Do I have to change my cat diet prior to treatment ?
Not specifically for radioiodine treatment. Please follow your referring veterinarian’s advice.

Should I stop thyroid medication (Tapazole®) prior to radioiodine treatment ?
It is important to not  abruptly withdraw Tapazole® (methimadozole) prior to scintigraphy and radioiodine therapy because:
- It imposes an unnecessary physiologic stress on the kidneys and heart, and may increase the risk of death during hospitalization when care is limited due to radiation exposure to personnel.
- It results in an overestimation of the size and function of the normal thyroid gland on a thyroid scan.¹ This can potentially lead to false positive diagnoses or overtreatment leading to hypothyroidism.
- It does not affect the thyroid scan findings of hyperfunctional tissue.²
- It does not improve the efficacy of radioiodine therapy because hyperfunctional tissues are autonomous. This means that radioiodine absorption in the abnormal thyroid tissue is not affected by medication.
- It can render normal thyroid cells undesirably susceptible to radioiodine damage by increasing radioiodine absorption in the normal thyroid.¹

Why is a thyroid scan necessary ?
Thyroid scintigraphy provides valuable information regarding both anatomy and physiology of hyperfunctional thyroid tissue and plays an integral role in the diagnosis and management of cats with hyperthyroidism. The thyroid scan serves as a screening procedure to discriminate between benign and malignant thyroid conditions. It also detects local extension and distant metastasis of thyroid malignancy or the presence of ectopic thyroid tissue in the thoracic cavity. It will also confirm the diagnosis in patients that have equivocal lab tests.

Will radioiodine treatment give my cat cancer ?
There is no conclusive evidence that radioiodine treatment of the thyroid can cause cancer in people.  No association between radioiodine treatment and increased risk of cancer has been reported in veterinary medicine

How long does it take to notice results after treatment ?
By one month there should be noticeable improvement in clinical signs and by 3-6 months, the radioiodine therapy should have had its full effect. Occasionally (in <1% of patients) the radioiodine therapy needs to be given again. The rule of thumb for retreatment is recurrence of clinical signs associated with failure of hormone levels to normalize, or an increasing thyroid hormone level in at least 2 blood work samples, at least 6 months after treatment.

Are there any side effects from radioiodine treatment?
As with most medical therapies, there are side effects, but they are extremely mild. Because the radioactivity from the radioiodine treatment is concentrated in the hyperfunctional thyroid tissue, very little radioactivity goes to other parts of the body. Therefore, a very small dose of radioactivity is usually required. As a result, the only direct side-effect is development of hypothyroidism, meaning that the thyroid gland is no longer producing adequate amounts of thyroid hormone. This side-effect occurs when the treatment dose of radioiodine was too high, or if the cat has no residual normal thyroid tissue. This can largely be prevented by tailoring the dose to each patient based on thyroid scan findings. If low blood thyroid levels are accompanied by clinical signs such as lethargy, obesity, and poor hair coat, then thyroid hormone supplementation may be required. This will correct the condition. Because we scan our cats, our incidence of post treatment hypothyroidism is <2%, vs. >15-20% in clinics that do not perform scans.

Can Radioiodine treatment induce kidney failure?
One of the most difficult issues in the treatment of hyperthyroidism is the unmasking of pre-existing renal disease. Hyperthyroidism results in increased renal filtration rate as well as increased cardiac output and blood pressure. These physiologic changes, while detrimental to long term renal health, may augment renal function to normal levels in some hyperthyroid cats, to the extent that their true renal status is obscured. Successful treatment of hyperthyroidism results in normalization of the individual cat’s renal filtration rate, which results in renal failure in some cats. This decline in function represents an unmasking of the cat’s true renal function, rather than nephrotoxic effects of treatment. Therefore, it has been recommended that a Tapazole trial, a reversible method of treatment, be used as a screening treatment in newly diagnosed cats. Then, if their renal function declines to an unacceptable level, treatment can be discontinued or decreased so that the cat is left mildly hyperthyroid.

How do I know if my cat is at risk for kidney failure after radioiodine treatment?
At NCVS, we request a Tapazole trial prior to treatment. The Tapazole trial is necessary to evaluate the kidney function in your cat. In short, hyperthyroidism may mask renal disease and we are attempting to determine if renal disease will be present after radioiodine therapy. After diagnosing your patient with hyperthyroidism, please start your cat on Tapazole as directed by your referring veterinarian. Your cat ideally should be kept on Tapazole until thyroid levels have decreased to <2.5ug/ml. This usually occurs in about one month at a Tapazole dose of 2.5 mg twice daily. After 2-4 weeks on medication, please have your cat’s thyroid levels, blood count and chemistries re-checked to evaluate your cat’s renal function and thyroid status, and because tapazole can rarely cause severe side effects, such as anemia.

Why does my cat have to stay in isolation after treatment with radioiodine?
 After administration, most of the radioiodine is taken up by the abnormal thyroid tissue that is responsible for the disease. The rest of it is eliminated primarily in the urine, but also in feces and saliva.
After treatment your cat will emit gamma radiation from its thyroid gland and excrete radioactive radioiodine over several weeks. In order to protect your health from radiation exposure and the environment from radioactive iodine contamination, your cat has to be housed in an shielded ward until enough radioiodine has been eliminated so that it is safe for you to take your cat home. The safety criteria set forth by the Code of Federal Regulation (Title 10) include that 1) the radiation exposure to you and your family from your cat must not exceed about 1/3 of the dose of radiation you get from natural radiation background (such as cosmic radiation or from ground, ie, concrete and granite) in 1 year, and 2) the total amount of radioactivity eliminated by your cat in the urine and feces does not exceed approximately 1/1000 of the administered dose. At NCVS, release dates are made in accordance with these standards, and is controlled by our radiation safety officer on-site.