Liver Function Tests Explained: ALT, AST, GGT, and More

Why Your Liver Panel Deserves More Attention

The liver is the body's primary metabolic processing hub — responsible for glucose storage, fat metabolism, protein synthesis, drug detoxification, bile production, and hundreds of other functions. Standard liver function tests provide a window into hepatocellular integrity that can reveal problems years before symptoms develop.

Nonalcoholic fatty liver disease (NAFLD) now affects approximately 25% of the global adult population and is the most common cause of chronic liver disease. It is almost entirely driven by diet, insulin resistance, and excess visceral fat — and it is almost entirely silent until advanced stages. Routine liver panels are one of the few tools that can catch it early.

The Key Liver Enzymes

ALT (Alanine Aminotransferase)

ALT is the most liver-specific enzyme in the standard panel. It is predominantly found in hepatocytes (liver cells), so elevated ALT is a relatively sensitive indicator of liver cell damage or stress.

Optimal range: Below 30 U/L for men; below 19 U/L for women (note: many labs use higher upper limits that are now considered outdated by hepatology guidelines).

Common causes of mild elevation (40–100 U/L): Early NAFLD, statin use, vigorous exercise, obesity, insulin resistance.

Causes of marked elevation (>300 U/L): Acute viral hepatitis, drug-induced liver injury, ischemic hepatitis.

AST (Aspartate Aminotransferase)

AST is found in both liver and muscle cells, making it less liver-specific than ALT. An AST:ALT ratio above 2:1 is a classical pattern for alcoholic liver disease. An AST:ALT ratio below 1 (ALT higher than AST) is more typical of NAFLD.

GGT (Gamma-Glutamyl Transferase)

GGT is the most sensitive marker of alcohol-related liver stress and is also elevated by fatty liver, certain medications, and biliary disease. Even within the "normal" range, higher GGT levels are independently associated with increased cardiovascular risk and metabolic syndrome.

Optimal GGT: Below 20 U/L for women; below 30 U/L for men.

ALP (Alkaline Phosphatase)

ALP is elevated in biliary tract disease (bile duct obstruction, primary biliary cholangitis) and bone disorders. When elevated in isolation — without elevated ALT or AST — biliary or bone pathology is more likely than hepatocellular disease.

Bilirubin

The breakdown product of hemoglobin. Elevated bilirubin produces jaundice (yellowing of skin and whites of eyes). Mildly elevated unconjugated bilirubin in the context of normal liver enzymes is often Gilbert's syndrome — a benign genetic variant affecting approximately 10% of the population and associated with reduced cardiovascular and cancer risk due to bilirubin's antioxidant properties.

The Fatty Liver Warning Pattern

Early NAFLD characteristically presents as:

• ALT mildly elevated (30–80 U/L), often higher than AST
• GGT mildly elevated
• Normal or mildly elevated triglycerides
• Slightly elevated fasting insulin
• Normal bilirubin and ALP

This pattern should prompt dietary intervention, not just reassurance. NAFLD progresses to fibrosis, cirrhosis, and hepatocellular carcinoma in a significant minority of untreated cases.