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Radioiodine Therapy for Feline Hyperthyroidism

Frequently Asked Questions

1.) What causes hyperthyroidism in cats?

2.) What are the options for treating cats with hyperthyroidism?

3.) Why is radioiodine therapy any better than giving those pills my veterinarian mentioned?

4.) What are the risks to my cat associated with radioiodine therapy for hyperthyroidism?

5.) What are the risks to me or other human family members associated with radioiodine therapy for my cat's hyperthyroidism?

6.) Why does the treatment take so long?

7.) Why does it cost so much?

8.) What kind of precautions are needed after my cat returns home?

9.) Will the furniture in my house be contaminated (radioactive) if my cat sits on them when it comes home after the radioiodine therapy?

10.) Can you accommodate multiple hyperthyroid cats from a single family needing radioiodine therapy simultaneously?


1.) What causes hyperthyroidism in cats?

In most cats (~99%), a small benign tumor (adenoma) or several small benign tumors (adenomas) are responsible for the overproduction of thyroid hormone that leads to the symptoms of hyperthyroidism. In rare cats (~1.0 %) a functional thyroid carcinoma (cancer) is responsible for the over-production of thyroid hormone that leads to the symptoms of hyperthyroidism. The most current data suggest that these tumors develop as the result of a mutation in the gene for the production of an inhibitory protein (G inhib ) that turns off the thyroid cell. In other words the thyroid cell loses its ability to regulate its growth and its production of thyroid hormone. What causes this mutation is still under investigation.

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2.) What are the options for treating cats with hyperthyroidism?

A.) Medical - Cats with hyperthyroidism can be given pills called methimazole (brand name = Tapazole®). Tapazole® works by suppressing the thyroid gland's production of thyroid hormone. It does not eliminate the thyroid tumor that causes hyperthyroidism nor does to slow the continued growth of the tumor.

B.) Surgical - Cats with hyperthyroidism can be treated by surgical removal of the thyroid tumor responsible for the disease. Unfortunately this generally requires the removal of the entire thyroid gland, necessitating the chronic administration of thyroid supplements for the remainder of the cat's life. Surgery also has other negatives including the risk of anesthesia, and probable damage to small glands attached to the thyroid gland called the parathyroid glands. These parathyroid glands are responsible for calcium balance in the body and damage to them can lead to low calcium levels, seizures and even death.

C.) Radioiodine - A single SQ (under the skin) injection of radioiodine can result in the destruction of the small tumor that causes hyperthyroidism without damage to other parts of the body. It is normal for iodine to be selectively taken up by thyroid cells. This allows us to target the hyperfunctional thyroid cells of a thyroid tumor for irradiation without being significantly invasive, excessively manipulative or traumatic to the patient.

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3.) Why is radioiodine therapy any better than giving those pills my veterinarian mentioned?

An alternative to radioiodine therapy for feline hyperthyroidism is the chronic administration of an oral medication called methimazole (brand name = Tapazole®).

A.) Unfortunately Tapazole® is not a cure for hyperthyroidism. Tapazole® works by suppressing the thyroid gland's production of thyroid hormone. While this can temporarily control the clinical symptoms of hyperthyroidism, it does not resolve the underlying abnormality in the thyroid that caused the hyperthyroidism. In most cases, the small benign tumor that causes hyperthyroidism continues to grow during Tapazole® therapy and can in some cases undergo a malignant transformation into a malignant thyroid tumor (cancer). In most cases, the dose of Tapazole® needed to control thyroid hormone levels increases over time, as the tumor continues to grow.

B.) While Tapazole® is a good drug, it has a large number of side effects associated with its use. Among the list of side effects are frequent gastrointestinal side effects (vomiting and/or diarrhea), liver dysfunction causing severe jaundice, anemia, decreased white blood cell levels, decreased platelet levels, and an allergic reaction that manifests itself in the form of red scabby facial skin lesions that itch profoundly. Radioiodine is the only curative option for feline hyperthyroidism that does not require an anesthetic. Radioiodine also limits its effect to the hyperfunctional adenoma (benign tumor) in the thyroid that is causing the hyperthyroidism. As a result, it does not damage the normal thyroid tissue. This means no chronic medications are needed following radioiodine therapy.

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4.) What are the risks to my cat associated with radioiodine therapy for hyperthyroidism?

