Effective Treatment of Feline Hyperthyroidism with I-131

I-131 Therapy Feline hyperthyroidism is one of the most common diseases of older cats, usually due to enlargement of the thyroid gland. The veterinarians of Red Bank Veterinary Hospital have experience in diagnosing and treating this disease, as well as the effective treatment option radioactive iodine or I-131.

Diagnosing Feline Hyperthyroidism

Symptoms of feline hyperthyroidism include weight loss, ravenous appetite, gastrointestinal signs, and hyperactivity. Excessive thyroid hormone affects every organ, leading to heart conditions and high blood pressure.

To diagnose feline hyperthyroidism, a thyroid hormone level blood test known as a T4 is performed. Some cats require additional blood testing or repeated thyroid levels.

Comprehensive Treatment

Therapies for hyperthyroidism include prescription diet, oral medication, removal of the thyroid gland, and radioactive iodine treatment.

  • Diet—An iodine-restricted prescription diet must be the only food given for effective reduction of the thyroid level. This treatment is not recommended in cats with pre-existing renal disease.
  • Oral medication—Methimazole reduces production of thyroid hormone when given twice daily in tablet form or compounded into a transdermal formulation. Gastrointestinal upset is common; less common side effects include liver toxicity, skin lesions, and decreased red blood cells and platelets. Side effects typically resolve when medication is stopped.
  • Surgery—Removal of the thyroid gland is rarely recommended. Although the surgery is fairly simple, anesthesia on an older hyperthyroid cat is considered high risk. Complications related to calcium regulation may also occur in the post-operative period.
  • Radioactive iodine—I-131 is an extremely effective therapy. In the body, iodine is naturally absorbed by the thyroid gland. Radioactive iodine is injected under the skin; the radioactivity is absorbed and destroys the abnormal thyroid cells, a permanent correction of hyperthyroidism. The patient is hospitalized for four to five days so levels of radioactivity can be measured. Once the level reaches an acceptable low range, the patient discharged.

Effectiveness of I-131

The response rate to I-131 therapy is excellent. Of all cats treated, 94% require only one injection and less than 3% need follow-up treatment. Hypothyroidism, a low level of thyroid hormone, is a rare side effect of using radioactive iodine. If a cat develops hypothyroidism as a result of treatment, medication can easily correct the thyroid level.

I-131 Appointments

If you are interested in radioactive iodine treatment for your cat, please speak with your primary care veterinarian or contact RBVH Internal Medicine Department at (732) 747-3636(732) 747-3636.

Patients are scheduled for admission and treatment on Mondays and are typically discharged on Fridays. While your cat is with us, we call you with daily updates and send daily updates to your primary care veterinarian.

Radioactive iodine is prescribed specifically for each cat and ordered when you schedule treatment. This order can only be cancelled by 3 p.m. on the Friday prior to the scheduled treatment.

Questions about I-131 Treatments

Is I-131 treatment safe?

Screening for other non thyroid-related diseases (i.e., renal and heart disease) is important prior to beginning I-131 therapy. Blood work, including a complete blood count, chemistry, and thyroid levels, should be performed within a month of treatment. Radiographs (X-rays) of the chest should also be done to evaluate the heart and lungs. Cardiac evaluation with an echocardiogram is also recommended. If all screening tests are normal, then your cat is a suitable candidate for I-131 therapy. In preparation for treatment, your cat will need to stop taking tapazole for 10–14 days beforehand.

Will I be exposed to radiation?

The dose of I-131 used for feline hyperthyroidism is very small. People and other animals in the household are at a very low risk from radiation. Cats will not be discharged until they reach an acceptable low level of radioactivity. However, there will be a low level of radioactivity for approximately 80 days post treatment. We will give you a list of guidelines upon your cat’s discharge from the hospital to help minimize your family’s exposure. These guidelines include not sleeping with your cat, washing your hands after touching your pet, and wearing plastic gloves when changing the litter. Since radioactive iodine is excreted in the cat’s urine, waste from your cat’s litter box should be bagged separately from the normal household trash for approximately two weeks after your pet’s discharge from the hospital. After this time period, the litter can be disposed of with the regular household trash if you choose. Pregnant women and children under the age of 18 should not be in contact with the family cat during this period.

Can my cat receive other procedures during this hospital stay?

Cats receiving I-131 are hospitalized in a protected area so their radioactive levels can be closely monitored. Since they are receiving a radioactive treatment, cats are unable to receive any elective procedures during their hospital stay.

Can I visit my cat?

For your protection, owner visitation is not allowed. Your cat will be hospitalized in a protected area of the hospital, specifically designed for I-I31 feline patients. We assure you that your pet will have multiple staff visits throughout the day by our certified I-131 technicians and veterinarians and will be given plenty of attention. You may also bring a disposable, small toy for your cat to play with during their hospitalization. Due to potential contamination, the toy will be properly disposed of by our hospital staff at the end of your cat’s stay with us.

What is the follow-up procedure?

Your cat’s follow-up exam should take place about 4 to 6 weeks after treatment. During this time, blood work will be performed to evaluate the thyroid level and renal values. You should see the symptoms of hyperthyroidism start to diminish before your recheck appointment.