Alanine aminotransferase (ALT) is an important marker of liver health. Keep reading to find out why this marker is often part of liver function tests – and when you should be concerned about your levels.
Healthy liver cells store most of ALT, but small amounts are also found in the kidneys, heart, muscles, fat tissue, intestines, and pancreas .
Normally, blood ALT levels are low. However, when liver cells are damaged due to illness, injury, or medication, they release ALT, increasing its blood levels .
Therefore, ALT blood levels are a marker of liver health: low levels typically indicate a healthy liver, while high levels suggest liver damage .
Before we talk about the ALT blood test, let’s take a look at what ALT normally does in the body.
- Assess liver health
- Investigate symptoms of liver disease, such as abnormally yellow skin or eyes (jaundice), or pain in the upper-right section of the abdomen
- Monitor progression of a liver disease
- Evaluate the effectiveness of a treatment for liver disease
- Determine if the liver is involved in or damaged by a health condition, such as diabetes or heart disease
Since ALT is an enzyme, its levels are typically determined by measuring its activity (the rate at which ALT transforms L-alanine and α-ketoglutarate into pyruvate and L-glutamate) .
ALT levels are often measured together with the liver enzyme aspartate transaminase (AST). The ratio of AST/ALT is also sometimes used as a marker of liver health.
While ALT levels can signal the presence of liver damage, they cannot determine the type of damage, such as scarring, infection, or inflammation .
ALT is measured in units per liter of blood or U/L.
The normal range is around 7-35 U/L in women and 7-40 U/L in men. There may be some lab-to-lab variability in ranges due to differences in equipment, techniques, and chemicals used.
Low ALT levels are expected and normal – they are just uncommon in the general population. This is because reference ranges are based on where 95% of a healthy population falls into, which means that there are 5% of the people who are healthy and not within the reference range!
There are some factors that can decrease ALT, but they are not common. These include:
- Vitamin B6 deficiency [20, 21]
- Smoking 
- Regular exercise 
- Oral contraceptives or hormone replacement therapy [22, 24]
- Chronic kidney disease [25, 26]
Find out more about low ALT levels and associated conditions here.
High ALT levels often signal a problem with the liver. However, a result that’s higher than normal, doesn’t necessarily mean that you have a health condition that needs treatment. Your doctor will interpret your ALT result, taking into account your medical history, symptoms, and other test results.
ALT can increase due to various underlying issues. Causes shown below are commonly associated with high ALT levels. Work with your doctor or another health care professional to get an accurate diagnosis. Aforementioned conditions include:
- Liver diseases, such as fatty liver, viral hepatitis, other infections that affect the liver (mononucleosis), or liver cancer [27, 28]
- Liver damage due to toxins such as lead, mercury, or pesticides [11, 29, 30]
- Liver damage due to prescription and over-the-counter drugs and supplements [11, 31, 32, 33]
- Alcohol abuse 
- Anorexia 
- Obesity [36, 34, 37]
- Gallstones and gallstone-induced inflammation of the pancreas (pancreatitis) [38, 39]
- Muscle damage due to strenuous exercise, injury, or muscle disease [40, 41, 42]
- Tissue damage due to surgery or burns [43, 44]
- Heart attack or heart failure [45, 31]
- Underactive thyroid gland (hypothyroidism) 
- Abnormal red blood cell destruction (hemolysis) [46, 47]
Read through this post to learn more about the causes of high ALT and what you can do to decrease high ALT levels.
While one elevated ALT test doesn’t necessarily mean that you have a health condition that needs treatment, several high ALT tests or a high ALT test in conjunction with symptoms of liver damage or disease are a reason for concern. Your doctor should work on finding the underlying cause and treatment options .
ALT is an enzyme your body needs to turn proteins into energy. Most of it is stored in your liver. Your levels will usually remain stable and relatively low if your liver is healthy, while liver damage causes ALT to leak into the bloodstream in higher amounts. Thus, doctors typically order ALT to check liver function. However, various other conditions beyond liver health can also affect your levels. That’s why doctors will analyze ALT alongside other markers of liver health like AST, ALP, bilirubin, and GGT.