What causes fatty liver and MASH: an endocrine disease
Fat does not end up in the liver by accident. It arrives because of how hormones — insulin above all — direct fuel through the body. Here are the metabolic and endocrine drivers of MASLD, the hormonal conditions that are easy to miss, and the genetic and lifestyle factors that decide who progresses.
Key takeaways
- Insulin resistance is the core driver: it floods the liver with fatty acids from fat tissue and switches on fat production inside the liver.
- Type 2 diabetes roughly doubles the risk of MASH with fibrosis; more than half of adults with type 2 diabetes have MASLD.
- Visceral (belly) fat matters more than weight alone — people with a normal BMI can have MASLD ("lean MASLD").
- Endocrine conditions — hypothyroidism, PCOS, low testosterone in men, menopause, growth-hormone deficiency, and cortisol excess — independently raise liver fat and are worth looking for.
What this means for you
If you have a fatty liver, the question is not "what did I do wrong" but "which hormonal and metabolic drivers are active, and which can we treat." For most people it is insulin resistance with some combination of weight, blood sugar and triglycerides — sometimes with an under-recognized thyroid or sex-hormone problem layered on top.
Insulin resistance and type 2 diabetes
Insulin normally does two things that protect the liver: it tells fat tissue to hold on to its fat, and it tells the liver to stop making glucose. In insulin resistance, both fail:
- Fat tissue leaks. Resistant fat cells release a steady stream of free fatty acids, and roughly 60% of liver fat in MASLD comes from this source.
- The liver makes its own fat. High insulin and high glucose — especially from sugar and fructose — switch on de novo lipogenesis, the liver's fat-making machinery.
- Fat clearance stalls. The liver can only burn or export so much fat. When intake exceeds capacity, toxic lipid intermediates injure liver cells, triggering the inflammation of MASH and the scarring that follows.
The relationship with type 2 diabetes runs in both directions. More than half of adults with type 2 diabetes have MASLD, around a third have MASH, and roughly one in six has advanced fibrosis in pooled studies. Diabetes is the strongest clinical predictor of progression to cirrhosis and liver cancer. And a fatty liver roughly doubles the risk of developing diabetes in the first place (see The two-way diabetes link).
Obesity and visceral fat
Obesity is the most common driver of MASLD, but where fat is stored matters more than how much. Visceral fat — the fat inside the abdomen — drains directly into the liver through the portal vein and is metabolically the most active. That is why waist size is part of the MASLD definition.
- Lean MASLD. About 10–20% of people with MASLD have a normal BMI. They typically have central fat, insulin resistance, or genetic risk — and they are not protected from progression.
- Weight trajectory. Gaining weight in adulthood, even within the "normal" range, raises risk; losing it lowers risk — the basis for treatment (see How much weight loss matters).
- Our sister site W8Experts covers medical weight management in depth.
Thyroid, sex hormones, and other endocrine drivers
This is the part of MASLD that endocrinologists are trained to find — and that general liver care often overlooks. Guidelines list these endocrine conditions as independently associated with MASLD:
| Condition | How it affects the liver | Where to read more |
|---|---|---|
| Hypothyroidism (including subclinical) | Thyroid hormone, via liver THR-β, drives fat burning and cholesterol clearance. Low thyroid signaling raises liver fat and LDL; the newest MASH drug, resmetirom, restores this pathway in the liver. | HashiExperts |
| Polycystic ovary syndrome (PCOS) | Insulin resistance plus androgen excess; MASLD is several times more common in women with PCOS, often at a young age. | DiaEndo |
| Low testosterone in men | Low testosterone and visceral fat reinforce each other; hypogonadal men have more liver fat and insulin resistance. | TestoExperts |
| Menopause | Loss of estrogen shifts fat to the abdomen and increases MASLD prevalence and fibrosis after menopause. | MenoExperts |
| Growth-hormone deficiency / hypopituitarism | GH normally limits liver fat; adults with GH deficiency have high rates of MASLD and MASH. | — |
| Cortisol excess (Cushing's, long-term glucocorticoids) | Cortisol drives central fat, insulin resistance and liver fat production. | — |
Finding one of these does not replace metabolic treatment, but it changes the plan. Treating overt hypothyroidism, for example, lowers LDL and can reduce liver fat, and a woman with PCOS or a man with hypogonadism has a second, treatable driver of insulin resistance.
For cliniciansWhich endocrine tests we add in MASLD
Beyond the standard metabolic panel, A1c and lipid profile, we check TSH (with free T4 if abnormal) in all patients, given the prevalence of subclinical hypothyroidism and its association with steatosis and LDL. We assess for PCOS in reproductive-age women with irregular cycles or hyperandrogenism, morning total testosterone in men with symptoms or central obesity, and consider screening for hypercortisolism when clinical features (proximal weakness, easy bruising, violaceous striae, resistant hypertension or diabetes) suggest it. In patients with pituitary disease or prior cranial irradiation, GH deficiency is part of the differential. Resmetirom trials enrolled patients on stable levothyroxine (about 14%), and the label does not require routine thyroid-function monitoring.
