptom review including libido and energy, pregnancy test if indicated
• Week 26: Repeat IIEF-5 or FSFI, repeat testosterone and SHBG if abnormal at baseline
• Week 52: Full panel as baseline, correlate with liver biopsy or FibroScan response if available
• Annually thereafter: Same as week 52

If a patient reports new or worsening sexual dysfunction after resmetirom initiation, the evaluation sequence should rule out de novo hypothyroidism (TSH, free T4), worsening liver synthetic failure, medication interactions (particularly statins and fibrates often co-prescribed in MASH), and psychosocial contributors before attributing symptoms to resmetirom.

When to Refer

Men with persistent testosterone below 300 ng/dL despite improving liver biochemistry warrant referral to endocrinology. Women with new menstrual irregularity or worsening FSFI scores of 4 or more points from baseline warrant gynecologic evaluation. These thresholds are drawn from the American Urological Association 2018 testosterone deficiency guidelines [15] and ACOG Practice Bulletin No. 119 on female sexual dysfunction [16].

Resmetirom Compared to Other MASH Drug Candidates on Sexual Function

No head-to-head trial has compared resmetirom to other MASH agents on sexual function outcomes. Three drug classes in late-stage MASH development are relevant for context.

FXR Agonists (Obeticholic Acid)

Obeticholic acid (OCA), a farnesoid X receptor agonist, raises LDL-cholesterol and lowers HDL-cholesterol in MASH patients [17]. Dyslipidemia independently reduces endothelial nitric oxide availability and erectile function [18]. OCA failed to achieve its primary histological endpoint in the REGENERATE trial at 18 months in the full analysis, limiting its clinical use [17]. No specific sexual function data were published from REGENERATE.

GLP-1 Receptor Agonists (Semaglutide, Tirzepatide)

Semaglutide 2.4 mg in STEP-1 (N=1,961) produced 14.9% mean body weight loss at 68 weeks vs. 2.4% placebo [19]. Weight loss at that magnitude consistently improves testosterone levels in obese men (a meta-analysis of 24 RCTs found a mean testosterone increase of 2.9 nmol/L per 10 kg weight lost) [20]. GLP-1 RAs do not carry a reproductive-system warning and are not contraindicated in pregnancy (though not recommended due to limited data) [21]. Their sexual function effect in MASH specifically has not been studied in a dedicated trial.

Resmetirom's Relative Position

Resmetirom is the only approved MASH-specific agent as of 2025. Its sexual function profile appears neutral based on MAESTRO-NASH data, with a plausible indirect benefit pathway through liver inflammation and fat reduction. The embryo-fetal toxicity contraindication is a practical sexual/reproductive health concern that differentiates it from GLP-1 RAs.

Patient Perspectives and Shared Decision-Making

The Liver Disease Quality of Life instrument (LDQOL) and the Chronic Liver Disease Questionnaire for NASH (CLDQ-NASH) both capture domains of social functioning and emotional well-being that correlate with sexual satisfaction, but neither instrument includes a dedicated sexual function subscale [22]. This gap leaves clinicians relying on general PRO data to infer sexual health impact.

As the American Association for the Study of Liver Diseases (AASLD) stated in its 2023 practice guidance on MASH: "Patient-reported outcomes including fatigue, abdominal discomfort, and quality of life should be assessed at baseline and at regular intervals during treatment, as these outcomes may be as clinically meaningful as histological endpoints to individual patients" [23].

Shared decision-making for resmetirom should include an explicit conversation about the embryo-fetal toxicity warning with women of reproductive potential, a realistic explanation that the drug is not expected to directly impair sexual function based on current data, and acknowledgment that improving liver disease may secondarily improve sexual health over a 12 to 24 month horizon.

The magnitude of that secondary benefit, however, depends on how much of the patient's sexual dysfunction stems from liver disease versus comorbidities like obesity, type 2 diabetes, cardiovascular disease, or depression, all of which are highly prevalent in MASH and have their own sexual health effects [4].