Liver Function Tests Explained: What a Liver Panel Shows and When to Get One

ALT (alanine aminotransferase). The most liver-specific enzyme. It leaks from liver cells when they are injured — by fat, alcohol, viruses, medications or autoimmune disease. Mild elevations (under 2–3× the upper limit) are most often fatty liver; the level tracks with weight, triglycerides and insulin resistance.

AST (aspartate aminotransferase). Also released by injured liver cells, but found in muscle and heart too, so hard exercise or a muscle injury raises it. The AST:ALT ratio carries information: ALT higher than AST suggests fatty liver; AST more than twice ALT suggests alcohol-related disease; the ratio also rises as any liver disease progresses to fibrosis.

ALP (alkaline phosphatase). Made in the bile ducts and in bone. A high ALP with a high GGT points to the bile ducts (stones, strictures, primary biliary cholangitis, some medications); a high ALP with a normal GGT points to bone (healing fracture, Paget's disease, vitamin D deficiency) or, in children and teenagers, normal growth.

Total and direct bilirubin. The pigment left over when red cells are recycled; the liver conjugates it and sends it into bile. Mostly indirect (unconjugated) bilirubin with normal enzymes is usually Gilbert's syndrome — a harmless inherited trait affecting about 5% of people — or increased red-cell breakdown. Direct (conjugated) bilirubin rises when the liver can't excrete it: bile-duct obstruction or advanced liver disease. Visible jaundice starts around 2.5–3 mg/dL.

Albumin and total protein. Albumin is made only by the liver and has a 20-day half-life, so a low value reflects weeks of reduced production (advanced liver disease), loss through the kidneys or gut, or inflammation and malnutrition. Total protein minus albumin gives the globulins, which rise in chronic inflammation and some liver diseases.

Pattern	Points toward	Common next step
ALT > AST, both mildly high; ALP and bilirubin normal	Fatty liver (MASLD), the most common finding in US adults	A1c, lipids, weight; repeat in 3–6 months; FIB-4 score
AST > 2× ALT, GGT high	Alcohol-related liver injury	Alcohol history; CBC (MCV); repeat after 4+ weeks abstinent
ALT and AST > 5–10× upper limit	Acute hepatitis (viral, drug, autoimmune, ischemic)	Same-day clinician review; hepatitis panel; medication and supplement list
ALP and GGT high, bilirubin high, enzymes modest	Bile-duct obstruction or cholestatic disease	Ultrasound; anti-mitochondrial antibody if no obstruction
Isolated indirect bilirubin, everything else normal	Gilbert's syndrome or hemolysis	Repeat fasting; CBC and reticulocyte count; often no action
Low albumin with high bilirubin and prolonged PT/INR	Reduced liver synthetic function (cirrhosis)	Hepatology referral

Reference ranges differ slightly by lab and sex; what matters is the multiple of the upper limit and the direction over time. A single mildly high ALT is repeated before anything is labeled — roughly a third normalize on the second draw.

• Anyone with metabolic risk — overweight, prediabetes or diabetes, high triglycerides, high blood pressure. Fatty liver affects about one in three US adults and is silent until advanced; a yearly ALT with a FIB-4 calculation is the accepted screen.
• Regular alcohol use above moderate levels — a liver panel plus GGT yearly, and after any period of heavier drinking.
• Medications and supplements that can injure the liver — statins (baseline, then as needed), methotrexate, isoniazid, antifungals, high-dose acetaminophen, anabolic steroids, and supplements such as kava, green-tea extract concentrates, and bodybuilding blends. A baseline before starting and a check at 6–12 weeks is standard for many of these.
• Peptide, GLP-1 or testosterone therapy — a baseline and periodic panel is part of responsible monitoring.
• Symptoms — fatigue, right-upper-abdominal discomfort, itching, dark urine, pale stools, yellowing of the eyes, easy bruising.
• Hepatitis exposure or risk — pair the panel with a hepatitis B and C screen; the CDC recommends every adult be screened for hepatitis C at least once.

Every comprehensive metabolic panel already includes ALT, AST, ALP, bilirubin and albumin, so an annual CMP covers routine screening; the dedicated hepatic panel adds direct bilirubin and total protein, and GGT is the useful add-on when alcohol or bile ducts are the question.

• No fasting required for the liver panel itself (fast 8–12 hours if it's bundled with glucose or lipids).
• Skip strenuous exercise for 24–48 hours — weight training and long runs raise AST (and CK) from muscle, which can look like a liver problem.
• No alcohol for 24 hours, ideally longer; a weekend of heavy drinking can lift GGT and AST for days.
• List every medication and supplement when you review results — acetaminophen, antibiotics, statins and herbal products are frequent, reversible causes of a high ALT.
• Hemolyzed samples falsely raise AST and bilirubin; if the values don't fit the picture, a redraw is reasonable.