Radioactive iodine therapy for feline hyperthyroidism

Systemic 131 Iodine Therapy In Cats with Hyperthyroidism


Systemic 131 Iodine therapy is a widely accepted treatment modality for cats with hyperthyroidism. Numerous reports in the veterinary literature advocate the use of radioiodine treatment for cats with thyroid adenoma and carcinoma. Advantages of systemic iodine therapy over medical or surgical treatment for cats with hyperthyroidism are that this procedure is noninvasive (compared to surgery) and usually only 1 dose is enough to return the animal to a euthyroid state (compared to daily administration of conventional anti-thyroid drugs.) It is expected that your cat will return to a euthryoid state (normal) between 2 and 4 months after being treated with 131 Iodine.

Hyperthyroidism is caused by hyperactive thyroid glands typically caused by a benign change to one or both thyroid glands where they stop responding to normal feedback mechanisms. This leads to the uncontrolled increase in thyroid hormone levels in the body. Rarely, this increase can be caused by a malignant tumor (thyroid carcinoma) of the thyroid gland (2-4%). Unfortunately, the excess of thyroid hormones leads to several changes in the body, which include heart disease (hypertrophic cardiomyopathy), systemic hypertension, cardiac arrhythmias, masking of chronic kidney disease, weight loss despite a ravenous appetite (rarely decreased appetite), increases in liver enzymes, unkempt hair coat, as well as gastrointestinal signs (e.g. vomiting and diarrhea).

I-131 is considered a permanent and effective treatment in more than 95% of cases. A small percentage of cats will require a second treatment or develop hypothyroidism (require thyroid hormone supplementation). Therefore, certain precautions should be taken before considering this permanent treatment. Excess thyroid hormone increases heart rate and effort leading to increased blood flow to the kidneys. Thus, it is possible that cats with underlying chronic kidney disease (loss of over 66-75% of kidney function) may have adequate urine concentration and normal kidney values due to the increased blood flow to the kidneys. Several attempts have been made in order to diagnose cats with underlying (masked) kidney disease before permanent I-131 treatment. However, the most reliable and predictable form of diagnosing underlying chronic kidney disease is treating hyperthyroid cats medically (temporary treatment) prior to I-131 therapy. It may take 1 to 3 months of medically controlled hyperthyroidism before chronic kidney disease is unmasked (normalization of blood flow to the kidneys). Even though hyperthyroidism is thought to cause further damage to the kidneys due to hyperfiltration because of increased blood flow to the kidneys, cats with the combination of hyperthyroidism and chronic kidney disease tend to eat well. The concern of permanent treatment (I-131) is that a cat that was initially eating well may develop decreased appetite and lethargy once chronic kidney disease is unmasked and kidney values may become elevated. The decreased appetite and lethargy is due to accumulation of toxins that were previously being excreted by the kidneys.

In summary, we recommend medically (methimazole) treating cats with hyperthyroidism for 3 months after becoming euthyroid (normal thyroid hormone level) to maximize the chances of unmasking kidney disease before I-131 treatment.

Unfortunately, methimazole can have deleterious side effects that need to be considered. Some cats can develop gastrointestinal signs (vomiting, diarrhea and decreased appetite), facial scratching, bone marrow toxicity [e.g. neutropenia (low white cell count), thrombocytopenia (low platelet count)], liver toxicity (increased liver enzymes), besides unmasking kidney disease. In this case, it is very important to monitor blood work (CBC and chemistry panel), urine specific gravity, total T4 (thyroid hormone) every 3 weeks until thyroid hormone is controlled. Gastrointestinal signs are more common when methimazole is given orally and can be circumvented by the use of topical methimazole. However, the other side effects occur regardless of the route of administration, thus the importance of discontinuing methimazole if these side effects occur. It is important to remember that topical methimazole takes on average 2 weeks longer to achieve thyroid hormone control.

Other important considerations in a cat with hyperthyroidism includes addressing possible cardiac abnormalities (systemic hypertension, hypertrophic cardiomyopathy, tachycardia (fast heart rate), and cardiac arrhythmias. In this case, we recommend getting a blood pressure measurement, ECG, and chest radiographs to assess cardiac function. We may recommend an evaluation by a cardiologist depending on these findings. This is especially important before considering I-131 treatment because it requires isolation. This is aimed at decreasing hospitalization risks. Even though the hypertrophic cardiomyopathy can be secondary to hyperthyroidism, it is important to know the severity of the cardiac disease and assess the risk of cardiac failure during hospitalization.

