ALT and AST are enzymes made in the liver. They are also known as transaminases. The liver uses these enzymes to metabolize amino acids and to make proteins. When liver cells are damaged or dying, ALT and AST leak into the bloodstream. Many different things can cause liver enzymes to rise above normal levels, including:

Viral hepatitis
Excessive alcohol intake/Alcoholic liver disease
Liver inflammation from medications and certain herbs,
Auto-immune hepatitis - a condition where a person's immune system mistakes the liver for an invader and attacks it,
Fatty liver- fat build -up in liver cells, called steatohepatitis when the fatty liver is inflamed
Inherited liver diseases
Liver tumors
Heart failure
ALT (also called alanine aminotransferase or SGPT) is found in the liver only. High levels of ALT in the bloodstream mean that there may be liver inflammation and/or damage. This test cannot predict liver damage or disease progression. It is simply a direct measurement of the amount of ALT in the person's bloodstream at the time of the test. The normal range of ALT levels is between 5 IU/L to 60 IU/L (International Units per Liter). ALT levels in people with HCV often rise and fall over time, so additional testing such as HCV RNA, HCV genotyping and a liver biopsy may be needed to help determine the cause and extent of liver damage.