Hyperthyroidism results from excessive production of thyroid hormones by the thyroid glands in the neck, and can affect the cardiovascular system, which includes the heart.

In most cases this is caused by benign enlargement of the glands rather than cancer. The increased hormone levels accelerate the cat’s metabolism, which explains many of the clinical signs you may see in your cat. Cats often lose weight despite eating well, and may appear restless or unusually active. Increased thirst, vomiting, and changes in coat quality are also common.

The cardiovascular system is particularly affected by excess thyroid hormone. Persistent stimulation of the heart leads to a faster heart rate and increased workload for the cardiac muscle. Over time this can cause thickening of the ventricular walls and other changes consistent with hypertrophic cardiomyopathy. Some cats develop heart murmurs or arrhythmias, and in more advanced cases signs of congestive heart failure may occur. Importantly, many of these cardiac changes are secondary to hyperthyroidism and can improve once thyroid hormone levels return to normal following treatment.