Feline Hyperthyroidism

cute cat picture The thyroid glands are located in the neck on either side of the trachea. They secrete hormones that help regulate the body's metabolic rate. Hyperthyroidism is a disease characterized by overproduction of thyroid hormone with a subsequent increase in metabolic rate. It is a common disease of older cats. At University Veterinary Hospital, approximately 10% of our geriatric feline patients have been diagnosed as hyperthyroid; however, the actual percentage of feline patients affected is probably greater since we commonly see cats as emergency patients who do not need to be tested for hyperthyroidism as part of their emergency care. Hyperthyroidism typically results from a benign growth in one or both glands. Less than 2% of hyperthyroid patients have hyperthyroidism as a result of a malignant, or cancerous, growth.

What cats are more likely to become hyperthyroid?

The likelihood that a cat will develop hyperthyroidism increases with age; average age at diagnosis is 12 years old. Environmental and dietary risk factors have been investigated, but at this time it is unknown what role these factors may play in the development of hyperthyroidism. No breed predisposition exists, however, Siamese cats seem to be over-represented in the hyperthyroid population.

What are the signs of hyperthyroidism?

Hyperthyroidism affects every organ and body system. Some of these changes can be recognized by pet owners; others must be diagnosed by a veterinarian through physical examination or bloodwork.

The most common sign is weight loss, often in the face of an increased appetite (although later in the disease, some cats become anorexic). Rate of weight loss varies among cats; some cats lose weight so gradually that their owners do not realize it is happening, and some cats lose weight very quickly. Affected cats often drink more water than normal, urinate frequently, have diarrhea, vomit, and have greasy or unkempt coats. These signs are also associated with diabetes and kidney disease and can only be distinguished by performing lab work.

Many cats with hyperthyroidism have heart murmurs and high blood pressure because excess thyroid hormone causes the heart muscle to become thick and stiff. This is called thyrotoxic cardiomyopathy or hypertrophic cardiomyopathy. Affected hearts do not pump blood efficiently like normal hearts. This can lead to blood clot formation in the chambers of the heart and retinal detachment due to high blood pressure due to high blood pressure.

Hyperthyroid cats may also be hyperactive, urinate or defecate outside the litter box, or have behavior/personality changes.

What causes hyperthyroidism?

Hyperthyroidism is caused by a tumor of the pituitary or thyroid glands. 98% of cats with hyperthyroidism have a benign tumor of one of the thyroid glands. The remaining 2% have malignant tumors. Hyperthyroidism caused by a pituitary tumor is rare.

How is it diagnosed?

In most cases diagnosis is relatively simple. A blood test is performed to measure the level of thyroxine (T4). Usually the T4 level is well above the reference range and no further testing is needed for diagnosis. However, some cats who have signs of hyperthyroidism have a T4 level in the high normal range. In these cases free T4 should be measured. If free T4 is also non-diagnostic, a triiodothyronine (T3) suppression test or a radionuclide scan can be performed or the free T4 can be repeated in 4 to 8 weeks.

What are my options for treatment?

Several options are available. The best treatment for a particular cat depends on the cat's overall health at the time of diagnosis, financial considerations, and willingness of the cat to take medications. Before any treatment is begun the overall health of the cat should be evaluated by a physical examination, complete blood count, serum chemistry profile, urinalysis, blood pressure measurement, and possibly x-rays and ultrasound of the heart.

Radioactive iodine - an injection is given which destroys the hyperfunctioning thyroid tissue while sparing normal tissue. This injection requires hospitalization at a veterinary hospital licensed to administer radioactive therapy. At University Veterinary Hospital we refer patients to Dr. Turrel at Veterinary Oncology Specialists. If you are outside the Bay Area, please contact your local veterinarian for a referral. Because of its high success rate, radioactive iodine is the preferred treatment. Rarely a cat may require two injections for cure or become hypothyroid after the injection. Cats treated once with radioactive iodine may have recurrence of the disease later in life if a new tumor grows.
Oral medication - - methimazole may be given orally once or twice a day in the form of a pill or a liquid. Methimazole blocks the formation of thyroid hormone but it does not affect the abnormal thyroid tissue directly; therefore, it is a lifelong treatment. Side effects are seen in less than 20% of cats and include vomiting, lethargy, anorexia, fever, anemia, and facial excoriations. Side effects may occur up to six months after initiating therapy. Blood tests must be performed every few weeks when treatment is begun to monitor for side effects and to make dosage adjustments as needed. Patients undergoing radioactive iodine therapy receive a few weeks of methimazole beforehand.
Methimazole in PLO gel - this form of methimazole can be applied topically to the ear. Absorption of the drug through the skin is not as predictable as absorption from the GI tract and may not work as well as oral formulations. This treatment is appropriate for cats who do not tolerate oral medication due to vomiting or difficulty of administration.
Surgery - the affected thyroid lobe(s) may be surgically removed. One or two surgeries may be needed depending on how much tissue is affected. There is always a risk whenever an animal is put under anesthesia; however, if the previously discussed tests to determine your pet's health status have been performed and the proper precautions are taken, such as placing an IV catheter and monitoring during surgery, the risk is minimal. Methimazole is usually administered orally for a few weeks before the surgery. If abnormal cells are left behind after the surgery, they can continue to cause hyperthyroidism, but this complication is uncommon. As with radioiodine therapy, new growths may occur in the future.
Ethanol injection - ethanol is injected into the abnormal thyroid tissue under ultrasound guidance. Due to potential complications such as damage to surrounding tissues and laryngeal nerve paralysis, this procedure is not recommended.

What is the prognosis?

Since most thyroid tumors are benign, any of the above recommended therapies usually results in a good to excellent prognosis. However, the specific prognosis for each individual cat depends on the duration of the disease, the cat's state of health, and possibly the cat's tolerance of medication.

Treating hyperthyroidism often unmasks kidney disease. This is because excess thyroid hormone causes an increased blood flow through the kidneys which compensates for loss of function. Once the blood flow to the kidneys is restored to normal, loss of function becomes evident. Kidney disease is often treatable and the two diseases are amenable to concurrent treatment.

Can hyperthyroidism be prevented?

At this time there are no known preventive measures. However, geriatric cats (those over 7 years old) should have physical exam every six to twelve months and yearly bloodwork in order to catch the disease early, before the cat gets sick enough to show signs.

Based in part on info from University of Gelph

Phone: (510) 841 - 4412 Fax: (510) 841 - 3019 email: staff AT uvhberkeley DOT com

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