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Hyperthyroidism is an overproduction of thyroid hormone by the thyroid glands. There are 2 thyroid glands located in the front of the neck. One or both of the glands can enlarge (98% have a benign tumor) and over produce thyroid hormone. This condition keeps the cat's
metabolic rate running at an abnormally high speed which over-stimulates virtually every organ system in the body. Hyperthyroidism occurs most commonly in older cats and is rare in dogs. The average age of cats with hyperthyroidism is 13 years, with only about 5% of hyperthyroid cats younger than 10 years. Hyperthyroidism is rare in cats less than 8 years old.


  • weight loss
  • increased appetite
  • hyperactivity and restlessness
  • behavioral changes such as aggressive or "cranky" behavior
  • extremely rapid heartbeat and/or arrhythmia
  • increased water consumption 
  • increased urination
  • periodic vomiting
  • increased amount of stool or diarrhea
  • fever
  • shedding/hair loss or unkempt haircoat
  • occasionally weakness, depression and poor appetite


The diagnosis of hyperthyroidism is a clinical diagnosis verified by laboratory tests. Because many factors affect thyroid laboratory test results, one normal value does not exclude the diagnosis in a symptomatic patient. Likewise, one abnormal result must be confirmed by a veterinary exam. A diagnosis of hyperthyroidism is made when the level of thyroid hormone (T4) is increased in the blood. Most hyperthyroid cats have very high T4 levels, 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 different tests called a free T4 (fT4) or a TSH level. In addition, the enlarged thyroid gland(s) can often be felt in the neck on a physical examination.

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. The heart may appear enlarged on x-ray or ultrasound and may show abnormal electrical activity on an electrocardiogram (EKG).  Cats with serious heart disease and hyperthyroidism need to be treated for both diseases. The heart disease and high blood pressure will reverse in many cats, but not all, after successful treatment of hyperthyroidism. 


Felines have the same three therapeutic options as human patients. All three treatments will reduce thyroid hormone levels and the signs of hyperthyroidism. If your cat is diagnosed with hyperthyroidism, we will discuss the three options with you in detail. If your cat has other diseases, one treatment may be better suited for your cat than another. The three options are:

· Anti-thyroid medication to suppress thyroid hormone production
· Surgical removal of the thyroid gland containing the tumor (Thyroidectomy)
· Radioactive Iodine (I-131) injection


Medical treatment is a lifelong commitment to giving your cat a pill once to twice a day. The anti-thyroid pill is methimazole, also known as Tapazole. Tapazole controls the disease, but it is not a cure. Regular blood tests are required to adjust the dosage and to check for dangerous side effects. The tumor often continues to grow and the dosage must be increased over time. 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 period of 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.


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. Parathyroid gland damage causes low blood calcium that may cause seizures.  Low blood calcium is treated with calcium or vitamin D.

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. Cats that have had surgery may have recurrence of hyperthyroidism, especially cats in which only one thyroid gland required removal.  Blood thyroid hormone levels should be measured once or twice a year to determine if hyperthyroidism has returned.


Radioactivity avoids the stress and complications of surgery and the side effects, hassle and on-going expense of pills. Just one injection can safely destroy the complete thyroid tumor – wherever its location – in roughly 98-99% of cases. Cats do not have to be placed under anesthesia for the procedure. The disadvantages of radioactive iodine treatment include the fact that treatment with radioactive iodine is only performed at selected specialty veterinary practices 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). Most cats do not need additional treatment, however approximately 2% may require re-treatment later because of interfering medications, a larger tumor mass, or a cancerous tumor instead of a benign one. Less than 1% of treated cats may require thyroid supplementation due to unusual sensitivity to I-131.




If your cat is exhibiting signs of hyperthyroidism, please call 516-379-6200 to schedule an appointment for an evaluation today.


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