Cardiomyopathy in Cats and Hyperthyroidism

cardiomyopathy in catsThyrotoxic cardiomyopathy in cats is a condition where the muscles of the heart become unable function at optimum levels. Hyperthyroidism can increase the severity of heart disease in cats.

There’s two secondary complications of hyperthyroidism in cats that can be significant. Hypertension (high blood pressure) and heart disease ( thyrotoxic cardiomyopathy).

A myopathy is a muscular disease in which the muscle fibers do not function for any one of many reasons, resulting in muscular weakness.

Thyrotoxic myopathy is a neuromuscular disorder that develops due to the overproduction of the thyroid hormone thyroxine.

Due to the increased pumping pressure and elevated heart rate that occurs with thyrotoxic cardiomyopathy (hyperthyroid cardiomyopathy), over time hypertension develops. Progressive cardiomyopathy leads to heart failure or other serious consequences.

In some cases, the high blood pressure can become so severe that retinal hemorrhage or retinal detachment will occur and result in sudden blindness. Heart conditions develop when the heart must enlarge and thicken to meet the increased metabolic demands.

Hypertrophic Cardiomyopathy in Cats (HCM)

HCM in cats causes thickening of the heart muscle resulting in poor function. As the ventricle (pumping chamber) becomesprogressively thicker, less blood enters the chamber and less blood is pushed out into the body.

The cause of HCM in cats is unknown, although certain breeds appear to be predisposed.

This disorder is most common in middle-aged male cats. HCM can develop secondary to other disorders such as hyperthyroidism and hypertension. A thyroid hormone test for hyperthroidism in cats should be done to exclude these secondary causes when cardiac hypertrophy (thickening) is diagnosed.

Most feline heart disease is hereditary. Usually cases are not linked to lifestyle, diet, or home care. Cardiomyopathy is a progressive disease, Unfortunately,  many cats do not exhibit a strong response to treatment. Although some do not show any symptoms or develop heart failure with hypertrophic cardiomyopathy in cats or develop heart failure, others die from the disease.

Symptoms of heart failure:

  • Decreased appetite and weight loss
  • Difficulty breathing – rapid breathing
  • Weakness
  • Coughing
  • Lethargy

Some cases of cardiomyopathy in cats suffer sudden paralysis of one or both hind legs. For other cats, the first symptom of heart disease is sudden death.

Hypertrophic cardiomyopathy in cats can cause blood clots to form in the heart and then flow into the rear legs. Leading to sudden paralysis of one or both hind legs. This serious condition is extremely difficult to treat and very painful and many cats are euthanized after suffering this complication.

Some felines with heart disease develop a feline heart murmur that can be detected during a physical exam. However, many cats exhibit no outward signs of disease. Because of this, many cases of cardiomyopathy are not diagnosed until the disease has progressed.

Studies have shown heart disease and cardiomyopathy in cats is believed to be largely hereditary. Siamese, Persians, and Maine Coon cats are at increased risk. Older felines are more likely to suffer from advanced heart disease than young cats.

Linked to one type of cardiomyopathy are diets with inadequate amounts of the amino acid taurine.  Most high quality commercial diets are supplemented with taurine, therefore, diet-related cardiomyopathy is very rare in some developed countries.


Cats suffering from cardiomyopathy should not be exposed to stressful situations and should not be exposed to extreme heat. Felines with heart complications are also at increased risk from anesthesia. Cats with cardiomyopathy have difficulty tolerating stress.  They may succumb to stress from heat, car travel, grooming, or veterinary procedures. Animals with heart disease sometimes die suddenly from acute massive heart failure.


Diagnosis of cardiomyopathy is a combination of diagnostic tests and physical examination including: electrocardiogram (ECG), X-rays, and ultrasound (echocardiogram). Usually Blood and urine tests are run to evaluate for hyperthyroidism and other concurrent illnesses. If thyroid disease is found, treatment is implemented to reduce its effects on the heart where some cats show clinical improvement, others do not show a marked response.

Success of therapy is based primarily upon its effect on the symptoms, for Cardiomyopathy in Cats.