Alanine aminotransferase, commonly called ALT, is an enzyme found mainly inside liver cells. When those cells are healthy, ALT stays put and only small amounts show up in your blood. When liver cells are damaged or inflamed, ALT leaks into the bloodstream, and a simple blood test can measure how much. That’s why ALT is one of the most commonly ordered lab tests for checking liver health.
What ALT Does in Your Body
ALT plays a specific role in metabolism: it transfers a chemical group from one amino acid (alanine) to another molecule, producing glutamate and pyruvate in the process. Both of those products feed into your body’s energy cycles. The enzyme sits in the main fluid compartment of liver cells, which is why the liver is its primary home. Small amounts also exist in kidney, heart, and muscle tissue, but the concentration in the liver is far higher than anywhere else. This makes ALT one of the most liver-specific markers available on a routine blood panel.
Normal ALT Ranges
ALT is measured in international units per liter (IU/L). The American College of Gastroenterology defines a healthy normal ALT as 29 to 33 IU/L for men and 19 to 25 IU/L for women. Many older lab reference ranges used higher cutoffs, sometimes up to 40 or even 56 IU/L, which means some people with genuinely elevated ALT were told their results were “normal.” If your lab report shows a range that looks different from these numbers, the updated thresholds are worth keeping in mind.
The gender gap starts early. In adolescents, the upper limit of normal is about 30 IU/L for boys and 21 IU/L for girls, with girls’ levels dipping slightly lower through the teenage years.
What Elevated ALT Means
An elevated ALT level signals that liver cells have been injured, but it doesn’t tell you why. The degree of elevation helps narrow things down. Clinicians generally categorize it this way:
- Borderline (less than 2x the upper limit of normal): Could reflect fatty liver disease, early-stage hepatitis, or even temporary causes like recent intense exercise or certain medications.
- Mild (2 to 5x normal): Often seen with chronic viral hepatitis, ongoing fatty liver inflammation, or medication-related liver stress.
- Moderate (5 to 15x normal): Points toward more significant liver injury, such as active hepatitis or toxic exposure.
- Severe (above 15x normal): Typical of acute viral hepatitis, drug-induced liver injury, or sudden loss of blood flow to the liver.
- Massive (above 10,000 IU/L): Rare, and usually caused by acute poisoning (such as acetaminophen overdose) or severe ischemic injury where the liver is suddenly starved of oxygen.
Common Causes of High ALT
The four most frequent reasons for a persistently elevated ALT are chronic alcohol use, medications, non-alcoholic fatty liver disease (sometimes called metabolic-associated fatty liver disease), and chronic viral hepatitis (hepatitis B or C). Of these, fatty liver disease has become the most common in many populations, driven by rising rates of obesity and insulin resistance.
Less common liver-related causes include autoimmune hepatitis, hereditary iron overload, Wilson disease, and celiac disease. ALT can also rise from sources outside the liver entirely. Muscle injuries, certain thyroid conditions, and even a heart attack can push the number up because small amounts of the enzyme exist in those tissues too.
The AST-to-ALT Ratio
Your blood work will usually report ALT alongside a related enzyme called AST. The ratio between the two provides a useful clue. In a healthy liver, AST divided by ALT is less than 1. When that ratio climbs above 2, it strongly suggests alcohol-related liver disease: 92% of patients with alcoholic liver disease have a ratio above 1, and 70% have a ratio above 2. A ratio below 1, on the other hand, is more typical of non-alcoholic fatty liver disease. The ratio isn’t definitive on its own, but it helps guide the next steps in evaluation.
Symptoms to Watch For
Most people with mildly elevated ALT feel completely fine. The enzyme bump often shows up on routine blood work with no warning signs at all. If liver damage is significant enough to cause symptoms, you might notice fatigue, loss of appetite, nausea, abdominal pain (particularly in the upper right side), dark urine, pale stools, itching, or yellowing of the skin and eyes. The presence of any of these alongside elevated ALT typically prompts more urgent investigation.
How the Test Works
The ALT test itself is a straightforward blood draw, usually bundled with other liver markers in what’s called a liver panel or comprehensive metabolic panel. You’ll typically need to fast for several hours beforehand because the other tests in the panel require it. Let your provider know about any supplements, over-the-counter pain relievers, or prescription medications you take, since some of these can independently raise ALT and may need to be paused before testing.
A single elevated reading doesn’t necessarily mean something is wrong. Intense exercise in the days before a blood draw, a recent illness, or a course of antibiotics can all cause a temporary spike. If your ALT comes back high, the usual next step is repeating the test after a few weeks to see whether the elevation persists. Persistent elevation is what triggers further workup, which might include imaging of the liver, additional blood tests for viral hepatitis or autoimmune markers, or in some cases a biopsy.
What Can Lower ALT Levels
Because fatty liver disease is the most common driver of elevated ALT in the general population, the most effective intervention for many people is weight loss. Losing even 5 to 10% of body weight can meaningfully reduce liver fat and bring ALT levels back toward normal. Reducing alcohol intake, managing blood sugar, and reviewing medications with a provider are other practical steps. Regular physical activity helps independently of weight loss by improving how the liver processes fat.
If a specific condition like hepatitis B or autoimmune hepatitis is causing the elevation, treating the underlying disease is what brings ALT down. The enzyme level itself isn’t the problem. It’s a signal pointing to whatever is injuring the liver, and addressing that root cause is what matters.