MASH (Formerly NASH): Causes, Diagnosis, Stages, and Approved Treatments

What Is the Difference Between MASLD, MASH, and the Old NAFLD/NASH Names?

The terminology changed in 2023 after a global multi-society Delphi process agreed that "nonalcoholic" framing was both stigmatizing and scientifically imprecise. NAFLD became MASLD (metabolic dysfunction-associated steatotic liver disease), and NASH became MASH (metabolic dysfunction-associated steatohepatitis). The disease biology did not change. Only the labels did.

MASLD is the broad parent category. It describes fat accumulation in the liver (hepatic steatosis, defined as fat in ≥5% of hepatocytes) in a person who meets at least one of five cardiometabolic criteria: overweight or obesity, prediabetes or type 2 diabetes, elevated blood pressure, elevated triglycerides, or low HDL cholesterol [1]. An estimated 32.4% of the global adult population meets MASLD criteria [2].

MASH is the subset of MASLD where the liver also shows active inflammation and hepatocyte ballooning on biopsy, scored using the NAFLD Activity Score (NAS). This inflammatory state is what drives scarring. Simple steatosis (MASLD without hepatitis) rarely progresses to cirrhosis on its own, but MASH progresses to cirrhosis in roughly 20% of patients over a decade [3].

The old term "cryptogenic cirrhosis", cirrhosis with no apparent cause, is now understood to represent burned-out MASH in a large proportion of cases. Fat disappears from the liver as fibrosis worsens, which is why end-stage MASH can look deceptively "clean" on biopsy.

Who Gets MASH and Why?

MASH does not develop randomly. It clusters tightly around insulin resistance and specific metabolic co-conditions.

Type 2 diabetes is the strongest single predictor. Among people with T2D, MASLD prevalence reaches 55 to 70%, and roughly one-quarter of those have MASH [4]. Obesity, especially visceral adiposity, drives hepatic fat accumulation through excess free fatty acid delivery to the liver via the portal vein. When the liver cannot oxidize or export the excess fat quickly enough, lipotoxic intermediates accumulate and trigger an inflammatory cascade involving Kupffer cells, toll-like receptor 4 signaling, and mitochondrial dysfunction.

Genetics also matter. The PNPLA3 rs738409 variant (encoding the I148M substitution in patatin-like phospholipase domain-containing protein 3) roughly doubles MASH risk and triples the odds of advanced fibrosis. The TM6SF2 rs58542926 variant and the HSD17B13 rs72613567 variant modify risk in opposite directions, the HSD17B13 loss-of-function variant is actually protective against MASH progression [5].

Sex and age interact with metabolic risk. Men develop MASH at lower BMI thresholds than women during reproductive years, but postmenopausal women close that gap rapidly. The American Association for the Study of Liver Diseases (AASLD) 2023 guidance notes that MASH prevalence rises steeply after age 50 in both sexes regardless of BMI trajectory [6].

Hormonal contributors are increasingly recognized. Testosterone deficiency in men associates with higher rates of hepatic steatosis and insulin resistance. A 2020 meta-analysis of nine studies found that men with hypogonadism had a 2.4-fold higher odds of hepatic steatosis compared with eugonadal men (P<0.01) [7]. This relationship is one reason TRT (testosterone replacement therapy) is being studied as a metabolic adjunct, though it is not currently an approved therapy for MASH specifically.

How MASH Is Diagnosed

Diagnosis requires demonstrating both steatosis and active hepatic inflammation, ideally on tissue.

Liver biopsy remains the regulatory gold standard. The NAFLD Activity Score grades steatosis (0, 3), lobular inflammation (0, 3), and hepatocyte ballooning (0, 2). A NAS of ≥5 with ballooning is considered diagnostic for MASH. Fibrosis is staged separately on the METAVIR scale from F0 to F4.

Non-invasive tests are now endorsed for initial risk stratification. The FIB-4 index (using age, AST, ALT, and platelet count) identifies low-risk patients (FIB-4 <1.30) who can be monitored without biopsy. The Enhanced Liver Fibrosis (ELF) score, which combines hyaluronic acid, PIIINP, and TIMP-1, performs similarly for detecting advanced fibrosis. The 2023 AASLD guidance recommends FIB-4 as the first-line non-invasive test for all patients with suspected MASLD/MASH [6].

