Alanine Aminotransferase (ALT)

A high ALT means some of your liver cells are leaking an enzyme called alanine aminotransferase into your blood, a sign of irritation or injury. In a US adult, a mild rise is far more often fatty liver than hepatitis, and the number is easy to misread, because many labs flag ALT as high only above about 40 U/L while the true healthy ceiling is closer to 29 to 33 U/L in men and 19 to 25 in women.

What is ALT in a blood test?

ALT, short for alanine aminotransferase, is an enzyme that lives inside liver cells and helps them process amino acids. When those cells are irritated or damaged, they leak ALT into the bloodstream, so a rising ALT is a signal of liver-cell injury. Unlike AST (aspartate aminotransferase), which also sits in heart, skeletal muscle, kidney, brain, and red blood cells, ALT is concentrated in the liver, which makes it the most liver-specific of the routine enzymes. ¹³ ALT also clears slowly, with a half-life of roughly two days, so it tends to stay up longer than AST after an injury. ⁵

What does a high ALT mean?

How worried to be depends less on the exact number than on how many times above the top of the range it sits. The 2017 ACG guideline groups aminotransferase elevations (the family that includes ALT and AST) by that multiple. ¹

Most raised ALT results people worry about fall in the borderline-to-mild range, and in a US adult that is fatty liver until proven otherwise. Fatty liver (now called MASLD, metabolic dysfunction-associated steatotic liver disease) is the most common chronic liver disease in the world, so a mild rise is common and usually metabolic, not infectious. ¹⁴ That is reassuring, and it is still a reason to look, because the scarring that matters can build quietly. See fatty liver for what it is and whether it is serious.

What is a normal ALT level?

For decades, labs set the upper limit of normal near 40 U/L. That number came from reference groups that, it turned out, included people with hidden fatty liver and undiagnosed viral hepatitis, which pushed the 'normal' ceiling up. When Prati and colleagues re-drew the range using only people with no risk factors, a normal body weight, and normal metabolic labs, the healthy ceiling dropped to about 30 U/L for men and 19 U/L for women. ² The 2017 ACG guideline adopted a similar range. ¹

The true healthy normal ALT level in prospectively studied populations without identifiable risk factors for liver disease ranges from 29 to 33 IU/L for males and 19 to 25 IU/L for females, and levels above this should be assessed by physicians.ACG Clinical Guideline, 2017 (Kwo et al.)

So a result inside your lab's printed range can still sit above the guideline's healthy ceiling, because the cutoff shifts with whichever population a lab used; see how reference ranges work. In practice, an ALT the lab calls normal is worth a second look if it sits above the 29 to 33 U/L healthy limit.

ALT vs AST: which one points to the liver?

Because AST is not liver-specific, a high AST is only meaningful next to ALT and CK. The full picture, including the AST/ALT ratio and the alcohol pattern, lives on the AST and AST/ALT ratio pages. In most liver disease ALT sits above AST; when the ratio climbs past 2 alongside a high GGT (gamma-glutamyl transferase, another liver enzyme), the pattern points to alcohol.

What causes high ALT levels?

• Fatty liver (MASLD): the single most common cause of a mildly high ALT in US adults. See fatty liver.
• Alcohol: regular or heavy drinking irritates liver cells; the pattern often shows up alongside a raised GGT and an AST/ALT ratio above 2.
• Medications and supplements: acetaminophen (Tylenol), some statins, certain antibiotics, and some herbal or bodybuilding supplements can raise ALT. Never stop a prescribed medicine on your own; ask your clinician.
• Viral hepatitis: hepatitis B and C can raise ALT, sometimes with no symptoms for years, which is why they are worth testing for when ALT stays up.
• Muscle injury: hard exercise or muscle damage raises AST more than ALT, so a high AST with a normal ALT and a raised CK usually means muscle.
• Less common: autoimmune hepatitis, thyroid disease, celiac disease, and inherited conditions such as hemochromatosis (iron overload) or Wilson disease.

How do I lower ALT levels?

There is no pill that lowers ALT directly; the number falls when the cause behind it eases. For fatty liver, the most common cause, gradual weight loss lowers liver fat and, with it, ALT. Cutting back or stopping alcohol helps if drinking is a factor, and reviewing your medications and supplements with your clinician can catch a drug that is nudging the enzyme up. Do not stop a prescribed medicine on your own.

1. 1Don't panic, but don't ignore itA mild, isolated ALT rise is common and usually fatty liver. It is a prompt to look, not an emergency.
2. 2Bring the number to your clinicianAsk whether to repeat the test in a few weeks to months, and whether to check for hepatitis B and C and for metabolic risk factors.
3. 3Work the usual leversAlcohol, body weight, and certain medications and supplements are the common movers. Any change to a prescribed medicine should be made with your clinician, not on your own.
4. 4Retest and track the trendA single value matters less than the direction over months. A falling ALT alongside weight loss or less alcohol is a good sign.

Does a normal ALT mean my liver is fine?

A normal ALT is reassuring, but it does not fully clear your liver. ALT can read normal in people with significant fatty liver, and even with advanced fibrosis (scarring), because the enzyme reflects active leakage from injured cells, not the total amount of damage already done. ⁴ A normal ALT tells you there is probably no active, ongoing injury today; it does not measure how well the liver is working, and it does not rule out scarring that built up earlier.

How ALT is tested and how often to retest

ALT comes bundled in the standard liver panel and in a comprehensive metabolic panel (CMP), so it is rarely ordered on its own and adds little cost; both are routine, low-cost tests usually covered by insurance. You do not need to fast for ALT itself, though it often rides along with a fasting glucose or cholesterol draw. For a mild, isolated rise, clinicians commonly repeat the test in a few weeks to a few months rather than acting on a single value, and people with fatty-liver risk factors (excess weight, type 2 diabetes, high triglycerides) are the ones most often screened. ¹

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