Iodine-131 therapy for feline hyperthroidism

What is hyperthyroidism?

Hyperthyroidism may be the single most commonly diagnosed hormonal disease in cats next to diabetes. It is generally a disease of older cats with an average age around 9-10+ years, but can be seen in cats as young as 5 years of age. In most cases, the disease is caused by a growth of abnormal, non-cancerous cells which secrete thyroid hormones in excess of the normal levels. If left untreated, a hyperthyroid cat can exhibit many, if not all, of the following signs: extreme weight loss, excessive appetite in most cases, but decreased appetite in some, muscle weakness, heart disease (increase in the size of the heart, increased rate, changes in heart rhythm, cardiac arrest), intolerance to stress, and eventually death.

How does radioiodine (131Iodine) work to treat the hyperthyroidism?

The thyroid gland is the only tissue in the body that concentrates iodine actively. However, the glands cannot differentiate between normal dietary iodine and radioactive iodine (131I). Therefore, the radioiodine is concentrated by the hyperactive, abnormal thyroid tissue. Because the normal thyroid tissue becomes atrophied (decreased in size and ability to function) in the hyperthyroid patient, the normal thyroid tissue itself should not concentrate the radioiodine.

How is the radioiodine administered?

The protocol for administering radioactive iodine at the University of Minnesota Veterinary Medical Center is by the subcutaneous route (under the skin). The radioactive iodine is administered as a single injection under the skin in the region of the inner thigh. Because of the nature of the injection, we generally will administer a small amount of sedation. This is to ensure the safety of both the patient and the staff members involved in the injection procedure as well as to ensure the entire dose is administered to the patient.

How long will my cat have to stay in the hospital after the treatment?

The hospitalization period varies from cat to cat, but is generally between 15-18 days. The effective half-life of the radioiodine (decay of radioactivity) can be quite variable depending upon each cat’s ability to excrete via the kidneys and the amount of time the radioiodine is bound to the thyroid. Due to the removal of iodine through the kidneys, cats with preexisting kidney disease may have to stay longer because the radioiodine may not be removed from the body as quickly. The patient will be monitored at regular intervals to determine when they are able to be released. The patient can be released from the hospital once the exposure rate from the radioactivity in the body reaches a level that is deemed to be safe to the general public (i.e., owners), which has been established by the University and the Minnesota Department of Health.

How will my cat be cared for during their stay?

Your cat will receive attention two times daily from one of our radioiodine therapy technicians. During this time your cat will receive routine care (feeding, watering, litter change, cage clean-up). We operate on a lights-on / lights-off basis. For normal daylight hours we provide natural lighting and music for our patients. From the period of 6 p.m. to 8 a.m. we provide lights out time, thus allowing for a somewhat natural life day.

It is possible for owners to bring in something familiar from home for their cats (something with their scent on it, such as a blanket, shirt, etc.), with the understanding that it will not be returned after the stay because of radiation safety considerations.

What happens once my cat is released from the hospital?

The care for your pet after being released from the hospital includes holding their litter out from normal trash for two weeks. To achieve this you can place it into a double garbage bag and place it in an unoccupied area or you can use flushable litter during the two week period. This is done to prevent others from being exposed to the radiation (i.e., sanitation workers) and also because most garbage facilities have radiation detectors at their plants and will return the garbage to you if radiation is detected. At the end of the two week period, the litter can then be disposed of with the normal trash. 

Pregnant women and children under 18-years-of-age should have absolutely no contact with the patient during the two week period. These two groups of people are the most susceptible to the hazards of radiation (the growing cells in the body of the child or unborn child are susceptible to radiation and can cause a stunt in the growth pattern).

You will need to limit contact with your pet during the two week period following discharge from the hospital. This will include having your pet sleep in an unoccupied room, restricting your pet from food preparation areas, and not letting your pet sit on your lap. Limited (less than one total hour per day) petting is acceptable. It is important that you always wash your hands after any contact with your pet or your pet’s excreta during the two weeks. This will help to prevent the spread of radiation to other regions of the house as well as to decrease the exposure to you from any possible contamination.

What type of monitoring should be done post radioiodine treatment?

Blood urea nitrogen (BUN) and creatinine (kidney values) should be measured at 1 month and 3 months after the radioiodine treatment. This helps monitor for kidney disease which can be unapparent while the patient has hyperthyroidism but can become clinically noticeable once the thyroid levels have returned to normal. The state of hyperthyroidism causes increased blood flow to the kidneys which can “mask” kidney disease that is already present in the older patient group in which both of these conditions are most likely to occur. Radioiodine therapy itself is not associated with causing kidney disease in cats.

Thyroid hormone level is also monitored at 1 month and 3 months after the radioiodine therapy to assess response to therapy. During this time, it is possible for patients to experience a period of subclinical hypothyroidism (low thyroid levels where the patient shows no signs of illness) that is almost always asymptomatic and does not require therapy in the majority of patients. In most patients, the atrophied (decreased in size and functional ability) thyroid tissue becomes functional, and the patient’s thyroid level returns to a more normal level. Cats that continue to have low thyroid levels may require thyroid supplementation.

Patients that continue to have high levels of thyroid hormones by 3 months after the radioiodine therapy probably will require re-treatment. This has only been noted in approximately 5% of cases.