ALT stands for alanine aminotransferase, an enzyme found mainly in your liver cells. When liver cells are damaged or inflamed, ALT leaks into your bloodstream, and a blood test picks up the elevated levels. A normal ALT range is 7 to 55 U/L for males and 7 to 45 U/L for females, though labs may vary slightly in their cutoffs.
What ALT Actually Does in Your Body
ALT is an enzyme that helps your liver process amino acids, the building blocks of protein. Specifically, it shuffles chemical groups between molecules to produce compounds your liver uses for energy. While ALT exists in small amounts in your kidneys, heart, and muscles, the liver contains the highest concentration by far. That’s what makes it a useful marker: when ALT shows up at high levels in your blood, the liver is the most likely source.
What a Normal Result Looks Like
Results are reported in units per liter (U/L). The standard reference ranges are:
- Males: 7 to 55 U/L
- Females: 7 to 45 U/L
A result within these ranges generally means your liver cells aren’t releasing unusual amounts of ALT. Keep in mind that “normal” can shift depending on the lab and the population it serves. Your doctor may flag a result that’s technically within range if it’s noticeably higher than your previous tests.
During pregnancy, the upper end of the normal range actually drops. This happens because blood volume increases significantly, diluting the concentration of ALT. Studies show ALT levels tend to decrease through the second and third trimesters, reaching their lowest point in late pregnancy before returning to pre-pregnancy levels within about a week after delivery.
How Doctors Classify Elevated ALT
Not all elevations carry the same weight. The American College of Gastroenterology uses multiples of the upper limit of normal (ULN) to categorize severity:
- Borderline: Less than 2 times the upper limit (for example, under 110 U/L in men)
- Mild: 2 to 5 times the upper limit
- Moderate: 5 to 15 times the upper limit
- Severe: More than 15 times the upper limit
- Massive: Above 10,000 U/L
A borderline or mildly elevated result is common and often warrants a recheck in a few weeks rather than immediate alarm. Moderate to severe elevations typically prompt further testing to identify the cause quickly.
Common Reasons ALT Goes Up
The most frequent cause of mildly elevated ALT in adults is fatty liver disease, now formally called metabolic dysfunction-associated steatotic liver disease (MASLD). It’s closely tied to being overweight, having high blood sugar, or carrying excess fat around the midsection. Many people with fatty liver have no symptoms at all and discover it only through routine blood work.
Viral hepatitis (types A, B, and C) is another major cause and can push ALT levels significantly higher, sometimes into the thousands. Alcohol-related liver damage, autoimmune hepatitis, and toxic hepatitis from medications or supplements also trigger ALT release. Certain over-the-counter pain relievers, herbal products, and prescription drugs are well-known liver irritants.
Beyond these, a surprisingly long list of conditions can raise ALT:
- Hemochromatosis: excess iron stored in the body
- Wilson’s disease: excess copper stored in the body
- Celiac disease: an immune reaction to gluten that damages the small intestine
- Thyroid disorders
- Heart failure
- Liver cancer
- Certain infections: mononucleosis, cytomegalovirus, Epstein-Barr virus
Non-Liver Causes Worth Knowing About
Because small amounts of ALT also live in muscle tissue, intense physical activity can raise your levels without any liver involvement. Studies on marathon and ultra-marathon runners have found significant ALT increases after races, with levels remaining elevated for days afterward. Bodybuilding, heavy weightlifting, and any new or recently intensified exercise routine can have the same effect, especially in people whose bodies aren’t yet adapted to the workload.
In cases of rhabdomyolysis, a condition where muscle fibers break down rapidly, ALT was abnormally elevated in about 75% of patients studied. If you’ve recently started a tough workout program or done something physically extreme, mention it to your doctor before assuming an elevated ALT points to your liver. A separate blood marker for muscle damage (creatine kinase, or CK) can help distinguish muscle injury from liver injury.
What the AST-to-ALT Ratio Tells You
ALT is often ordered alongside AST (aspartate aminotransferase), a related liver enzyme. The ratio between the two provides an extra diagnostic clue. When AST divided by ALT is less than 1, liver injury is generally considered mild. When the ratio exceeds 1, it suggests more serious liver cell damage. This ratio, first described in 1957 by the researcher Fernando De Ritis, is still used routinely to help distinguish between types of liver disease and gauge severity.
In alcohol-related liver disease specifically, the AST-to-ALT ratio often climbs above 2, which can help doctors differentiate it from other causes of elevated enzymes.
What Happens After an Abnormal Result
A single elevated ALT reading rarely leads to a diagnosis on its own. Your doctor will typically look at the full picture: your other liver enzymes, your medical history, medications, alcohol use, and body weight. If the elevation is borderline, a repeat test in a few weeks is standard, since temporary spikes from exercise, illness, or a new supplement are common.
For persistent elevations, the next steps usually involve additional blood tests to screen for hepatitis, autoimmune conditions, or iron and copper storage disorders. An ultrasound of the liver can check for fatty liver, structural changes, or tumors. In some cases, a specialized imaging scan or a liver biopsy may follow, but most people get answers well before that stage.
If your result came back elevated and you’re wondering whether it’s serious, the degree of elevation matters most. A value slightly above the upper limit in someone who just ran a half-marathon or started a new medication tells a very different story than a level five or ten times the normal range with no obvious explanation.