Genetics, alcohol, and other factors
- Genetics. Variants in PNPLA3 and TM6SF2 increase liver fat and the risk of fibrosis and liver cancer, and explain part of the higher MASLD rates in Hispanic Americans. We do not routinely test, but family history of cirrhosis raises our level of concern.
- Alcohol. Alcohol and metabolic dysfunction multiply each other's damage. Moderate drinking with metabolic risk factors is classified as MetALD; anyone with significant fibrosis should avoid alcohol completely (see Alcohol & coffee).
- Diet. Excess calories, sugar-sweetened drinks and fructose, and ultra-processed foods drive liver fat production.
- Sleep apnea. Intermittent low oxygen at night is associated with more advanced MASH.
- Medications. Some drugs promote liver fat, including glucocorticoids, tamoxifen, methotrexate, amiodarone and some antipsychotics.
- Other liver diseases — viral hepatitis, autoimmune disease, hemochromatosis — must be excluded when the picture does not fit; they are managed by hepatology.
What we know
- Insulin resistance is the central mechanism; type 2 diabetes accelerates progression.
- Visceral fat, not BMI alone, tracks with liver fat.
- Several endocrine disorders independently raise liver fat.
What we don't know
- Whether correcting subclinical hypothyroidism or low testosterone reverses MASH (limited trial data).
- How best to use genetic risk in routine care.
- Why some people with the same risk profile progress and others don't.
Questions to ask your doctor
- Is insulin resistance driving my fatty liver, and how do we treat it?
- Has my thyroid been checked? Could PCOS, low testosterone, or menopause be contributing?
- Do any of my medications add to liver fat?
- How much alcohol, if any, is safe for my liver?
How we grade evidence. Every intervention carries a plain label — from established, guideline-supported care (A) through moderate (B) and mixed or limited (C) to experimental (D) and insufficient (E). An improvement on a liver biopsy or scan is not the same as fewer cirrhosis cases, heart attacks, or deaths, and we say which we mean.
Real questions we hear about what causes fatty liver.
I'm not overweight — how can I have a fatty liver? Grade B
What the evidence shows
About 10–20% of people with MASLD are not obese; they share the same metabolic risk and can progress to fibrosis.
In our practice
We measure the metabolic drivers directly — waist, glucose and insulin resistance, lipids, thyroid and sex hormones — rather than going by weight.
Can my underactive thyroid cause a fatty liver? Grade B
What the evidence shows
Observational studies link low thyroid function to MASLD independent of weight; the thyroid-hormone receptor-β pathway is the target of resmetirom.
In our practice
We check TSH in every patient with MASLD and treat hypothyroidism when present — see HashiExperts.
I have type 2 diabetes. Does that change my liver risk? Grade A
What the evidence shows
Guidelines from the ADA, AACE and AASLD all recommend fibrosis screening with FIB-4 for adults with type 2 diabetes.
In our practice
Every patient with type 2 diabetes in our practice gets a liver fibrosis risk assessment.
References
- Rinella ME, et al. AASLD Practice Guidance on the clinical assessment and management of nonalcoholic fatty liver disease. Hepatology. 2023;77(5):1797–1835.
- Cusi K, et al. American Association of Clinical Endocrinology clinical practice guideline for the diagnosis and management of nonalcoholic fatty liver disease in primary care and endocrinology clinical settings. Endocr Pract. 2022;28(5):528–562.
- Younossi ZM, et al. The global epidemiology of NAFLD and NASH in patients with type 2 diabetes: a systematic review and meta-analysis. J Hepatol. 2019;71(4):793–801.
- Lomonaco R, et al. Advanced liver fibrosis is common in patients with type 2 diabetes followed in the outpatient setting. Diabetes Care. 2021;44(2):399–406.
- Mantovani A, et al. Non-alcoholic fatty liver disease and risk of incident diabetes mellitus: an updated meta-analysis of 501 022 adult individuals. Gut. 2021;70(5):962–969.
- Sinha RA, Singh BK, Yen PM. Direct effects of thyroid hormones on hepatic lipid metabolism. Nat Rev Endocrinol. 2018;14(5):259–269.
- Rinella ME, et al. A multisociety Delphi consensus statement on new fatty liver disease nomenclature. Hepatology. 2023;78(6):1966–1986.
- EASL–EASD–EASO Clinical Practice Guidelines on the management of metabolic dysfunction-associated steatotic liver disease (MASLD). J Hepatol. 2024;81(3):492–542.
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