Hyperthyroidism & Heart Mumurs in Cats

"My heart only murmurs for you."
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If your vet detects a heart murmur in your older kitty, it's possible she'll recommend further testing for hyperthyroidism as well as heart disease. Heart murmurs can result from underlying diseases such as these, which eventually leads to hypertrophic cardiomyopathy, or thickening of the heart muscle.

Heart Murmurs

When your vet listens to Puffy's heart with a stethoscope, she's monitoring the sounds it makes. If she detects a heart murmur, that means she hears something abnormal. Vets grade heart murmurs on a scale of 1 to 4, based on loudness, where they occur in the heart's beating cycle and how long they last. Most feline heart murmurs occur during the systolic phase, when the heart pumps blood out. While a grade 1 murmur might be difficult for your vet to hear, a grade IV is so loud she can feel it if she puts her hand on Puffy's chest. The next step is finding out the reason for the murmur. In elderly cats, it might be hyperthyroidism.


Hyperthyroidism results when your cat's thyroid glands head into overdrive, producing too much thyroid hormone. This generally involves a benign tumor on the thyroid glands. Since these hormones help regulate much of Puffy's metabolism, overproduction can lead to problems not only with his heart, but with his kidneys and liver. In addition to heart disease, cats suffering from hyperthyroidism might be diagnosed with diabetes and high blood pressure. If your cat loses weight even though he's always hungry; drinks water excessively and exhibits negative personality changes, suspect hyperthyroidism, especially if he's a senior citizen.

Hypertrophic Cardiomyopathy

If your cat has a heart murmur and your vet suspects hypertrophic cardiomyopathy because of hyperthyroidism, consider yourself lucky. That's because your cat was diagnosed. Unfortunately, for many cat owners the first sign of hypertrophic cardiomyopathy is finding their pet dead. The cat's heart walls thicken, as scar tissue encroaches on the heart's chambers, making pumping action more difficult. Besides certain symptoms of hyperthyroidism, cats affected with hypertrophic cardiomyopathy become lethargic. However, while cats whose hyperthyroidism hasn't seriously affected the heart lose weight even though constantly hungry, felines with hypertrophic cardiomyopathy lose their appetites.


Your vet takes blood samples to test your cat's thyroid levels to determine hyperthyroidism, as well as feeling the glands in the neck to check for enlargement. To diagnose cardiac disease after hearing the heart murmur, she'll check your cat's heart via chest X-ray or ultrasound, as well as an electrocardiogram.


According to VCA Animal Hospitals, "Hyperthyroidism is the most treatable cause of cardiomyopathy since complete resolution of the heart disease is possible if diagnosed and treated early." Treatment can include surgery to remove the thyroid glands, but that's probably not an option in a cat diagnosed with heart disease. Your vet can prescribe methimazole, marketed under the brand name Tapazole, a pill you must give Puffy two or more times daily for the rest of his life, along with frequent vet visits to check his thyroid levels. The gold standard for hyperthyroid treatment is radioactive iodine therapy, which offers a cure. Only certain facilities offer this option, as your cat is injected with radioactive iodine that destroys malfunctioning thyroid tissue. While there are few, if any, side effects to this treatment, your cat must remain quarantined at the facility for a period of time until he is no longer radioactive. You can't have contact with him during that time frame.

Always check with your veterinarian before changing your pet’s diet, medication, or physical activity routines. This information is not a substitute for a vet’s opinion.

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