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Hyperthyroidism in Cats

Comprehensive information relative to the health of your kitty.

Hyperthyroidism in Cats

Hyperthyroidism is the overproduction of thyroid hormone. It is a common syndrome and generally affects older cats. There are 2 thyroid glands located in the neck. One or both of the glands can enlarge and overproduce thyroid hormone. Involvement of both glands is more common than involvement of one gland. Thyroid hormone affects the function of most organs in the body, so the signs of hyperthyroidism are quite variable.

Signs of hyperthyroidism can include:

  • weight loss
  • increased appetite
  • increased activity and restlessness
  • aggressive or "cranky" behavior
  • a poor hair coat
  • a fast heart rate
  • increased water drinking
  • increased urination
  • periodic vomiting
  • increased amount of stool or diarrhea
  • occasionally difficulty breathing
  • occasionally weakness
  • occasionally depression

A diagnosis of hyperthyroidism is made when the level of thyroid hormone is increased in the blood. Since many of these symptoms can occur with multiple diseases or syndromes, a blood sample is required to differentiate whether or not hyperthyroidism may be the cause. Most hyperthyroid cats have very high levels of hormone but some cats will have signs of hyperthyroidism with normal or only slightly increased levels of thyroid hormone. Thyroid hormone levels can vary over time so it may be necessary to check blood levels several times or perform supplemental thyroid tests. If the diagnosis is not obvious by blood tests, a nuclear medicine scan of the thyroid glands can be performed at certain specialty veterinary practices. A blood panel and urinalysis are also performed to screen for abnormalities in other organs such as liver and kidney that may be present due to the advanced age of the animal.

High levels of thyroid hormone may cause heart disease and/or high blood pressure (hypertension). The heart may appear enlarged on x-ray or ultrasound and may show abnormal electrical activity on an ECG (electrocardiogram). Heart disease may cause fluid to build up in or around the lungs. Cats with serious heart disease or hypertension and hyperthyroidism need to be treated for both diseases. The heart disease and hypertension will reverse in many cats after successful treatment of hyperthyroidism.

There are four types of treatment for hyperthyroidism:

  • Anti-thyroid medications
  • Surgical removal of affected thyroid glands
  • Treatment with radioactive iodine
  • Y/D diet by Hill's Pet Nutrition

All four treatments will reduce thyroid hormone levels and the signs of hyperthyroidism. Discuss the four options with your veterinarian. If your pet has other diseases, one treatment may be better for your cat than another.

Anti-thyroid medication:

The anti-thyroid pill used mostly commonly today is methimazole, also known as Tapazole. Methimazole is given one to three times daily and must be continued life long. Compounded flavored chews or liquid or transdermal gel can be used for cats that will not easily take pills. It takes several weeks for methimazole to reduce blood thyroid hormone levels to normal. If methimazole is discontinued, thyroid hormone levels will return to high levels over a few weeks. Methimazole may be used to reduce thyroid hormone levels to normal before surgically removing the thyroid gland(s). Cats with heart disease may be too sick and fragile to anesthetize for surgery in which case methimazole can be given until the heart improves and the cat is stronger. Some owners (and their cats) find it difficult to give pills daily and may decide, after starting anti-thyroid pills, to later have their cat treated with radioactive iodine or surgery. Methimazole may produce side effects in cats including depression, vomiting and lack of appetite. These signs usually resolve without stopping the medication. A more serious side effect is the development of low blood cell counts which are more likely to develop during the first 3 months of treatment. Blood cell counts should be evaluated regularly during the first 3 months. If blood cell counts decrease, methimazole is stopped and another treatment method should be considered.


Enlarged thyroid glands can be surgically removed. Methimazole is given for 1 to 2 months before surgery so that thyroid hormone levels are normal at the time of surgery. If both glands are enlarged, they can both be removed and most cats will still produce enough thyroid hormone by a few thyroid cells scattered through out the body to prevent hypothyroidism (abnormally low thyroid hormone levels). A few cats will become hypothyroid and may need to take thyroid pills. Surgical removal of the thyroid gland(s) can usually be performed without complications. Occasionally complications may develop including damage to the parathyroid glands, which are closely attached to the thyroid gland, damage to nerves close to the thyroid gland or damage to the voice box. Parathyroid gland damage causes low blood calcium that may cause seizures. Low blood calcium is treated with calcium or vitamin D. Nerve damage causes abnormal size of the pupils of the eyes and droopy eyelids. Damage to the voice box causes a change in voice.

Radioactive iodine:

Some cats will remain hyperthyroid after surgical removal of the thyroid glands. These cats have thyroid cells in abnormal locations, including inside the chest cavity where surgical removal is difficult. This extra thyroid tissue is called ectopic thyroid. If you and your veterinarian decide that surgery is the best treatment option for your cat, a nuclear medicine scan (described in the diagnosis section) could be performed at a specialty veterinary practice before surgery to see if your cat has ectopic thyroid tissue. If ectopic thyroid tissue is seen on the nuclear medicine scan, then a different treatment, either methimazole or radioactive thyroid treatment should be selected. Treatment with radioactive iodine is only performed at selected specialty veterinary practices. Radioactive iodine is given by injection and will accumulate in the abnormal thyroid producing tissue killing the abnormal thyroid cells but sparing the normal thyroid cells. Radioactive iodine will accumulate in ectopic thyroid tissue. Radioactive iodine treatment is very effective and rarely causes hypothyroidism. The cats do not have to be placed under anesthesia for the procedure. The disadvantages of radioactive iodine treatment include the need to travel to a facility that offers this treatment and the need for the cat to remain hospitalized until the level of radioactivity decreases to a safe level as determined by the state radiation control office (usually 1 to 3 weeks).

Y/D diet by Hill's:

Y/D is an iodine deficient diet that works to lower the circulating thyroid hormone levels. Iodine is one of the essential building blocks needed for thyroid hormone synthesis, so feeding a diet extremely low in iodine prevents thyroid hormone production (therefore preventing the hormone to be elevated). This regulation method requires the patient's diet to consist 100% of y/d, meaning not a single bite ever of anything else can be fed (because there is enough iodine in one bite of any other food -even a single cat treat- to allow hormone production, even in excess).

Older cats with hyperthyroidism often also have kidney disease. Treatment of these cats is a delicate balancing act. Hyperthyroidism can actually improve kidney function by increasing blood flow to the kidneys. Some cats with kidney disease will show a worsening of kidney function after treatment for hyperthyroidism. Monitoring kidney function in hyperthyroid cats is imperative. Because the level of thyroid hormone can change with other medical issues, treatment dosing may need to be altered. Regular lab tests and exams will help assess the proper medication dosage.

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Remember that many human medications are very toxic to cats... NEVER give aspirin, tylenol, or any other medications to your pet without specific instructions!
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