ALT blood test: what high (and low) ALT levels mean

Clinicians grade a high ALT by how far above the upper limit it sits. Under two times the limit is mild and extremely common; it is most often fatty liver, alcohol, a medicine or a transient viral illness, and roughly a third of mild rises are normal on repeat. Two to five times is moderate and justifies a full work-up. Over 10–15 times (above roughly 500–1,000 U/L) means acute hepatitis from a virus, drug or toxin, a gallstone suddenly blocking the bile duct, or a loss of blood flow to the liver.

ALT higher than AST is the usual pattern in fatty liver and viral hepatitis. AST higher than ALT, especially with a high GGT, points toward alcohol or advanced scarring. A high ALT with raised bilirubin, low albumin or low platelets is more concerning than the same ALT with everything else normal, because it suggests the liver's function and structure are affected, not just its cells.

Common causes

1. 1.

   Non-alcoholic fatty liver disease (MASLD)

   Excess fat in liver cells, driven by insulin resistance, causes mild inflammation and leakage of ALT, usually to 40–100 U/L with ALT above AST. It affects around a third of US adults and is the leading cause of a raised ALT; weight, waist size, triglycerides and A1C usually point to it.

2. 2.

   Alcohol

   Alcohol injures liver cells and raises ALT, although AST typically rises more because alcohol depletes vitamin B6 and damages mitochondria. Even moderate regular drinking can keep ALT at the top of or above the normal range.

3. 3.

   Medications

   Acetaminophen (particularly above 3–4 g a day or combined with alcohol), statins, NSAIDs, amoxicillin-clavulanate and other antibiotics, antifungals, anti-tuberculosis drugs, anti-seizure drugs, methotrexate, isotretinoin and some antidepressants are common culprits. Most cause a mild, reversible rise; a few cause serious injury.

4. 4.

   Supplements and herbal products

   Green tea extract (in weight-loss products), kava, anabolic steroids and 'prohormones', some bodybuilding and energy supplements, and high-dose niacin are well-documented causes of ALT rises and occasionally severe liver injury. Always list them for your clinician.

5. 5.

   Viral hepatitis

   Hepatitis B and C cause chronic, fluctuating ALT elevation and are checked in everyone with an unexplained rise; hepatitis A and E, Epstein-Barr virus (mono) and cytomegalovirus cause short-lived, sometimes very high values. Many people with chronic hepatitis B or C have no symptoms for years.

6. 6.

   Strenuous exercise and muscle injury

   Muscle contains some ALT, so a heavy weight session or endurance event can raise it modestly for several days, usually with a bigger rise in AST and CK. Rhabdomyolysis can raise ALT substantially.

7. 7.

   Celiac disease

   Untreated celiac disease causes a mild, unexplained ALT rise in a meaningful minority of patients, through gut inflammation and increased gut permeability. It resolves on a gluten-free diet, which is why celiac antibodies are part of the standard work-up.

8. 8.

   Hemochromatosis, autoimmune hepatitis and Wilson disease

   Iron overload (checked with ferritin and transferrin saturation), immune attack on liver cells (autoantibodies and raised IgG) and copper accumulation in people under 40 (ceruloplasmin) are less common but important because each has a specific treatment.

9. 9.

   Thyroid disease

   Hyperthyroidism increases the liver's metabolic workload and hypothyroidism can raise ALT through muscle and metabolic effects; both are mild and improve with treatment, so TSH is checked routinely.

10. 10.

    Bile-duct blockage, heart failure and low blood flow

    A gallstone passing through the duct can briefly send ALT above 1,000 U/L; right-sided heart failure congests the liver; and shock or an arrhythmia can starve it of oxygen ('ischaemic hepatitis'), causing a very high but rapidly falling ALT.

Symptoms that can go with it

• Usually none; most people with a high ALT feel entirely well
• Tiredness and reduced appetite
• Discomfort or fullness under the right ribs
• Nausea, particularly with viral or drug-related hepatitis
• Dark urine, pale stools, itching or yellow skin and eyes if bilirubin is also high
• Easy bruising, swollen ankles or a swollen abdomen in advanced liver disease

A low ALT is almost never a problem and most results below the lower limit are normal variation in people with low muscle mass or a small frame. The one practical point is that ALT drifts lower with age, kidney failure and frailty, so an 'average' ALT in an older adult or someone on dialysis may actually be relatively high for them and can hide fatty liver or hepatitis. Vitamin B6 deficiency can also lower measured ALT, because the enzyme needs it to function.

Common causes

1. 1.

   Normal variation and low muscle mass

   Reference ranges capture the middle 95% of people, so some healthy adults always test below the lower limit. Smaller, leaner people and women tend to sit lower.

2. 2.

   Older age and frailty

   ALT falls in later life as liver and muscle mass decline, and very low ALT in older adults is associated with frailty in population studies. It is a marker, not a cause, of reduced reserve.

3. 3.

   Vitamin B6 deficiency and dialysis

   ALT needs pyridoxal phosphate to work. People on dialysis, with alcohol dependence, or on isoniazid can have reduced measurable ALT activity, which may mask liver disease.

Symptoms that can go with it

• None from the low ALT itself
• Tingling, cracked lips or low mood if vitamin B6 is deficient