According to MedlinePlus, the normal levels of GGT in the blood are between 0 and 51 international units per liter. GGT stands for gamma-glutamyl transpeptidase.
Gamma-glutamyl transpeptidase, or GGT, measures functioning of the liver and bile ducts. If GGT is too high, it can be due to diabetes, alcohol abuse, lung or pancreas disease, or liver problems. Heart disease and hepatitis are also culprits. Testing for GGT involves a simple blood test where a needle is inserted into a vein and blood is removed and tested for GGT levels. Some drugs can alter the test results, such as phenobarbital and birth control tablets.