This article is from the Interpretation Of Lab Test Profiles, by Ed Uthman firstname.lastname@example.org with numerous contributions by others.
Gamma-glutamyltransferase is markedly increased in lesions
which cause intrahepatic or extrahepatic obstruction of bile
ducts, including parenchymatous liver diseases with a major
cholestatic component (e.g., cholestatic hepatitis). Lesser
elevations of gamma-GT are seen in other liver diseases, and in
infectious mononucleosis, hyperthyroidism, myotonic dystrophy,
and after renal allograft. Drugs causing hepatocellular damage
and cholestasis may also cause gamma-GT elevation (see under
"Total bilirubin," below).
Gamma-GT is a very sensitive test for liver damage, and
unexpected, unexplained mild elevations are common. Alcohol
consumption is a common culprit.
Decreased gamma-GT is not clinically significant.