MRI-PDFF (proton density fat fraction) quantifies hepatic fat with high accuracy (correlation r = 0.99 vs. biopsy) and is used increasingly in clinical trials to confirm steatosis response [8]. Magnetic resonance elastography (MRE) grades fibrosis non-invasively and outperforms transient elastography (FibroScan) in head-to-head comparisons.

Blood tests alone cannot diagnose MASH. Elevated ALT is common but neither sensitive nor specific. Roughly 25% of patients with biopsy-confirmed MASH have normal ALT at the time of diagnosis [9].

The Fibrosis Progression Problem: Why Stage Matters More Than Inflammation Score

Fibrosis stage, not inflammation grade, is the dominant predictor of mortality in MASH. This is a key clinical fact that changes how clinicians prioritize treatment urgency.

A landmark 2015 analysis by Angulo et al. (N = 619 patients with long-term follow-up) showed that each one-stage increase in liver fibrosis was associated with a 2.59-fold increase in all-cause mortality (hazard ratio 2.59 to 95% CI 1.46, 5.10, P<0.001) [10]. Patients with stage F3 fibrosis progress to cirrhosis at approximately 2 to 3% per year. At F4 (established cirrhosis), annual rates of hepatic decompensation run 3 to 5%, and hepatocellular carcinoma develops in 1 to 2% per year even in the absence of viral hepatitis.

MASH-related cirrhosis is now the second-leading indication for liver transplantation in the United States, trailing only alcohol-related liver disease [11]. Because MASH progresses silently, most patients reach F3 or F4 without ever experiencing jaundice, ascites, or encephalopathy.

The table below outlines the HealthRX clinical triage framework based on FIB-4 score and co-conditions. The medical team applies this in telehealth intake to decide whether to route a patient directly to gastroenterology or manage initial risk-factor modification in primary care.

HealthRX MASH Triage Framework

| FIB-4 Score | Risk Category | Recommended Action | |---|---|---| | <1.30 | Low | Annual FIB-4 recheck; lifestyle counseling | | 1.30, 2.67 | Indeterminate | ELF score or FibroScan within 3 months | | >2.67 | High | Urgent hepatology referral; consider liver biopsy | | Any + T2D + ALT >2x ULN | Elevated regardless of FIB-4 | Hepatology referral within 4 weeks |

Approved Pharmacotherapy: Resmetirom (Rezdiffra)

The FDA approved resmetirom on March 14, 2024, making it the first drug with an indication specifically for MASH with moderate-to-advanced liver fibrosis (stages F2, F3) in adults with a body mass index of ≥27 kg/m².

Resmetirom is a liver-directed, selective thyroid hormone receptor beta (THR-β) agonist. It mimics the metabolic effects of thyroid hormone in the liver, reducing lipogenesis and increasing fatty acid oxidation, without the cardiac side effects of systemic thyroid hormone excess.

The approval was based on the MAESTRO-NASH trial (N = 966). At 52 weeks, the primary endpoints were MASH resolution without fibrosis worsening, and a one-stage or greater fibrosis improvement without MASH worsening. Results:

• MASH resolution endpoint: 25.9% of patients on resmetirom 80 mg vs. 14.2% placebo (P<0.001) [12].
• Fibrosis improvement endpoint: 24.2% on resmetirom 80 mg vs. 14.2% placebo (P<0.001) [12].
• The 100 mg dose performed similarly: 29.9% MASH resolution and 25.9% fibrosis improvement.

The FDA label approves both the 80 mg and 100 mg once-daily oral doses. Common adverse effects include nausea (28.4% vs. 16.3% placebo) and diarrhea (27.7% vs. 10.1% placebo), generally mild and transient in the first weeks of treatment.

Resmetirom does not yet have cardiovascular outcomes data. The ongoing MAESTRO-NASH OUTCOMES trial (estimated completion 2027) is designed to confirm a reduction in liver-related clinical events.

Off-Label and Lifestyle-Based Approaches

No other drug currently holds FDA approval specifically for MASH, but several agents are used off-label or as adjuncts, and lifestyle intervention remains the most effective strategy for early-stage disease.

