RADIOACTIVE IODINE THERAPY CENTER

Hyperthyroidism in cats

Hyperthyroidism is a common medical condition affecting cats generally over six years of age. In most cases, the condition results from a benign tumor of the thyroid gland, which causes excess thyroid hormone secretion.

Excessive hormone secretion causes accelerations of bodily processes and clinical signs that are apparent to owners. Such signs include increased appetite, weight loss, rapid heart rate, muscle weakness, restlessness, vocalization, vomiting, diarrhea, increased thirst/urination, and heat intolerance.

Over time, the accelerated metabolism leads to deleterious effects on the cat's internal organs. Resultant heart disease, kidney disease, and/or intestinal disease can ultimately lead to death. Fortunately the disease is very familiar to veterinarians and is most often easily and rapidly diagnosed by measuring the level of thyroid hormone (T4) in the bloodstream.

Treatment for Hyperthyroidism in cats

There are three options for treatment of hyperthyroidism. All three are identical treatment forms to those for human patients with hyperthyroidism.

Oral anti-thyroid medications

This medication (methimazole) blocks the production of thyroid hormones by the thyroid gland. This oral medication does not cure hyperthyroidism, and is usually required twice daily (lifelong) to control the disease. Methimazole can be useful in the treatment of hyperthyroidism in cats but it is not an innocuous drug. Regularly scheduled blood tests are required to adjust dosages and to determine if potentially harmful side effects are present. Owners frequently find that oral administration of this drug to their cat is costly and difficult over time.

Thyroidectomy

Surgical removal of the thyroid tumor (s) is performed under general anesthesia. This procedure usually results in a return to normal thyroid function for the cat though the risk of anesthesia must be given careful consideration. If both lobes of the thyroid gland are not removed, approximately 70% of cats will eventually develop a functional benign tumor of the remaining tissue, requiring additional treatment or surgery.

Alternatively, removing both thyroid lobes during the same surgery increases the risks of disturbing calcium metabolism, which is governed by the 4 small, adjacent parathyroid glands. Because affected patients are usually geriatric, and under-conditioned, they must be monitored for post-surgical side effects including low calcium levels (hypocalcemia), and kidney dysfunction. They are commonly hospitalized from 2-5 days. To lessen anesthetic and surgical risk to the patient, a cat may be required to undergo medical therapy with methimazole until physical condition improves. Occasionally, hyperthyroid cats are found to have functioning thyroid tumors in the chest cavity, where surgery is not feasible.

Radioiodine I-131

Of the three treatment options, radioiodine is considered by many to be the treatment of choice for most hyperthyroid cats. Overall, radioiodine provides a simple, effective, and safe cure for cats with hyperthyroidism. This form of therapy has been used successfully for over 50 years in human medicine, and nearly 20 years in veterinary medicine. It requires no anesthesia and can be offered to medically stable patients, regardless of their age!

 

How does it Work?

Thyroid function requires the uptake of the element iodine in the body in order to produce normal thyroid hormones. If a radioactive form (I-131) of iodine is administered to hyperthyroid cats, it accumulates in thyroid tissue wherever it occurs in the body. Thyroid tumors accumulate the greatest amount of radioactive iodine. Once inside the tissue, the radioactive iodine emits radiation, which destroys the overactive thyroid cells.

The radioactive iodine not trapped in the thyroid is excreted in the urine and to some degree in the feces. The amount of radioactivity emitted by the compound naturally decreases by half, every 8 days. Thus, the radioactivity remaining in the cat's thyroid tumor tissue will painlessly dissipate on its own. Normal thyroid tissue tends to be automatically protected from the effects of radioiodine since the uninvolved thyroid tissue is suppressed and receives only a small dose of radiation. As an added patient benefit, there is no injury risk to the adjacent parathyroid glands. The residual (normal) thyroid tissue resumes full function within 1-3 months after treatment. An average of 95-98% of I-131 treated cats are permanently and safely cured with a single injection!

Is it Safe?

"Radioactive" iodine, despite its somewhat scary title, is considered the "gold standard" for safety and efficacy in treating hyperthyroid cats. I-131 administration is a safe and effective treatment for feline hyperthyroidism. This therapy has been successful in large numbers of cats, and the only recognized deleterious side effect has been hypothyroidism (underactive thyroid gland). This occurs in an extremely small percentage of cats and almost never requires specific treatment. The greatest risks are to the doctors and staff who work in the thyroid unit on a long-term basis. However, with stringent safety regulations, protocols, and monitoring, this form of therapy can be safe for cats and the caregivers!

Radiotherapy: What happens to my cat?

On or before the day of admission, you and your cat will meet with a veterinary medical specialist at PVS. Your cat will be thoroughly examined and the medical records will be reviewed. The doctor will discuss any admission tests required (which can be done on-site prior to treatment) to ensure that radioiodine therapy is the best option for your cat. The results of any tests performed at PVS will be discussed with you before proceeding with treatment. These tests usually include:

• A complete blood count
• A thyroid hormone level (T4 or free T4)
• Serum biochemistry analysis
• Urinalysis
• Ultrasonography (cardiac ultrasound to evaluate function)

If your cat is judged to be medically stable, he or she will be admitted to the radiotherapy unit within 24-48 hours of your appointment. The unit is specially constructed for this use and houses only cats that are receiving radioactive iodine.

The quiet accommodations include "Southwest" decor and housing in roomy and cheery cat condos (by Snyder Manufacturing, Inc.). These condos have separate bathrooms and shelves for snoozing. The unit includes windows for natural lighting, music, and heated floors. The patients enjoy watching patrons of our bird and squirrel feeders.

Once the dose of radioactive iodine for your cat has been determined, it is injected painlessly under the skin (subcutaneous) exactly like a routine vaccination.

From that point on, your cat need do nothing else but sleep, eat and play while the radiation dissipates to safe levels (usually 5-8 days). We like to spoil all our patients as much as safely permissible. This brief separation is likely to be harder for the owners than the patients!

Your cat will be monitored daily while in our care. By daily monitoring of your cat's radiation level, we can determine when this level has declined to that allowable by law. At this time, your cat can be released to you. You will be contacted daily with updates during your cat's stay in the radiotherapy unit. If you have questions or concerns, do not hesitate to call us.