Summary
Alanine Aminotransferase (ALT) is the most liver-specific enzyme in routine blood tests. When liver cells are damaged — from fatty liver, alcohol, viral hepatitis, or medication — ALT leaks into the bloodstream, making it the earliest and most reliable marker of liver injury. Elevated ALT is one of the most common abnormal findings in routine health screening.
ALT is found predominantly in liver cells (hepatocytes), with smaller amounts in kidney and muscle. When hepatocytes are injured or inflamed, ALT is released into the blood, where levels rise within hours of injury and may remain elevated for weeks.
Mildly elevated ALT (1–3× upper limit of normal) is extremely common in the UK and most frequently reflects non-alcoholic fatty liver disease (NAFLD), which affects an estimated 25–30% of UK adults. Moderate to markedly elevated ALT (> 5× ULN) warrants urgent investigation for viral hepatitis, alcoholic hepatitis, or drug-induced liver injury.
ALT is more liver-specific than AST (aspartate aminotransferase) and is preferred as the primary liver enzyme marker in most UK guidelines.
What It Is
Alanine aminotransferase (ALT) is a cytoplasmic enzyme that catalyses the transfer of an amino group from alanine to α-ketoglutarate, producing pyruvate and glutamate in the glucose-alanine cycle. It is present in high concentrations in hepatocyte cytoplasm and is released into the circulation whenever the plasma membrane of a liver cell is compromised.
The degree of elevation provides prognostic information: mild elevation (1–3× ULN, typically 56–168 U/L) is most often seen in NAFLD, metabolic syndrome, and alcohol-related liver disease. Moderate elevation (3–10× ULN) suggests more active hepatitis or drug injury. Massive elevation (> 100× ULN) is characteristic of acute viral hepatitis, ischaemic hepatitis, or acute paracetamol toxicity.
Reference ranges vary by laboratory and sex — most UK labs quote 7–40 U/L for women and 7–56 U/L for men, though some studies suggest a tighter upper limit (> 30 U/L in women, > 40 U/L in men) is more appropriate for detecting early NAFLD.
Functions
Liver injury sentinel
ALT is the primary marker of hepatocyte damage — it rises before symptoms develop and is the standard first-line test for liver health.
Disease severity indicator
The degree of ALT elevation correlates with the extent of liver inflammation — tracking ALT over time monitors disease progression or treatment response.
NAFLD screening marker
Elevated ALT in an asymptomatic person with metabolic risk factors (obesity, diabetes, dyslipidaemia) is the most common presentation of non-alcoholic fatty liver disease.
Treatment monitoring tool
ALT is used to monitor the response to liver-protective interventions (weight loss, alcohol reduction, antiviral therapy) and to detect drug-induced liver injury.
Reference Ranges
Alanine Aminotransferase (ALT)
Measured in U/L| Status | Range (U/L) | What it means |
|---|---|---|
| Normal | 7–56 | No significant liver cell damage detected. |
| Mild | 57–168 | Mildly elevated — investigate for NAFLD, alcohol use, or medication effects. |
| Moderate | 169–560 | Moderate elevation — active hepatitis, drug injury, or alcohol-related liver disease likely. |
| Markedly elevated | > 560 | Urgent investigation required — acute viral hepatitis, ischaemic injury, or toxicity. |
Reference ranges vary between laboratories and by sex. Some guidelines recommend lower upper limits (30 U/L women, 40 U/L men) for detecting early NAFLD. Strenuous exercise raises ALT — avoid 24h before testing.
Symptoms of Imbalance
Liver disease is often asymptomatic until advanced stages — elevated ALT may be the only early sign. Symptoms, when present, suggest significant hepatic injury.
- Low ALT with liver disease symptoms may indicate end-stage liver failure with few remaining hepatocytes.
- Fatigue and malaise
- Right upper abdominal discomfort
- Nausea and loss of appetite
- Jaundice (yellow skin/eyes) in severe cases
- Dark urine
- Pale stools
- Itching (cholestasis)
Causes of Imbalance
- Low ALT is normal. Very low ALT occasionally seen in severe malnutrition, uraemia, or hypothyroidism.
- Non-alcoholic fatty liver disease (NAFLD) — most common cause in the UK
- Alcohol-related liver disease
- Viral hepatitis (hepatitis A, B, C, E)
- Drug or supplement-induced liver injury
- Autoimmune hepatitis
- Coeliac disease (ALT normalises on gluten-free diet)
- Haemochromatosis
- Intense exercise (transient elevation)
FAQs
In most cases, mildly elevated ALT (up to 3× the upper limit of normal) in an otherwise healthy person reflects non-alcoholic fatty liver disease (NAFLD). Other common causes include alcohol use, certain medications, intense exercise, and coeliac disease. A repeat test after lifestyle changes and abstaining from alcohol, followed by ultrasound if still elevated, is the standard approach.
Yes. Drug-induced and supplement-induced liver injury (DILI and SILI) are significant causes of elevated ALT. Common culprits include high-dose vitamin A, anabolic steroids, herbal supplements (green tea extract, kava, comfrey), niacin, and statins. Always inform your doctor of all supplements when investigating elevated ALT.
Simple fatty liver (NAFLD) is reversible with weight loss and lifestyle change. However, in around 20% of cases it progresses to non-alcoholic steatohepatitis (NASH) — active inflammation — which can lead to fibrosis and cirrhosis over years. Regular monitoring of ALT and liver imaging is important once NAFLD is diagnosed.
In alcohol-related liver disease, ALT typically falls significantly within 2–4 weeks of complete alcohol cessation. GGT normalises more slowly (6–8 weeks). Persistently elevated ALT despite abstinence suggests structural liver disease (fibrosis or cirrhosis) and requires further investigation.
Yes. Strenuous exercise — particularly high-intensity training, weight lifting, or running — can raise ALT and AST from micro-muscle damage within hours. The effect usually resolves within 24–72 hours. For accurate liver enzyme testing, avoid strenuous exercise for at least 24 hours before your blood test.
References
- Pratt DS, Kaplan MM. Evaluation of abnormal liver-enzyme results in asymptomatic patients. N Engl J Med. 2000;342(17):1266–1271. View source
- NICE. Non-alcoholic fatty liver disease (NAFLD): assessment and management. NG49. 2016. View source
- European Association for the Study of the Liver. EASL–EASD–EASO Clinical Practice Guidelines for the management of non-alcoholic fatty liver disease. J Hepatol. 2016;64(6):1388–1402. View source