Feline Hyperthyroidism

What is hyperthyroidism?

The thyroid is a gland located in the neck. It plays a very important role in regulating the body's rate of metabolism. Hyperthyroidism is a disorder characterized by the overproduction of thyroid hormone. When excessive amounts of thyroid hormone are in the circulation, the body's metabolism speeds up greatly.

Hyperthyroidism is a fairly common disease of older cats. Although the thyroid gland enlarges, it is usually a non malignant (benign) change. Less than 5% of hyperthyroid cases involve a malignant change in the gland.

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 What does this do to the cat?

The typical cat with hyperthyroidism is 4 years of aged or older; on the average, affected cats are about 10 years of age. The rapid rate of metabolism causes the cat to lose weight. The cat tries to compensate for this with an increased appetite. In fact, some of these cats have a ravenous appetite and will literally eat anything in sight! Despite the increased intake of food, most cats gradually lose weight. The weight loss may be so gradual that some owners will not even realize it has occurred. Affected cats usually drink a lot of water and urinate a lot. There may be periodic diarrhea, and the hair coat may be unkempt. As the disease progresses, the cat's appetite may decline to the point of anorexia.

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How is hyperthyroidism diagnosed?

The disease is most commonly diagnosed by determining the blood level of one of the thyroid hormones; the hormone most frequently measured is T4. Usually, the T4 level is so high that there is no question as to the diagnosis. Occasionally, a cat suspected of having hyperthyroidism will have T4 levels within the range of normal cats. In this case, a second test, called a Free T4 Test, is performed. If this is not diagnostic, a thyroid scan can be performed at a veterinary referral center.

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Is this disease treatable?

Because less than 5% of these cats have cancerous growths of the thyroid gland, treatment is usually very successful. There are three choices for treatment; any one of them could be the best choice in certain situations. Many factors must come into consideration when choosing the therapeutic option for a particular cat.

1) Radioactive iodine. Radioactive iodine therapy is considered to be the most effective way to destroy the abnormal tissue. This requires three to five days or more of hospitalization at a veterinary clinic licensed to administer radiation therapy. This treatment is often limited to veterinary teaching institutions because of governmental regulations regarding radioactive materials. Currently this form of treatment is available at a private clinic in Edmonds. Usually, the expense will be greater than for the other options.

2) Surgery. Surgical removal of the affected thyroid lobe(s) (thyroidectomy) is also very effective. Because hyperthyroid cats are usually over 10 years of age, there is a degree of risk involved. However, the risk is much less than most people think, as long as the cat is otherwise healthy. Tests are done before surgery to evaluate the cat and predict the chances for complications. If the disease involves both lobes of the thyroid gland, two surgeries may be required, depending on the surgeon’s choice of procedures. In many cats, only one thyroid lobe is abnormal, so only one surgery is needed.

3) Oral medication. Administration of an oral drug, methimazole, can control the effects of the overactive thyroid gland. Some cats have reactions to the drug, but that number is fairly small (less than 20%). However, the side effects may begin as late as six months after the beginning of treatment and can include vomiting, lethargy, anorexia, fever, and anemia . Methimazole does not destroy the abnormal thyroid tissue, but rather prevents the production of excess thyroid hormone. Therefore, the drug must be given for the remainder of the cat's life. Periodic blood tests must be done to keep the dosage regulated and monitor for possible side effects. This type of treatment is appropriate for the cat who is a poor surgical risk due to other health problems.

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If I elect to have surgery for my cat, what is the procedure?

If surgery is the treatment method chosen, the cat may be put on methimazole for one to four weeks before surgery. This treatment should cause the ravenous appetite to subside, and your cat will probably gain weight. Some cats also have a very fast heart rate and may be medicated before surgery with another drug. After one to two weeks, another T4 level in the blood is measured.

The operation is performed in a sterile operating room and the cat is under general anesthesia. An incision is made along the neck just below the throat and the enlarged thyroid gland is removed. The skin is sutured together.

The cat is generally hospitalized for one night following surgery and returns home feeling quite well. He or she should eat normally after returning home.

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Can hyperthyroidism occur again?

Recurrence is a possibility in some cats. Recurrence is uncommon after radioactive iodine therapy. When surgery is performed, the chance of recurrence is slightly greater. It is usually not possible to surgically remove all of the cells from the abnormal thyroid gland. If those remaining cells grow, the disease may recur. However, this occurs less than 10% of the time and usually after 2-4 years. Another possibility is that one side of the thyroid gland appeared normal at the time of surgery so it was not removed. Then, months or years later, it may become abnormal.

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Is my cat is too old for anything but treatment with the oral medication?

Many owners of cats with hyperthyroidism are hesitant to have radiation therapy or surgery because of their cat's advanced age. But remember, old age is not a disease. The outcomes following both surgery and radiation therapy are usually very positive, and most cats have a very good chance of returning to an excellent state of health.

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