Feline Hyperthyroidism and Radioactive Iodine (I-131) Therapy

Hyperthyroidism is a disease common to middle-aged and older cats caused by thyroid hormone overproduction by the thyroid glands. Hyperthyroidism affects a cat’s metabolism, and causes clinical signs that affect multiple body systems, including:

• Increased appetite
• Weight loss
• Vomiting
• Diarrhea
• Hyperactivity
• Increased heart rate
• Heart murmurs
• High blood pressure

What treatment options are available for cats with hyperthyroidism?

If your cat has been diagnosed with hyperthyroidism, she will need prompt treatment to normalize her thyroid hormone levels and prevent systemic disease effects. The goal of hyperthyroidism treatment is to reduce thyroid hormone production, which can be accomplished by one of three methods:

• Antithyroid medication treatment — Antithyroid medications, such as methimazole, reduce production and release of thyroid hormones. Medication is typically administered twice daily, and treatment is lifelong. Medication administration can be particularly challenging in cats, and busy owners may find the daily treatments difficult to maintain. Antithyroid medications only manage, and do not cure, hyperthyroidism. 
• Thyroidectomy — Surgical thyroid gland removal involves anesthetizing your cat, and cats with hyperthyroidism may have systemic disease effects or concurrent health problems that make anesthetic complications more likely. Surgical removal is curative, but the nearby parathyroid glands may be damaged, with resulting calcium level abnormalities. Since medical therapy and radioactive iodine treatment are less invasive, thyroidectomy is rarely performed.
• Radioactive iodine (I-131) therapy — Radioactive iodine is a simple, non-invasive therapy that provides a lifelong hyperthyroidism cure. The preferred treatment option for most cats, Avets’ internal medicine department offers radioactive iodine treatment, which is less invasive than thyroidectomy, offers a lifelong cure, and does not require long-term medication administration.

Can all cats receive radioactive iodine therapy?

Before radioactive iodine is administered, a pre-treatment consultation will be conducted to confirm your cat is a good candidate, and will include diagnostic tests, such as:

• Complete blood count
• Blood chemistry
• Thyroid testing
• Urinalysis

If test results indicate your cat can receive radioactive iodine therapy, treatment can typically be started right away.

How is radioactive iodine administered?

Radioactive iodine is administered by a single injection under your cat’s skin. The thyroid gland uses iodine for hormone production and readily takes up the radioactive iodine, which destroys hormone-producing thyroid cells and shrinks the gland. At Avets, our board-certified internal medicine specialist will administer your cat’s I-131 treatment and oversee her care.

How is human exposure to radioactive iodine prevented?

The dose of I-131 administered to each cat is low, as is the human-exposure risk. To be safe, your cat will be hospitalized in a special ward until her radioactivity reaches an acceptable level, which typically takes two to three days. During her hospitalization, your cat will be cared for by our internal medicine staff, who are trained to limit human radiation exposure. You will not be able to visit her.

When your cat returns home, you will be given specific instructions to prevent excessive exposure and monitor her progress. Cats can have residual radioactivity for up to 12 weeks, during which time the level will steadily decrease. Aftercare typically involves precautions to minimize human exposure, such as:

• Limiting interactions with people and other pets — During her first two weeks home, your cat should be confined inside, preferably to an unoccupied area where she will have minimal interaction with people and other pets. You should not hold your cat, or allow her to sit or sleep with you. People under 18 years of age and pregnant women should not have direct contact with your cat during this time.
• Proper waste disposal — Radiation will be excreted in your cat’s urine and feces, and you must properly dispose of all waste. Line litter boxes with plastic liners to prevent radioactivity accumulation,  and flush all soiled litter down the toilet into the sewer system, or double-bag and store litter in a low-traffic area for three months before discarding it in the trash.
• Avoiding contact with contaminated objects — All bedding, toys, and other potentially contaminated material should by bagged and stored for three months, or thoroughly washed.  

What follow-up veterinary care will be required after radioactive iodine therapy?

Any additional medical care that your cat may need during the initial restriction period should be provided by our veterinary staff to avoid potentially exposing radiation to other veterinary professionals, who may not be used to handling cats after I-131 therapy.

Blood tests will be performed to measure your cat’s thyroid hormone levels at one, three, and six months after radioactive iodine treatment. Although we are always available to help with your cat’s aftercare, your family veterinarian can perform the blood testing. 

What is the expected outcome of radioactive iodine therapy?

More than 95% of cats treated with radioactive iodine need no additional hyperthyroid treatments. Few cats need a second treatment. Occasionally, the excessive hormone production is caused by a malignant tumor, which must be surgically removed, or treated with higher I-131 doses.

There is a slight risk your cat could become hypothyroid (i.e., develop low thyroid hormone levels) from treatment. In a study of more than 500 cats treated with radioactive iodine, only 2% developed this complication. Cats with hypothyroidism are treated with daily thyroid hormone supplementation.

If your family veterinarian has diagnosed hyperthyroidism in your cat, contact us to learn more about radioactive iodine therapy.