The radioiodine treat involves a single injection of a very small volume (<1ml) administered subcutaneously (under the skin). Because the treatment does not require any anesthetic or even extensive manipulation of the patient, the risks are virtually nonexistent. Rarely cats will demonstrate evidence of a sore throat following the treatment. This is more common in cats with large thyroid masses receiving large doses of radioiodine. Recent reports (Survival times for cats with hyperthyroidism treated with iodine 131, methimazole, or both: 167 cases (1996–2003)) reveal that cats treated with radioiodine live approximately twice as long as cats treated with methimazole. So the real risk is associated with not treating a hyperthyroid cat with radioiodine, as chronic medical management reduces the cat's life expectancy by approximately 2 years on average. Follow this link for a more detailed discussion on radiation risks.

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5.) What are the risks to me or other human family members associated with radioiodine therapy for my cat's hyperthyroidism?

Hyperthyroid cats are hospitalized following I-131 therapy to allow them to excrete the vast majority of the radioactive iodine before being discharged. At discharge clients are advised to limit close contact with their cat for approximately 12 days. These precautions are designed to limit exposure to adults in the household to less than 1 mSv (100 mrem). Using the most conservative model of radiation exposure risks, namely the linear no-threshold model, exposures to 1 mSv of radiation increase the lifetime risk of malignancy by less than 1/100th of 1 percent. Follow this link for a more detailed discussion on radiation risks.

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6.) Why does the treatment take so long?

The actual treatment with radioiodine actually takes less than one minute. However, federal regulations governing the use of radioactive materials require that we contain the cats in our facility until levels of radioiodine in their bodies fall below federal guidelines. Typically cats treated with radioiodine in our facility spend a total of 3-6 days with us following the radioiodine therapy. The majority of cats are discharged 3 days following their radioiodine therapy. Some cats with larger tumors require larger radioiodine doses and therefore require longer hospitalization periods prior to release.

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7.) Why does it cost so much?

Actually this treatment is the most cost effective option available. While the single expense of the radioiodine therapy is significant, it is less than 1.5 years of Tapazole® administration and similarly priced when compared to surgical therapies performed by specialists. In addition, once a cure has been accomplished with the radioiodine therapy, the need for ongoing medications is gone. With Tapazole® it is necessary to medicate the cat at least once daily for life. Following surgical thyroidectomy, most cats need chronic thyroid hormone supplementation to replace the hormone no longer made by the absent thyroid gland.

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8.) What kind of precautions are needed after my cat returns home?

For a period of about 2 weeks the contents of the cat's litter box must be disposed of by flushing down the toilet into the sanitary sewer. To facilitate this goal we suggest the use of special cat box fillers made for this purpose* (e.g. SwheatScoop, World's Best Cat Litter). It is also necessary to limit the amount of physical contact between people and the cat for a similar period. Brief periods of contact are allowed to ensure the continued emotional and physical well being of the patient.

* Despite what numerous individuals have been told by employees of the PetSmart chain, it is NOT illegal to flush cat waste into the sanitary sewer in California. California Assembly Bill #AB2485 merely requires the placement of a statement on the packages of flushable cat litters sold within the state of California that reflect the erroneous views of the authors of that bill that flushing household cat waste into the sanitary sewer is contributing to the incidence of a disease called Toxoplasmosis in the California Sea Otter population.

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9.) Will the furniture in my house be contaminated (radioactive) if my cat sits on it when he/she comes home after the radioiodine therapy?

Nothing that the cat contacts is made radioactive. It is impossible for the cat to make anything in your house radioactive unless it urinates on something. By the time the cat is discharged from the hospital, the radioiodine is only being eliminated from the cats body by the kidneys into the urine. This is why the cats are discharged with instructions for the use of a clumpable and flushable litter box fillers (e.g. SwheatScoop, World's Best Cat Litter) allowing the elimination of the radioiodine by disposal into the sanitary sewer. (FYI - This is also what happens when people are treated with radioiodine and then discharged from the hospital to go home...minus the litter box of course!)

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10.) Can AVMI accommodate multiple hyperthyroid cats from a single family needing radioiodine therapy simultaneously?

Our Snyder Cat Cottages allow us to create a suite out of two adjoining Cottages thus enabling two cats from the same family to share space by opening a passage way between the Cottages. Families with more than 2 hyperthyroid cats requiring simultaneous radioiodine therapy can be accomodated by housing cats from a single family in a large enclosure the size of a small room (8 feet deep x 4 feet wide x 7 feet tall). The extra space available in this accomodation allows the establishment of bedroom, bathroom and dining areas for up to 4 cats from a single family to use during their hospitalization for radioiodine therapy.


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