What is Feline Hyperthyroidism?

Feline hyperthyroidism is the most common endocrine disease in older cats, occurring in about 10 percent of cats over 10 years old. It is caused by excessive circulation of thyroid hormone produced by an overactive thyroid gland. Since the thyroid is responsible for metabolism in the body, the symptoms of hyperthyroidism are usually related to an overactive metabolism: weight loss, normal to increased appetite, increased vomiting, diarrhea, increased vocalization, and restlessness. A small percentage of cats are lethargic, or show no signs at all.

Thankfully, hyperthyroidism in cats is usually quite treatable. Cats that are diagnosed early in the course of the disease are less likely to have serious complications. Left untreated, there can be progression of clinical signs and potential damage to the cat’s heart, kidneys, and other organ systems.

On regular physical exams, your veterinarian can monitor your cat for weight loss, and may notice an increased heart rate and an enlarged thyroid gland in his or her neck. If advanced, there may be significant weight loss and hypertension. Thankfully, routine lab screenings can diagnose hyperthyroidism. When done on an annual basis for older cats, these lab screenings can help diagnose the disease before it causes permanent damage to other organs.

If you or your veterinarian have questions about whether or not I131 treatment is right for your particular cat, please contact us. We can help go over the case and the lab results and help guide your decision making. We are happy to help advise, and will absolutely tell you if radioiodine is not the best choice for your cat.

What is Radioactive Iodine Therapy?

During treatment, radioactive iodine (I-131) is administered as an injection under the skin and is rapidly absorbed into the bloodstream. The abnormal thyroid tissue absorbs the emitted iodine, and is destroyed, leaving normal healthy thyroid tissue behind to begin functioning properly. The advantages of I-131 treatment are that the procedure can be curative, carries minimal risks for you and your cat, and it does not require surgery or anesthesia. The disadvantage is that the treatment is only permitted at facilities specially licensed to use radioisotopes. The radioactivity carries no serious risk for the cat, but careful protective measures are required for people who come into close contact with the cat. So, a cat who is being treated has to continue to be hospitalized until their radiation level has fallen to within adequate limits. Owner visitation during the cat’s stay is therefore not permitted.

What happens when my cat comes home?

After treatment your cat will have a very low amount of radiation in its body—about 50 times less than you would have if you were treated with I-131, or as little as you would receive if you flew across the country in a plane. Because of this radiation, state regulations require the following:

  • Your cat must ride home in a carrier.
  • For the first 2 weeks use rubber gloves when cleaning the litter box. Use flushable litter, dispose of your cat’s waste in the toilet, not the garbage. For 2-4 weeks following treatment, your cat will excrete unabsorbed I–131 and radioactive waste products in its urine and stool. If your septic system won’t handle flushable litter we can give you instructions to allow safe storage and disposal.
  • Minimize snuggling and holding your cat to under 30 minutes a day.
  • Pregnant women and people receiving chemotherapy should not clean the cat’s litter box or medicate the cat.
  • If your cat usually is allowed outside, he or she must stay inside for two weeks following treatment.
  • A follow up visit should be scheduled with your family veterinarian at 1 and 3 months post treatment.