GLP-1 receptor agonists. Semaglutide 2.4 mg weekly (Wegovy) produced MASH resolution in 59% of patients vs. 17% placebo in a phase 2 trial (N = 320 to 72 weeks) [13]. Fibrosis did not improve significantly in that phase 2 cohort, though the ongoing ESSENCE trial (phase 3, N = 1,200) is powered for fibrosis outcomes and reports in late 2025. Liraglutide 1.8 mg daily showed similar MASH resolution (39% vs. 9% placebo) in the LEAN trial (N = 52) [14].

SGLT2 inhibitors. Empagliflozin and dapagliflozin reduce liver fat on MRI-PDFF by roughly 3, 4 percentage points in MASLD patients with T2D, but biopsy-confirmed fibrosis data remain limited [15].

Vitamin E. The PIVENS trial (N = 247) showed that vitamin E 800 IU/day produced MASH resolution in 36% vs. 21% placebo (P = 0.001) in non-diabetic adults [16]. AASLD guidelines support vitamin E as an option in non-diabetic MASH, with the caveat that long-term safety data beyond two years are limited.

Weight loss. A 7 to 10% reduction in body weight correlates with MASH resolution in approximately 50% of patients. A 10%+ reduction achieves fibrosis improvement in roughly 45% [17]. The mechanism is primarily reduction in hepatic free fatty acid flux driven by improved adipose tissue insulin sensitivity.

Alcohol. Any regular alcohol use accelerates fibrosis progression in MASH. The 2023 AASLD guidance recommends complete abstinence for patients with stage F2 or higher fibrosis. Even light drinking (1, 2 drinks per day) in patients with MASH doubles the rate of fibrosis progression compared with abstinence [6].

MASH and Cryptogenic Cirrhosis

Cryptogenic cirrhosis, historically defined as cirrhosis with no identifiable viral, autoimmune, or genetic cause, is now understood to represent burned-out MASH in 60 to 75% of cases [18]. As MASH progresses to cirrhosis, fat content drops below the 5% threshold needed for a steatosis diagnosis, and inflammatory activity often resolves. The liver at this stage can appear histologically "bland," masking the metabolic etiology.

Patients with cryptogenic cirrhosis who have obesity, T2D, or dyslipidemia should be evaluated as probable MASH-related cirrhosis. This reclassification matters for listing decisions, post-transplant outcomes, and recurrence risk: MASH recurs in the transplanted liver in up to 30% of recipients within five years if metabolic risk factors are not controlled [19].

MASH and Cardiovascular Risk

Cardiovascular disease, not liver failure, is the leading cause of death in patients with MASH who have not yet reached F3/F4 fibrosis. A 2021 meta-analysis (N = 36,902 MASLD patients across 16 studies) found that MASLD approximately doubled the risk of fatal cardiovascular events compared with the general population [20].

The metabolic drivers of MASH, insulin resistance, dyslipidemia, systemic inflammation, are the same drivers of atherosclerosis. Resmetirom itself reduces LDL cholesterol by approximately 13% and triglycerides by approximately 22% at the 100 mg dose, a secondary benefit with potential cardiovascular implications [12].

Statin use is safe in MASH and MASLD. The concern that statins worsen liver enzymes in fatty liver disease was effectively settled by the GREACE trial and subsequent analyses: statins reduce cardiovascular events in MASLD patients without worsening liver histology, and ALT elevations on statins in MASLD patients are rarely drug-related hepatotoxicity [21].

Monitoring Intervals for Established MASH

Once MASH is diagnosed, monitoring frequency depends on fibrosis stage:

• F0, F1: Annual FIB-4, liver ultrasound every 12 to 24 months, metabolic risk factor review.
• F2, F3: FIB-4 and ELF score every 6 to 12 months, hepatology co-management, hepatocellular carcinoma (HCC) surveillance not yet required.
• F4 (cirrhosis): Liver ultrasound plus AFP every 6 months for HCC surveillance per AASLD guidelines, upper endoscopy every 1 to 3 years for varices screening, and semi-annual clinical assessment for decompensation [6].

Patients starting resmetirom should have ALT and AST checked at 3 months, then every 6 months. The drug carries an FDA label warning for potential hepatotoxicity; treatment should be discontinued if ALT rises to more than 3 times the upper limit of normal and is confirmed on repeat testing within one week.