ALT (alanine aminotransferase) is an enzyme concentrated in your cat’s liver cells, and it spills into the bloodstream when those cells are damaged or inflamed. The normal range for cats is roughly 28 to 109 U/L, based on Cornell University’s reference intervals. An elevated result doesn’t point to one specific disease. It signals that something is injuring liver cells, and the list of possible causes ranges from a primary liver problem to a thyroid condition to a medication side effect.
How ALT Works as a Liver Marker
ALT sits inside hepatocytes, the main working cells of the liver. When those cells are stressed, swollen, or dying, their membranes become leaky and ALT escapes into the blood. The higher the number, the more cells are affected at that moment. In cats, ALT has a very short half-life of roughly 3 to 4 hours, which means levels can rise and fall quickly. After an acute injury, ALT typically peaks around 48 hours and then starts declining if the damage stops. This short half-life is actually useful: if your vet rechecks the value a few days later and it’s dropping, that’s a sign the liver is recovering.
The degree of elevation matters. A mild bump just above the reference range can be clinically insignificant or reflect a minor, self-limiting issue. Moderate to marked increases, especially several times the upper limit, suggest active liver disease and usually prompt further testing such as ultrasound or biopsy.
Fatty Liver Disease (Hepatic Lipidosis)
Hepatic lipidosis is one of the most common liver diseases in cats and a frequent reason for elevated ALT. It happens when a cat stops eating for several days, often because of stress, illness, or a sudden diet change. The body floods the liver with fat for energy processing, but feline livers handle this poorly, and the fat accumulates inside the liver cells until they stop functioning normally.
ALT and AST are typically mildly to moderately elevated in most cases. Interestingly, if ALT rises to levels equal to or greater than another liver enzyme called ALP, that pattern suggests something beyond simple fatty liver may be going on, such as cholangitis or a tumor, and your vet will likely investigate further. Overweight cats and cats with diabetes are at higher risk, partly because poorly controlled diabetic cats tend to develop the fat and cholesterol changes that set the stage for lipidosis.
Cholangitis and Inflammatory Liver Disease
Cholangitis is inflammation of the bile ducts inside the liver, and it’s the other major primary liver disease in cats. It comes in several forms: a bacterial infection that causes acute illness, a chronic lymphocytic form driven by the immune system, and a destructive form associated with ongoing bile duct damage.
All forms tend to produce moderate to marked increases in ALT and AST, with more variable changes in ALP and GGT. Cats with cholangitis may show vomiting, weight loss, jaundice (a yellow tint to the gums or whites of the eyes), or lethargy. The disease often waxes and wanes, so enzyme levels can fluctuate with the activity of the inflammation. Diagnosis usually requires ultrasound and sometimes a liver biopsy to identify which type is present, since treatment differs for each.
Hyperthyroidism
This is one of the most overlooked causes of high ALT in cats, especially in middle-aged and older animals. A study of 217 hyperthyroid cats found that nearly 57% had elevated ALT. The thyroid gland produces excess hormone, which ramps up the body’s metabolic rate. The liver, working overtime to keep up with increased oxygen and energy demands, sustains low-grade damage in the process.
The exact mechanism isn’t fully understood, but the leading theory involves relative oxygen starvation in liver tissue as the overactive metabolism outpaces blood supply. The good news: these enzyme elevations generally improve once the hyperthyroidism is brought under control, whether through medication, radioactive iodine therapy, or other approaches. The elevations appear to be a consequence of the thyroid condition itself rather than a sign of independent liver disease.
Medications and Toxins
Several drugs commonly prescribed for cats can raise ALT as a side effect. Methimazole, ironically the most widely used medication for hyperthyroidism, can cause an immune-mediated liver reaction in some cats, leading to loss of appetite, jaundice, and rising ALT and AST. This is an idiosyncratic reaction, meaning it’s unpredictable and not dose-dependent.
Diazepam (commonly known as Valium) and related sedatives like oxazepam and alprazolam can trigger fulminant liver failure in cats. This is a rare but potentially fatal reaction where massive areas of liver tissue die. Griseofulvin, an antifungal medication, can also cause lethargy, poor appetite, and increased ALT in cats. Because cats lack certain liver enzymes that other species use to detoxify drugs, they’re more vulnerable to medication-related liver injury than dogs or humans.
Common household toxins are another concern. Lilies, certain essential oils, and acetaminophen (Tylenol) are all severely toxic to cats and can cause dramatic ALT spikes as liver cells are destroyed.
Infections That Affect the Liver
Feline infectious peritonitis (FIP) is a viral disease that can cause significant liver inflammation. In cats with FIP, the liver becomes a source of inflammatory signaling molecules, and tissue examination typically reveals characteristic lesions: patches of granulomatous inflammation and tissue death surrounded by inflammatory debris. The liver also ramps up production of acute phase proteins like alpha-1 acid glycoprotein, which is actually used as part of the diagnostic workup for FIP.
Other infections that can elevate ALT include toxoplasmosis, bacterial infections that spread to the liver, and in some regions, liver flukes. Systemic bacterial infections, including sepsis, can raise ALT both through direct liver involvement and through the general inflammatory stress they place on the body.
Muscle Damage as a Source
ALT is considered liver-specific in cats, but skeletal muscle does contain small amounts of the enzyme. Severe muscle injury from trauma, blood clot events (like saddle thromboembolism, where a clot blocks blood flow to the hind legs), or prolonged seizures can push ALT up, though usually not as dramatically as liver disease does.
The way to tell the difference is by checking another enzyme called creatine kinase (CK). CK is almost exclusively a muscle enzyme. If CK is extremely elevated, especially above 30 times the upper limit, alongside a modest ALT increase, the ALT is likely leaking from damaged muscle rather than the liver. If CK is normal and ALT is high, the liver is the source.
Poorly Controlled Diabetes
Cats with diabetes that isn’t well managed tend to have higher ALT levels than cats whose blood sugar is under good control. A large retrospective study comparing well-controlled and poorly controlled diabetic cats found that the poorly controlled group had significantly higher ALT, along with elevated cholesterol and triglycerides. This pattern suggests that the metabolic disruption of uncontrolled diabetes promotes fat accumulation in the liver, essentially creating the conditions for hepatic lipidosis. Getting blood sugar regulated often improves liver values over time.
What the Degree of Elevation Tells You
Not all ALT elevations carry the same urgency. A value slightly above the reference range on an otherwise normal blood panel in a cat that feels fine may just need rechecking in a few weeks. Because ALT’s half-life in cats is only 3 to 4 hours, a persistently elevated level means the liver is experiencing ongoing damage, not just residual enzyme from a one-time event.
Mild elevations (less than two to three times the upper limit) are common with fatty liver disease, early hyperthyroidism, or minor medication effects. Moderate to marked elevations, particularly when ALT climbs well above ALP, point more toward active inflammatory liver disease like cholangitis, toxin exposure, or aggressive infections. Your vet will interpret the ALT number alongside other enzymes, bilirubin, protein levels, and imaging findings to narrow down the cause. A single elevated ALT value is a starting point, not a diagnosis.