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Radioactive Iodine Treatment for Cats with Hyperthyroidism

In older cats, the overproduction of thyroid hormone is one of the most commonly diagnosed diseases. One in 300 cats overall is hyperthyroid, with an average age of 13. Hyperthyroidism is caused by a benign growth in the thyroid gland that is over-producing thyroid hormone. These tumors are almost always benign and represent a form of goiter rather than a form of cancer. Only 1% to 2% of hyperthyroid cats have a cancerous thyroid growth. When a cat produces too much thyroid hormone, its metabolic rate soars to the point where it can burn off more than half of its body weight. Affected cats will have weight loss despite excellent appetite. Increased water consumption, increased urination, vomiting and diarrhea are also symptoms that are seen in cats with hyperthyroidism. If thyroid production is not kept in check, hypertension, heart and liver problems develop, and the cat dies.

Fortunately, hyperthyroidism can be treated successfully. Due to the seriousness of the disease, most veterinarians now screen all cats above the age of 8 years for elevated thyroid levels. The earlier the disease can be diagnosed and treated the better the chance of avoiding permanent damage from elevated thyroid hormone levels. There are three options for treating hyperthyroidism in cats. None of these options is inexpensive with costs around $900 to $1200.

Options for treating hyperthyroidism in cats

Surgery: A thyroidectomy, the surgical removal of the thyroid gland is one option for treatment. This option often results in a cure. However, the surgery is delicate, with a chance that other problems may be caused as a result of the surgery, such as calcium deficiencies. In addition, there is often thyroid tissue scattered throughout the neck and chest area. The removal of the enlarged thyroid glands in the central neck does not guarantee that this ectopic thyroid tissue will not continue to produce high levels of thyroid hormone resulting in the cat, once again, becoming hyperthyroid. But more significantly is the age and general health of the cat: older pets are at greater risk while under anesthesia.

Medication: Hyperthyroidism can be treated with the oral drug Tapazol (methimazole). This drug is used to “control” hyperthyroidism – it
is not a cure – and must be given twice daily to achieve and maintain its effect. However, some cats do not tolerate the drug and some owners are not up to the task of administering pills to the cat for the remainder of its life. This therapy is often used to stabilize a cat prior to other treatments and to address the immediate health problems caused by hyperthyroidism until a long-term solution can be put into place.

Radioactive Iodine Therapy: This is the preferred treatment for hyperthyroidism in the cat and is the treatment used to treat humans with hyperthyroidism. The benefits are significant with a cure rate of 90 – 95%, with no further treatment. The cat gets one injection of radioactive iodine that kills the overproducing cells without harming any other of the body’s functions. It is a one-time painless injection, but the treatment creates a radioactive cat that must be kept in a special ward until the radioactivity drops to safe levels. The cat will typically stay in the hospital for 10 to 14 days.

Advantages of Radioactive Iodine Treatment

  • Treatment is a one-time event (only about 5% of cats require a second treatment) and no on-going therapy is required.
  • The disease is not simply managed but is actually cured!
  • No anesthesia is required. Treatment amounts to one painless injection followed by 1 to 2 weeks of boarding. Very non-stressful for older cats.

Disadvantages of Radioactive Iodine Treatment

  • Owner and pet are separated during the quarantine.
  • This is a relatively expensive therapy.
  • Radiotherapy may not be a good idea for a cat with poor kidney function.

Common Questions Regarding Radioactive Iodine Treatment

How does Radioactive Iodine Treatment cure hyperthyroidism?
Iodine 131 is used to destroy thyroid tissue. Normally, iodine is joined to the amino acid tyrosine in the thyroid gland to create T4. Iodine 131 is carried directly to the thyroid gland as though it were regular iodine. Iodine 131, being radioactive, emits high-speed electrons, which kill the surrounding abnormal thyroid tissue. Because these electrons penetrate only fractions of an inch, only the thyroid glands experiences the radiation and the rest of the body is spared.
How many treatments are needed?
Iodine 131 is administered by injection, need not be repeated and no additional therapy is required. However, while humans receiving similar treatment are promptly allowed to go home post-treatment, the process is different with cats. The Nuclear Regulatory Commission requires cats to remain hospitalized in a special isolation ward until the iodine 131 is gone from the body. This amounts to about 10 to 14 days of hospitalization. Blood work is monitored following treatment to ensure return to normal thyroid status. Occasionally, a single course of radiotherapy is inadequate and a second course is needed.
Why does kidney function need to be checked before treatment?
Kidney failure is often hidden in hyperthyroid cats. If kidney function is not thoroughly investigated prior to Iodine 131 therapy, hidden kidney failure may be unmasked irreversibly by this therapy. Radioactive Iodine Treatment does not cause kidney failure but may "unmask" it. This can be avoided simply by screening potential candidates for kidney failure prior to radiotherapy.
Will I have to give my cat thyroid supplements after treatment?
Normal thyroid cells become dormant when the benign growth causing the hyperthyroidism is present. The cells that are killed are those that are functioning, therefore, the normal dormant cells are spared preventing hypothyroidism (low thyroid) in most cats. After treatment these normal cells resume producing low levels of thyroid hormone. Normal thyroid levels are achieved within 6 months in 95% of treated cats. Hypothyroidism will develop in about 2% of treated cats and may require administering a thyroid supplement.
How do I know if my cat is a candidate for Radioactive Iodine Therapy?
Most cats with hyperthyroidism are excellent candidates for this treatment. However, during the hospitalization period handling must be kept to a minimum. Cats with other health issues that require treatment during the hospital stay may not be candidates for this therapy. This can be discussed with our veterinarians prior to treatment. After treated cats are released, there is still a low level of radioactivity present. Cats that live in a household with a pregnant woman or very small children may not be candidates unless temporary housing elsewhere is provided for the cat for 2 weeks after release from the hospital.

Additional Information about Radioactive Iodine therapy

For more information regarding Radioactive Iodine Therapy please contact:

Feline Specialties Veterinary Hospital
9702 S Riverside Drive
Tulsa OK 74137
(918) 299-8222