The most common cause of an enlarged heart in cats is hypertrophic cardiomyopathy (HCM), a condition where the muscular walls of the heart thicken abnormally. Studies of apparently healthy cats estimate HCM prevalence at roughly 14 to 16%, making it far more common than most owners realize. Beyond HCM, several other conditions can cause the heart to enlarge, including overactive thyroid glands, high blood pressure, nutritional deficiencies, and less common forms of heart muscle disease.
Hypertrophic Cardiomyopathy: The Leading Cause
HCM causes the walls of the left ventricle to grow thicker than normal. A healthy cat’s heart wall measures less than 6 millimeters on ultrasound; anything at or above that threshold raises concern. As the walls thicken, the chamber inside shrinks, meaning the heart holds less blood with each beat and has to work harder to keep up. Over time, this strain can cause the left atrium (the chamber above the ventricle) to stretch and enlarge as blood backs up.
The cause of most HCM cases is unknown, though many are believed to be genetic. In some breeds, specific mutations have been identified. Ragdolls carry a mutation in the MYBPC3 gene (known as R820W) that alters a protein critical for normal heart muscle development. Maine Coons have a different mutation in the same gene, called A31P. Both are inherited in an autosomal dominant pattern with incomplete penetrance, meaning a cat can carry the mutation and never develop obvious disease, or it can develop severe disease relatively young. The mutations have also been detected in Ragamuffins, American Bobtail Longhairs, Highlanders, and Munchkins.
Genetic testing is available for both known mutations through veterinary genetics labs, but a negative test result doesn’t guarantee a cat is free from HCM. Many cats, including mixed breeds, develop the condition through mutations that haven’t been identified yet.
Hyperthyroidism and Heart Enlargement
An overactive thyroid gland is one of the most common hormonal disorders in older cats, and it places enormous strain on the heart. Excess thyroid hormones drive up the body’s overall metabolic rate, forcing the heart to beat faster and pump harder. This sustained overwork causes the heart muscle to thicken in a pattern that looks very similar to HCM on an ultrasound.
The effects go beyond simple overwork. Thyroid hormones directly alter how heart muscle cells function at the genetic level, changing the expression of genes responsible for cell structure and contraction strength. They also increase the sensitivity of the heart to adrenaline-like signals, pushing the heart rate even higher. If untreated, this cascade can progress to left atrial enlargement and eventually congestive heart failure. The good news is that heart changes caused by hyperthyroidism are often at least partially reversible once thyroid levels are brought under control.
High Blood Pressure
Chronic high blood pressure (hypertension) forces the heart to pump against greater resistance with every beat. In response, the left ventricle thickens, a change called concentric hypertrophy. Veterinarians often pick up signs of this during a routine exam: abnormal heart sounds, including gallop rhythms (an extra heart sound that creates a three-beat pattern), are common in hypertensive cats.
Interestingly, the degree of heart wall thickening doesn’t always match how high the blood pressure is. Some cats with moderately elevated pressure develop significant thickening, while others with very high readings show less change. Hypertension in cats is usually secondary to another condition, most often kidney disease or hyperthyroidism, so treating the underlying problem is a key part of protecting the heart.
Taurine Deficiency and Dilated Cardiomyopathy
Unlike the thickening seen in HCM, dilated cardiomyopathy (DCM) causes the heart chambers to stretch and balloon outward, making the heart larger overall but weaker with each contraction. Before 1987, DCM was a major killer of cats. Researchers discovered that most cases were caused by a deficiency in taurine, an amino acid essential for heart muscle function. Since then, all commercial cat foods have been supplemented with taurine, and the incidence of DCM has dropped dramatically.
Cases still surface occasionally, almost always in cats fed noncommercial diets: vegetarian formulations, baby food, or home-cooked meals that haven’t been properly balanced. The prognosis depends heavily on timing. Cats with taurine-responsive DCM face a high risk of death in the early stages, but those that survive the first two to three weeks of supplementation often recover completely, as the heart damage is fully reversible once taurine levels are restored. Most DCM cases diagnosed in cats today, however, are not taurine-responsive and have no identifiable cause.
Restrictive Cardiomyopathy and Other Forms
Restrictive cardiomyopathy (RCM) is less common but still seen in cats. In this form, excessive scar tissue builds up on the inner lining and within the muscle of the ventricle. The scarring prevents the heart from relaxing fully between beats, so it can’t fill with enough blood. Over time, blood backs up into the atria, causing them to enlarge even though the ventricle walls may not be dramatically thickened.
Other potential triggers for heart enlargement include inflammation of the heart muscle (myocarditis) and, rarely, excess growth hormone. For the majority of feline cardiomyopathies, however, the specific cause remains unknown. Cornell University’s Feline Health Center notes that many cases are either confirmed or presumed to be genetically driven.
Signs You Might Notice at Home
Enlarged hearts in cats are notoriously hard to detect early because cats are masters at hiding illness. Many cats with significant heart disease show no symptoms at all until the condition is advanced. When signs do appear, they often relate to fluid buildup or poor circulation.
One of the most reliable things you can monitor at home is your cat’s resting breathing rate. A healthy cat at rest or asleep breathes between 15 and 30 times per minute. Rates consistently above 30 breaths per minute are abnormal and can signal that fluid is accumulating in or around the lungs due to heart failure. Count breaths for 15 seconds and multiply by four while your cat is calm or sleeping. Other signs include lethargy, reduced appetite, hiding, and open-mouth breathing.
The Risk of Blood Clots
One of the most dangerous complications of an enlarged heart in cats is arterial thromboembolism, often called a “saddle thrombus.” When the heart chambers enlarge, blood can pool and form clots inside the heart. If a fragment breaks loose, it travels through the bloodstream until it lodges in an artery, cutting off blood flow to whatever tissue lies downstream. In roughly 90% of cases, the clot lodges at the base of the aorta where it splits to supply the hind legs.
The result is sudden, severe pain and paralysis of one or both hind legs. The affected legs may feel cold, and the paw pads may look pale or bluish. Studies estimate that 6 to 17% of cats with underlying heart disease eventually develop this complication. It is a medical emergency, and it can be the first sign that a cat has heart disease at all.
How Enlarged Hearts Are Diagnosed
Echocardiography, an ultrasound of the heart, is the gold standard for diagnosing cardiomyopathy in cats. It allows a veterinarian to measure wall thickness, chamber size, and how well the heart contracts and relaxes. Chest X-rays are commonly used alongside ultrasound to check for signs of congestive heart failure, such as fluid in or around the lungs. On X-rays, veterinarians use a measurement called the vertebral heart score (VHS) to assess heart size. A normal VHS in a cat ranges from 6.9 to 8.1 on a side-lying view; values above that range suggest the heart is enlarged.
A blood test measuring a protein called NT-proBNP can serve as a screening tool when echocardiography isn’t immediately available. Elevated levels suggest the heart is under stress, though the test isn’t definitive on its own. For suspected hyperthyroidism or kidney disease, additional bloodwork helps identify whether the heart changes are secondary to another condition that can be treated directly.