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Feline Hyperthyroidism

The thyroid gland is a gland in the neck region of mammals that produces hormones which are very important in everyday metabolism.  Hyperthyroidism is a disease that occurs when a tumor of the thyroid gland causes production of too much thyroid hormone and leads to an extremely high metabolic rate.  This is one of the most common diseases to occur in a geriatric cat.

 There are many different symptoms that can be caused by this disease.  The most frequent symptom is weight loss.  Even though a cat may be eating a normal amount of food each day, the excess thyroid hormone causes a cat’s body to require even more calories than what the cat ate.  This leads to a breakdown of the fat stores in the cat’s body and therefore weight loss.  Some other common side effects include an increased appetite, diarrhea, and an increased heart rate. 

 The increased thyroid hormone can be toxic to certain body organs.  The excess hormone causes a very rapid heart rate, which can lead to cardiomyopathy (an enlarged heart) and thrombosis (blood clot formation).  The thyroid hormone needs to be processed by the liver and can lead to liver failure because the liver is being overworked.

 One interesting side effect is that the increased thyroid hormone can cause an increase of blood flow to the kidneys.  Often older cats can have some degree of kidney failure along with hyperthyroidism.  The increased blood flow is actually beneficial to the kidneys because it helps them work, even when they are failing.  Diagnosis of concurrent kidney disease can be difficult because the thyroid hormone makes it seem like a cat’s kidneys are working at normal capacity, even when they might not be.

 Hyperthyroidism is a treatable disease.  There are three forms of treatment.  One is management of the disease with medication.  This means that the cat will need to have medication given daily for the rest of his/her life to keep the thyroid disease under control.  Another form of treatment is by a subcutaneous injection of radioactive iodine.  This is done in specialty hospitals with specialized equipment to monitor the radioactivity of the cat.  The cat needs to stay in the specialty hospital for 5 - 7 days while the radioactivity wears off.  This is a cure for the hyperthyroidism and further treatment for the thyroid disease is usually not necessary.  It is common for treatment with the medication for a period of time to allow the liver or heart to heal before radioactive iodine therapy is performed.  The third form of treatment is surgery to remove the thyroid gland.  This can be risky surgery since the cat may already have some liver and heart damage from the disease.  Your veterinarian will be able to recommend the best treatment for your cat based on physical exam and laboratory test findings.

For additional information, please e-mail kpercival@cathospitalofaustin.com.

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