Feline Hyperthyroidism

The thyroid is a two-lobed gland located in the neck of people, dogs, cats, and other animals. One lobe is located on each side of the trachea (windpipe). The thyroid gland produces thyroid hormone, a substance transported via the blood to every cell in the body. The primary function of thyroid hormone is to control the rate at which cells function: too much thyroid hormone makes cells work very fast and too little causes cell function to slow down. Low thyroid function (hypothyroidism) is relatively common in dogs and quite rare in cats. Excess thyroid function (hyperthyroidism) is rare in dogs, but is one of the most common diseases diagnosed in cats 8 years of age or older.

Each cat may respond to hyperthyroidism a little differently, causing abnormalities to vary from cat to cat. Among the most common owner observations are:

  • Weight loss
  • Increases in appetite
  • Patchy hair loss, failure to groom, or excessive grooming
  • Increases in water intake
  • Increases in urine output
  • Restlessness and/or nervousness
  • Increased vocalization
  • Increased respiratory rate
  • Heat avoidance

It is not known exactly why cats develop hyperthyroidism. About 15% of hyperthyroid cats have a single thyroid tumor (not a cancerous tumor), and about 80% have excess activity in both lobes (a benign condition called hyperplasia). Only 3-5% of hyperthyroid cats have a malignant (cancerous) thyroid tumor.

The diagnosis of hyperthyroidism is relatively easy. Most hyperthyroid cats have too much thyroid hormone (thyroxine or “T4”) in their blood, and this can be confirmed with a blood test. A small percentage of hyperthyroid cats do not have a “diagnostic” blood T4 level, and in this situation, your veterinarian may wish to repeat the test a few days or a few weeks later or use a different thyroid test (i.e.: free T4 test and/or T3 suppression test).

Three common treatments are available for managing hyperthyroidism in cats. Each treatment has advantages and disadvantages. Your veterinarian will explain the choices to you, allowing you to decide what is best for you and your cat. Hyperthyroid cats not treated tend to become more and more ill, whereas treatment will usually either return your cat to a reasonable state of good health or cure your cat.

Treatment Option #1:

One treatment option is the use of a medication called methimazole (Tapazole®), a drug available in pill, liquid, or transdermal forms. This medication works by preventing the thyroid gland from producing T4. The drug is readily available, but is fairly expensive. The major negative aspect of this drug is that some cats are not the best pill or liquid takers. Side effects include vomiting and loss of appetite, excessive scratching, and decreases in red blood cells and white blood cells. Although these side effects are alarming, they are not common, and are reversible if the medication is discontinued. This treatment does not cure hyperthyroidism, but rather helps to continually manage the condition.

Treatment Option #2:

The second treatment option is the use of radioactive iodine. Iodine is the primary component of thyroid hormone, and it is concentrated in the thyroid gland. Radioactive iodine can destroy abnormal thyroid cells, and this is an extremely effective treatment that cures the hyperthyroidism quickly. Radioactive iodine therapy is considered the gold standard for the treatment of feline hyperthyroidism for most cats. The condition almost never recurs, and it requires no anesthesia or pills. The negative aspects of this form of treatment are:

  1. Sophisticated facilities are needed, as this treatment is not available everywhere.
  1. This procedure has a relatively large initial financial component. However, the need for recheck examinations is minimal, and as such, there is minimal additional financial expense. Treatment options #1 and #2 potentially have extensive follow-up requirements, making all treatment options ultimately very similar in cost.
  1. The procedure requires 5 days of quarantine for the cat in a special facility to eliminate the radioactivity in urine and feces (otherwise people might become exposed to this radioactivity). You may not visit your pet during this time period.

Potential Complications of Successful Treatment of Hyperthyroidism

Occasionally, cats with apparently normal renal function prior to treatment seemingly and suddenly become ill due to renal failure after treatment for hyperthyroidism. The increased blood supply to the kidneys as a result of elevated blood thyroid hormone levels will dramatically decrease after successful treatment for hyperthyroidism. Please know chronic renal disease is also commonly found in older cats; however signs associated with this condition may be masked by hyperthyroidism. Therefore, pretreatment with methimazole (Tapazole®) is utilized prior to surgery or radioactive iodine, as the effects of the pills are reversible, and lend invaluable information regarding kidney function post-therapy. If decreasing the thyroid hormone level reveals kidney disease, a permanent form of treatment (surgery, radioactive iodine) is avoided. If the thyroid hormone levels decrease, and the kidneys show no signs of concurrent disease, permanent treatment in an option.