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Diagnosis: Hyperthyroidism


Your cat has been acting pretty cranky and you just got word from your veterinarian that she has hyperthyroidism. Hyperthyroidism is an endocrine disease that affects older cats primarily. It is a commonly diagnosed problem in veterinary practice.


The thyroid gland is made up of two small glands that lie on either side of the trachea (windpipe) in the center of the neck. Hyperthyroidism is most commonly caused by a benign tumor that grows in the thyroid gland. Sometimes, the gland is enlarged to the point that your veterinarian will be able to feel what is called a “thyroid blip” when feeling your cat’s neck.


As mentioned in the anatomy section, hyperthyroidism is most commonly caused by a benign tumor growing in the thyroid gland. It can also be caused by a generalized increase in the gland and its production of thyroid hormone.


The symptoms of hyperthyroidism include weight loss, big appetite, vomiting, dull hair coat, dandruff, irritability, and excessive vocalization. These symptoms can also indicate many other problems, so veterinary care is critical.


Diagnosis of hyperthyroidism is based on clinical symptoms and results of a blood test. Usually a diagnosis can be made based on elevated T4 levels in the blood, but sometimes a test called a Free T4 confirms a case with borderline T4 results.


There are three treatment options for hyperthyroidism. Oral medication to suppress the thyroid gland can be administered. This medication is called methimazole. One disadvantage to this treatment is that pills must be given daily for the lifetime of the cat. Blood levels of thyroid hormone should be monitored three weeks after therapy is begun to determine correct dosage. Sometimes, blood cell counts will be decreased as a side effect of the medication. Blood cell counts should be measured every 2 weeks for the first 3 months of treatment. Surgery can also be done to remove the abnormal thyroid. There are usually enough thyroid cells left behind to produce normal levels of thyroid hormone. If not, hypothyroidism can result. Cats that are going to have their thyroid gland surgically removed should receive oral methimazole to reduce circulating thyroid hormone prior to surgery. Damage to the delicate parathyroid glands or to the structures of the neck (nerves, voicebox) can cause additional side effects. Radioactive iodine therapy can be done at referral hospitals or veterinary teaching hospitals. An injection of radioactive iodine is administered intravenously. The thyroid gland takes up the iodine and the radioactivity destroys the abnormal parts of the gland. The normal thyroid cells are not destroyed. This treatment is costly, but there are few side effects and lifelong medication is not required.


Cats with hyperthyroidism often have other medical concerns, such as kidney disease. Laboratory monitoring for thyroid hormone levels, blood cell counts, and organ function are important parts of follow-up care.


Prognosis with treatment is fair to good. Cats with hyperthyroidism are typically older so can suffer from multiple health problems. Prognosis without treatment is poor.

Please note that this information does not replace professional veterinary care. It is solely for educational purposes. Your pet's medical condition should be evaluated by a veterinarian before any medical decisions are implemented. If there is a potentially life-threatening emergency involving your pet, take your pet to a veterinarian or veterinary facility immediately.

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