Radioactive Iodine

Hyperthyroidism is a relatively common disease of older cats (usually over 10 years of age) that is caused by excess production of thyroid hormone from the thyroid glands.

In most cases hyperthyroidism is caused by a benign tumor-like growth of the thyroid gland. In up to 4% of cats, however, the condition is caused by a malignant thyroid cancer, which can spread to other parts of the body. If left untreated, a hyperthyroid cat can exhibit many, if not all, of the following signs: extreme weight loss, muscle weakness, heart disease (which includes an increase in the size of the heart, increased heart rate, changes in heart rhythm, and cardiac arrest), intolerance to stress, and eventually death.

Whenever your pet is showing signs of a health issue, your first step is to contact your primary care veterinarian. If it is indicated that your pet may suffer from hyperthyroidism or another serious condition, a veterinary specialist is available at an ExpertVet certified hospital.

Clinical Signs and Diagnosis

Excess thyroid hormone levels can cause a variety of symptoms in your older cat including weight loss, voracious appetite, poor hair coat, increased thirst and urination, vomiting and diarrhea, and hyperactivity. The disease can also cause high blood pressure and damage the heart. If left untreated, the signs of hyperthyroidism will progress, eventually causing death.

Your veterinarian can diagnose hyperthyroidism by using blood tests to search for elevated thyroid hormone levels.


Options for treating hyperthyroidism in your cat include the lifelong daily administration of tablets, surgical excision of the thyroid glands, or the administration of a single injection of radioactive iodine.

Each treatment option has advantages and disadvantages. Oral tablets are relatively inexpensive but do not cure the condition. In addition, they must be given twice daily indefinitely and can cause serious side effects.

Surgical removal of the thyroid glands can cure the benign form of the disease but requires anesthesia and risks damage to nerves and nearby calcium regulating glands in the neck.

An injection of radioactive iodine, meanwhile, will cure 97% of hyperthyroid cats and has rare side effects, but state laws require that treated cats be hospitalized in quarantine until their radioactivity decreases to regulation levels.

Whenever your pet is showing signs of a health issue your first step is to contact your primary care veterinarian. If it is indicated that your pet may suffer from Hyperthyroidism or another serious condition, a veterinary specialist is available at an ExpertVet certified hospital.

Radioactive Iodine (I-131) Therapy

Radioactive Iodine

Even with such regulations, radioactive iodine is considered to be the safest and most effective treatment for hyperthyroidism. It works by using iodine to target radiation to the thyroid gland. The thyroid gland uses iodine to produce thyroid hormone and is the only tissue in the body that concentrates iodine actively. However, the thyroid gland cannot differentiate normal iodine from radioactive iodine. As a result, the thyroid gland will absorb radioactive iodine, which then kills the hyperactive tumor cells. Because only the thyroid gland utilizes iodine, the radioactive isotope does not damage other parts of the body.

Radioactive iodine treatment also avoids some of the unpleasant side effects of radiation, such as hair loss or increase in skin pigmentation. Though at the beginning of therapy some cats seem to experience mild discomfort of the thyroid region (thyroiditis), it seems to resolve spontaneously. Overall, side effects are extremely rare.

People with hyperthyroidism, called Graves' disease, are also treated with radioactive iodine and are immediately sent home. Unfortunately, because the radioactivity is excreted from the body through the urine, it is impossible to send treated cats home without the risk of them exposing their human families.

For instance, while humans treated with radioactive iodine are instructed to stay away from children and to flush the toilet twice after urinating, the same protocols can’t be expected from cats.

Consequently, several states have passed laws that require treated cats to be quarantined for three to seven days until the radioactivity in their body has decreased to desired levels. Only specially trained personnel are allowed to handle treated cats during this quarantine period.

The hospitalization period can vary between cats depending on the rate of radioactive decay. The effective half-life of the radioactive iodine is variable depending upon each cat’s ability to excrete it via the kidneys and how long the radioactive iodine is bound to the thyroid. Cats with preexisting kidney disease may have an extended quarantine period because the iodine will remain bound to the tissue longer.

Your cat will be monitored daily and released from the hospital once the exposure rate from the radioactivity in the thyroid gland reaches a level that is deemed to be safe to the general public, if not already dictated by local law as previously mentioned.