Furthermore, we recommend stopping methimazole for 1 to 2 weeks (ideally) prior to I-131 treatment. Hyperthyroidism leads to atrophy of the normal thyroid glands because these glands respond to normal feedback mechanisms. Once hyperthyroidism is controlled via medical therapy, these glands go back to secreting thyroid hormone. Thus, they will also be affected (destroyed) if I-131 is given to cats that are normothyroid. Otherwise, atrophied glands are protected from I-131 therapy since I-131 has preferential uptake for thyroid secreting glands. Therefore, theoretically these cats are more predisposed to developing hypothyroidism after I-131. Recently, it has been shown that hypothyroidism is particularly detrimental if cats have concurrent chronic kidney disease. Cats with chronic kidney disease and post-treatment hypothyroidism live on average significantly less than cats with chronic kidney disease that are euthyroid (normal thyroid function). The normal thyroid glands in hyperthyroid (uncontrolled hyperthyroidism) cats are atrophied, therefore will be less affected by I-131 treatment, thus it will be less likely for these cats to become hypothyroid after I-131 treatment.

Thyroid scintigraphy has been recommended as the gold standard test for the diagnosis of hyperthyroidism. There is some controversy, but it is thought that normal cats undergoing methimazole therapy may appear hyperthyroid on thyroid scintigraphy. Besides the benefit of hyperthyroidism diagnosis, thyroid scintigraphy also provides further information regarding the number of thyroid glands affected, ectopic thyroid tissue, the potential for metastatic thyroid carcinoma (malignant cancer), and evidence of metastasis (spread) carcinoma if present. Even though thyroid scintigraphy is not a sensitive method in diagnosing thyroid carcinoma, it provides some support regarding this possibility. This is especially important because conventional I-131 treatment is not effective for thyroid carcinomas. Thyroid carcinomas can be treated via I-131, but much higher doses and hospitalization are required. Furthermore, there is some evidence that thyroid scintigraphy (using thyroid to background ratio) may predict I-131 treatment failure. Anecdotally, it has been suggested that cats with high thyroid to background ratios should receive a higher dose of I-131.

Even though I-131 treatment is very safe and effective, several considerations should be made prior to seeking this therapy. These recommendations are aimed to maximize treatment efficacy and minimize risks.

Joao Felipe de Brito Galvao, MV, MS, DACVIM (SAIM)

I. Scheduling systemic 131 Iodine therapy and pretreatment evaluation

Due to limited available space and trained personnel, and to limit human exposure, the number of animals treated with systemic 131 Iodine therapy at the VCA Arboretum View Animal Hospital is limited to 4 per week. Scheduling animals for 131 Iodine treatment should be made through the Internal Medicine Service (630) 963-0424.

Documentation of hyperthyroidism and evaluation of your cat's health must be performed prior to 131 Iodine treatment. These diagnostics may be conducted with your regular veterinarian or at VCA Arboretum View Animal Hospital. Pre-treatment evaluation consists of (not included in treatment cost estimate):

  • Blood work (CBC, Chemical profile, Plasma T4 concentration)
  • Urinalysis
  • Blood pressure
  • Occasionally ECG, chest radiographs (x-rays), and echocardiogram

The cost of I-131 treatment does not include pre-treatment diagnostics that your cat may require. I-131 treatment is estimated at $1185 (without thyroid scan) or $1696 (including the thyroid scan). Treatment cost includes:

  • Consultation with the Internal Medicine Specialist
  • Hospitalization with daily visual assessments and measured food intake (4-10 days)
  • 99m TC-pertechnate thyroid scintigraphy (included with thyroid scan)
  • Sedation procedure and sedation drugs (included with thyroid scan)
  • Monitoring Equipment used during sedation (included with thyroid scan)
  • Radiologist interpretation of scintigraphy (included with thyroid scan)
  • Radioactive Isotope delivery, administration and disposal

131 Iodine treatment will be performed only if pre-treatment evaluation confirms hyperthyroidism and if the animal's health status is compatible with isolation. Animals will not be treated if they are not considered to be safe candidates for treatment. 131 Iodine treatment will not be recommended if kidney function is not adequate unless the pet does not tolerate medical treatment with methimazole.

