Feline Hyperthyroidism, a Disease among Older Cats

Other Health-Related Articles:

Six years ago my fourteen-year-old orange and white domestic long-haired cat named "Spice" began exhibiting unusual and unpleasant behavior, namely urinating regularly on my dog's bed and defecating on my bed. In addition he had experienced a profound weight loss over the past year and a half. Spice had also recovered almost completely from a total loss of use of his limbs two years earlier. The only vestige of this mysterious ailment was a crooked left front paw that never straightened, and which he used to navigate with somewhat akin to a club foot. I mention this condition, which will be the subject of a future article, only to indicate that Spice had suffered more than his share of illnesses over the course of his rather long life.

A visit to my veterinarian Dr. Gary Shaw resulted in having a complete battery of blood tests performed on Spice. The weight loss in itself was indicative of a number of possible treatable illnesses prevalent among older cats, such as diabetes, kidney disease, and hyperthyroidism. The blood chemistry panel performed in the veterinarian's own facility proved inconclusive, although ruling out diabetes. However, the test for thyroid hormone performed by an outside lab indicated the presence of hyperthyroidism, which is caused by an increase in production and secretion of thyroid hormone by the thyroid gland. With subsequent research I learned that hyperthyroidism is the most common endocrine (hormonal) disease among cats, and one that is almost never diagnosed in cats under eight years of age.

Symptoms: The symptoms of this condition may include:

1. weight loss
2. increased appetite
3. hyperactivity and restlessness
4. moderate elevation of body temperature
5. increased heart rate
6. increased frequency and defecation with larger stools
7. increased thirst and urination
8. occasional vomiting
9. reddening of mucous membranes and skin
10. matted, greasy and unkempt coat

Diagnosis: The diagnosis of hyperthyroidism is most often based on the measurement of elevated levels of the thyroid hormones thyroxin (T4) and, less commonly triiodothyronine (T3), in the blood. A small benign tumor (thyroid adenoma) involving one or both thyroid lobes is the most common cause; thyroid carcinoma (cancer) is rarely diagnosed. Planar thyroid scintigraphy (or thyroid imaging) is another useful although less common means of detecting hyperfunctional thyroid tissue causing the disease. Its major value is in determining the extent of the gland involvement.

Early diagnosis is essential since the elevated levels of thyroid hormone in the blood can lead to other complications including heart and kidney damage, as well as accelerating tumors or other illnesses by the presence of a higher metabolic rate.

Treatment: If your cat is diagnosed with this illness your veterinarian will discuss with you the three possible treatment options, and depending upon the age of your cat and other medical conditions, particularly kidney function impairment and heart condition that may coexist, will recommend one of the options. The three treatment options include treatment with radioactive iodine, surgical removal of the gland, and treatment with antithyroid medications.

In the case of Spice, an elderly frail cat, surgery was definitely not an option. Surgical removal of the thyroid gland, known as thyroidectomy can be performed by a skilled veterinary surgeon, although anesthesia represents a risk especially in those cats with other concurrent conditions such as renal (kidney) failure. One or both of the thyroid lobes may be removed and there is generally a low recurrence of hyperthyroidism after surgery.

Treatment with radioactive iodine (131) I is sometimes a treatment of choice since it does not require sedation and may be administered orally or by injection. The treatment is permanently curative and has no serious side-effects; however, it does require the cat to be hospitalized for a period of time determined by local regulations regarding radioactivity. This is necessary to allow adequate decay of the radioactive iodine. The cat may not be excessively handled after this treatment for several days. The treatment can be quite costly, depending upon the length of hospitalization required.* There are only about two dozen facilities currently providing this kind of treatment in the United States. Because of the cost and Spice's advanced age as well as weakness this was also not an option; however, for a cat with a life expectancy of several years it should most definitely be considered.

Treatment by administering the oral medication Methimazole (Tapazol) was really the only viable option for Spice. However, the antithyroid drugs do not destroy the gland and act by interfering with production and secretion of thyroid hormone. The therapy, once commenced, must continue for the life of the cat, twice daily. Symptoms of the disease will recur rapidly if the treatment is discontinued. The cost of medication can be as high as a dollar a day, so for a cat with a life expectancy of more than five years it could be the most expensive alternative in the long run. Also, administering the drug twice daily can pose serious difficulty for many owners and cats. This therapy also does require rechecking of the blood periodically to determine and maintain the correct dosage. Where kidney malfunction exists or is suspected this is sometimes the treatment of choice, since it allows the veterinarian better control over the concurrent kidney disease.

Spice improved dramatically in the two months following the beginning of drug administration. He gained several pounds in a very short time and became a much happier cat, and I a happier owner. Unfortunately, in the third month of treatment his improvement reversed sharply, and he began eating frequently but only in tiny quantities. I took him in for repeat blood sampling; however, my veterinarian felt a large stomach tumor which had not been observable previously, and which was preventing his normal food consumption. In his weakened condition the only humane option was euthanasia. The photo on this page was taken on the last day of his life and shows a ravaged body but a fine spirit.

* In Southern California this treatment is available in three locations through Veterinary Nuclear Imaging. Their current total charges usually run approximately $750. This price includes the consultation, thyroid scintigraphy (scan), I-131 treatment as well as the hospitalization of 1 week. Click on the image for a map showing the locations.

References: Cat Fancier's Association: CFA Health Committee - Feline Hyperthyroidism.