Radioactive Iodine Treatment (I-131) For Cats

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Radioactive iodine (I-131) therapy for catsRadioactive iodine (radioiodine or I-131) is used in the treatment of hyperthyroidism which is the most common endocrine (hormonal) disorder to affect cats. Ten percent of cats over the age of ten in the United States are affected. The disease is caused by a benign, hormone-secreting tumour on the thyroid gland which is located in the cat’s neck, these hormones control metabolism and growth.

There are several treatment options for cats with hyperthyroidism which include:

Prescription diet

Hills y/d is low in iodine, which is required by the thyroid gland to produce its hormones, low iodene=reduced hormone production. It can take a few weeks for this food to take effect.

Medications

Methimazole or carbimazole which block the synthesis of thyroid hormones.

Surgery

Removal of the enlarged thyroid lobe(s), known as thyroidectomy. In some cases, the parathyroid gland may be accidentally damaged or removed during this procedure, resulting in hypoparathyroidism.

Radioactive iodine

To destroy the tumour which is the focus of this article.

Some of these treatments manage the condition (medications and diet), while others cure it (surgery or radioactive iodine).

What happens if hyperthyroidism goes untreated?

Feline hyperthyroidism

An increase in levels of thyroid hormones speeds up the cat’s metabolism (hypermetabolic state) which leads to a hyperdynamic cardiovascular state in which the heart beats faster which eventually causes secondary hypertropic cardiomyopathy and congestive heart failure. Collectively called cardiac thyrotoxicosis. These are late manifestations of hyperthyroidism which highlight the importance of an early diagnosis and bi-annual health checks in cats over the age of 7.

High blood pressure (hypertension) is another common side effect of hyperthyroidism in cats. Hypertension has a serious effect on several organ functions. Swelling and bleeding into the eyes can lead to retinal detachment and blindness.

Other side effects include hardening of the arteries and an increased risk of stroke.

How does radioactive treatment work?

The thyroid gland uses iodine-which is in many types of food-to make the thyroid hormones. Cats with hyperthyroidism have abnormal and hyperactive thyroid tissue as well as atrophied normal thyroid tissue. I-131 is a radioactive form of iodine, which the diseased thyroid takes up. Penetration is only 1-2 mm deep, which spares the underlying healthy thyroid cells.

Your veterinarian will refer your cat to a specialist veterinary centre for treatment.

Are all cats good candidates for radioactive iodine treatment?

Approximately 5.8% of cats will have both chronic kidney disease and hyperthyroidism. Hyperthyroidism leads to an increased blood flow to the kidneys, which can mask kidney disease.

Cats who are in poor health due to cardiac thyrotoxicosis are also not be suitable candidates for treatment.

Benefits of radioactive iodine treatment:

  • Treatment has a cure rate of 95-98%, although a small percentage of cats will require a second treatment
  • Cats treated with radioactive iodine live twice as long as those treated with medications
  • Radioactive iodine requires no anesthetic or surgery
  • No ongoing treatment such as medications or diet
  • Preserves the parathyroid gland
  • Is cost effective over the long term
  • There are few to no side effects

Preparing a cat for radioactive treatment: 

Baseline tests: These include biochemical profile, complete blood count and urinalysis to evaluate the overall health of the cat and determine how the liver and kidneys are functioning.

Methimazole or carbimazole trial:  As we know, these medications decrease thyroid hormone production and can unmask kidney problems before the cat receives irreversible radioactive iodine. Kidney function is assessed prior to commencement of medications and then once again once normal thyroid function (euthyroidism) occurs. If kidney disease is revealed, the medication can be discontinued. If no kidney disease is unmasked, the specialist veterinarian can proceed with radioactive treatment.

Electrocardiograph: A recording of the electrical activity of the heart to look for abnormalities. If found, an echocardiogram  (ultrasound of the heart) will be necessary.

Stop medications and/or special diets: The cat will stop methimazole (Tapazole) or carbimazole or Hills Y/D (a prescription diet low in iodine) 7-10 days before radioactive iodine treatment.

Administration of radioactive iodine:

The cat will receive a single injection subcutaneously (under the skin) of radioactive iodine. It  quickly absorbed into the bloodstream and taken up by the diseased thyroid tissue.

Cats who are difficult to handle may require sedation prior to administration of radioactive iodine.

How quickly will thyroid levels take to return to normal?

In most cases, thyroid levels should return to normal within a week, although this may take longer in cats with poor kidney function.

How long will the cat remain in hospital?  

The cat will be in isolation for 3-8 days. 95% of the radioactive iodine is excreted via the cat’s urine, with lesser amounts through the feces. Exposure to radioactive urine poses risks to veterinary staff and caregivers therefore careful removal of urine is necessary at an approved facility. Pet owners will not be permitted visitation during the cat’s hospital stay, however, the veterinarian will maintain daily phone contact with updates on the cat’s progress. 

Discharge occurs when radiation levels in the urine are no longer a risk to the general population.

Note: If you choose to send the cat to hospital with an item of bedding or a favourite toy, it can not be returned due to radioactive contamination.

Side effects: 

Some cats can become temporarily hypothyroid (decreased levels of thyroid hormone) in the period after treatment, but this should resolve.

Home care: 

Once discharged, additional precautions will be necessary as the cat will continue have low levels of radioactivity.

  • Pregnant women and children under 18 should avoid contact with the cat, its urine, feces, food bowls and toys for 2 weeks.
  • Use rubber gloves when cleaning the litter tray.
  • Avoid prolonged contact with the cat such as sleeping with the cat on your bed, or allowing the cat on your lap.
  • Always wash your hands after contact with litter trays and the cat.
  • Keep the cat separate from other household pets.
  • Do not allow the cat outside for 3 weeks after discharge.
  • Double bag cat litter and store in an outside location, in a well-ventilated space for a further two weeks, then dispose of in the garbage. This extra time allows for further decay of radioactivity.
  • Do not allow cats to walk on bench tops, if they do, wash thoroughly with soap and water.
  • Avoid contact with the cat’s saliva which contains a small amount of radioactivity.

The veterinarian will schedule a follow up appointment 4 weeks after treatment to check thyroid hormone levels, kidney function and blood pressure.

What is the survival time of cats who receive radioactive iodine treatment?

The median survival time of cats without concurrent kidney disease is up to 5.3 years.

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