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Prepared and provided by: Dr. Robert Franklin, D.V.M.

Feline Hyperthyroidism is the most common endocrine disease seen in cats. The cause is excessive thyroid hormone produced most commonly from adenomatous hyperplasia (abnormal increase in the number of normal cells) or adenoma (benign tumor) of the thyroid gland. This will result in a multisystemic disease.

Clinical Signs:

Feline Hyperthyroidism occurs in middle-aged to older cats. The clinical signs can vary from very mild to severe. The most common signs of hyperthyroidism are weight loss with an increased appetite. Other signs include hyperactivity, poor hair coat, excessive drinking and urination, and intermittent vomiting and diarrhea.


The diagnosis is made from blood work measuring thyroxine T4 levels. Often, enlarged glands can be detected during routine physical examination.


There are currently 4 possible treatments for hyperthyroidism.

a) Medical Treatment– consists of the administration of methimazole (Felimazole®) up to three times daily. The drug blocks the secretion of T4 but does not remove the tumorous thyroid gland. The drug is given for the rest of your cat’s life.

b) Surgery– removes the affected thyroid gland or glands and will cure hyperthyroidism.

c) Low Iodine Diet– Hill’s Prescription y/d. Feeding the y/d diet will help to manage your cat’s hyperthyroidism but will not cure the disease.

d) Radioactive Iodine I-131– will cure hyperthyroidism and is a single, under the skin injection.  

Reasons for radioactive iodine therapy:

Medical therapy may not be the best choice for several reasons. Some cats can be extremely difficult to pill. Mild reactions are common, such as vomiting and loss of appetite. Severe adverse reactions can be seen in 5% of cats and include blood or liver problems. Periodic blood testing is necessary to monitor the cat’s condition Surgery is elective but does require anesthesia.

Because many cats with hyperthyroidism have heart problems secondary to their elevated T4 levels, they have a higher anesthetic risk. There is a chance that the parathyroid glands, vital structures associated with thyroid glands, can temporarily be damaged during surgery, resulting in low blood calcium. This is a life-threatening condition that may result in extra hospitalization and cost. Radioactive iodine does not require anesthesia or risk of parathyroid injury. Methimazole is not needed and must be discontinued for at least one week prior to treatment. Your cat will be hospitalized for 4 days after the injection. Iodine is used by the thyroid gland to produce thyroid hormones. When injected, a large percentage of the I-131 accumulates in the thyroid glands. The rest is excreted in the urine and, to a lesser degree the feces. Once the radioactivity (Iodine 131 ) is taken up by the thyroid gland, gamma and beta rays are released and kill the abnormal thyroid cells, electively treating the hyperthyroid condition. Iodine 131 is 95% elective in curing hyperthyroidism. In approximately 5% of treated cases, low T4 levels (hypothyroidism) can develop. This is easily controlled with supplementation and may not be permanent.

Responsibilities at home:

Upon discharge from the hospital, your cat will still be minimally radioactive. This level is very low but you still need to exercise a little caution and common sense. The radioactivity in your cat will gradually disappear over the next 2 to 4 weeks. During this time you will need to use a flushable litter that can be disposed of via your toilet. You cannot throw the litter out with the garbage. It is also recommended not to sleep with your cat during this time and to limit close contact (less than 12”) with your cat to one hour per day for the first 2 weeks and 2 hours per day for the following 2 weeks. Your cat will need to be contained to your house, garage, or yard during this time.


The amount of radiation in your cat is extremely low. If you were being treated for hyperthyroidism, you would receive up to 10 times the dose your cat receives and still go home on the same day of treatment. The amount of radiation you might receive from your cat would be roughly equivalent to that received when you fly roundtrip across the country. You do not need to worry about exposure to your other pets. It can take from 1 to 12 weeks for the thyroid level to return to normal. Blood work is recommended 4 to 6 weeks after treatment. If the thyroid level is still elevated, repeat blood work is again recommended 12 weeks after treatment. If the thyroid level is still elevated then the treatment is repeated at no charge. Once blood work is normal, only the routine care recommended and provided by your regular veterinarian is necessary.