II. Patient's care during treatment

After administration of 131 Iodine, your cat is transported to an isolation ward to comply with radiation safety rules. During this time, the owner is not allowed to visit. Dr. Galvao and the Internal Medicine staff will take care of your cat. Care will consist of a daily visual inspection, cleaning the cage and providing food and water. The patient will be monitored on a regular basis to measure the exposure rate. Usually, cats with benign thyroid adenomas stay an average of 4-7 days in isolation. Cats are allowed to go home when their radiation exposure level is below 0.5 mR/hr at 1 meter of the patient (50 mR/hr at the skin surface over the thyroid glands).

III. Patient's care after release - Owner consent form

Cats are released from the hospital with residual radioactivity that will last approximately 2 weeks. In addition, your cat will continue to excrete radioactive material in the urine and feces for approximately one month. To comply with radiation safety rules, owners are asked to implement radiation safety precautions (see owner consent form) when the cat is back home.

Before release from the hospital, an interview with the owner is conducted by the Internal Medicine staff and Dr. Galvao in order to insure that the radiation instructions are understood. Instructions are fully explained to the owner and included in the owner consent form signed before release of the patient. Failure or unwillingness to complete the instructions will result in continuing hospitalization of the patient until the exposure rate at the skin surface over the thyroid glands has reached 2 mR/hr at thyroid level (approximately 15 more days). Additional costs will apply.

IV. Post-treatment evaluation

It is recommended to monitor improvement in clinical signs and recheck serum total T4 and renal function (creatinine, BUN, urine specific gravity) 1, 3 and 6 months after Iodine administration.

If the patient requires medical care within 1 month following the administration of 131 Iodine the owner should follow the instructions found in the owner consent form.

Owner consent form

Your cat has been treated with radioactive iodine (131I). The total activity of radioactive iodine administered was mCi. When released to your care, your pet will still be radioactive with an exposure rate of no more than 0.4 mR/hr (at 1 meter) and special radiation safety precautions will be needed for a period of 30 days after its release. After the date listed below, your pet will contain no detectable level of radioactivity.

No special radiation safety precautions will be needed after 1 month.

The radioactive material that has been administered to your pet is part of the treatment of his/her condition and is beneficial to him/her. However, it is important that no one be unnecessarily exposed to radiation. Therefore, radiation safety precautions have to be taken until the date listed above. By signing this owner consent form, you are accepting responsibility for the protection of yourself and other persons who may come in contact with your cat. Please comply with the following instructions:

  1. Your cat must continuously wear a collar warning of the presence of radioactivity.
  2. Your cat must be strictly confined to your house or residence with no access to bedrooms.
  3. You must limit close contact with your pet to brief periods of time for its necessary care. Spend no more than 10 minutes per day for 15 days after release from the hospital. Wash your hands carefully after handling your pet, her/his food dish and litter pan.
  4. Your cat must not be in contact with children or pregnant women for 30 days after its release.
  5. Provide your cat a clean litter box in an area where children and other pets cannot be exposed. Small amounts of radioactivity will be excreted in the urine and feces during the next 30 days and special care must be taken for disposal. Soiled litter needs to be discarded every day in the sanitary sewage system (toilet). Please use a partially covered litter box, small quantities of dust-free and flushable litter (Swheat Scoop® Natural Wheat Litter), and disposable liners. If a clumping type of litter is used, it must be scooped daily and solid waste discarded in the toilet.
  6. If your cat must see a veterinarian prior to the date listed on this form, please inform the doctor of the type of treatment your pet has received. Show this form to the veterinarian prior to examination.

Your cooperation is needed to comply with the federal regulations regarding radiation safety, reduce human exposure to a minimum, and allow continuing availability of this type of treatment to patients who need it at the VCA Arboretum View Animal Hospital. Please, don't hesitate to contact Dr. Galvao should you have any questions or specific problem regarding your pet's treatment and these instructions.

I have read this form and the information contained in it has been explained to me. I understand the radiation safety precautions that I must follow until the date listed above.

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