Radioactive iodine therapy (I131) for hyperthyroid cats
What is hyperthyroidism?
Hyperthyroidism is the most common hormonal disease in cats other than diabetes. This disease is caused by high circulating amounts of thyroid hormone. Typically, it is a disease of older cats with an average age of 9-10 years, but can occasionally be seen in cats as young as 5 years. Most of the time, the disease is caused by growth of benign cells (adenomas) in the thyroid gland, but rarely (about 1-2% of the time), the disease is caused by cancerous cells (carcinoma). The high levels of thyroid hormone speed up the body’s metabolism and causes many clinical signs. The most common sign is weight loss often in the face of a ravenous appetite. The weight loss can be rapid or gradual depending on the degree of hyperthyroidism. Other signs include vomiting, diarrhea, anxiety or nervousness which could be manifested as yowling or pacing the house, rapid heart rate, increased thirst or unkempt hair. If hyperthyroidism is not treated, severe problems can occur such as heart failure, kidney disease, emaciation or possibly even death.
How is hyperthyroidism diagnosed?
Hyperthyroidism is diagnosed based on a combination of history, physical exam findings and an elevated thyroid level. Most cats with hyperthyroidism have a palpable thyroid nodule.
What are treatment options for hyperthyroidism?
Treatment options include methimazole, radioactive iodine (I131) and surgery. Surgery is rarely recommended unless a malignant thyroid tumor is suspected.
Methimazole is an oral or transdermal medication that can be used to treat hyperthyroidism but it is only a mechanism of controlling the disease rather than curing it. Methimazole is a good option for treatment but has a higher likelihood of side-effects and requires twice daily administration. Routine monitoring will be required for life because often the dose needs to be escalated. If the thyroid levels are not well controlled, then the cat is more likely to have chronic kidney effects.
Radioactive iodine (I131) is considered to be the treatment of choice for hyperthyroidism because it cures the disease rather than just managing it. It is a one-time subcutaneous injection and has no side-effects. There is much less monitoring required long-term. Overall, this treatment is less expense than methimazole over the lifetime of the cat and it has also been shown to have the longest survival rates.
How does I131 work?
The thyroid gland is the only tissue in the body that concentrates iodine. The gland cannot differentiate between normal iodine consumed in food and radioactive iodine (I131). Radioactive iodine is given as a single SQ injection. After the injection, the radioiodine is concentrated in the hyperactive abnormal thyroid tissue thus destroying it. The normal thyroid tissue has typically become inactive during the hyperthyroid state so it is spared from the effects of I131 and will become active again a few weeks after treatment
What is the success of radioiodine (I131) treatment?
Success rate is 95-98%. A few patients (about 5%) will require a second treatment if they have very high levels of T4 prior to treatment. A small percentage of cats can become hypothyroid after treatment and require thyroid supplementation in the form of an oral pill.
About our program
If you are interested in radioactive iodine therapy for your cat, we ask that you schedule a consult to go over the treatment with any SAGE internal medicine specialist. At this appointment, your cat will be evaluated for any other problems and different treatment options will be discussed. Prior to treatment, your cat will need to have full bloodwork, urinalysis and T4 as well as chest xrays performed. Once you have decided to treat, you can let your internist know and we will call you to schedule a treatment date and to further go over the process and answer any questions about the treatment and care required after treatment.
We will admit cats on Tuesdays and they will be treated that day. They will typically be released on Friday afternoon. While they are at SAGE, they will be fed twice daily and receive routine care such as litter change, water, cage clean-up. We also play videos and music during the day. They are in an isolated area that only our radioiodine technicians are allowed to enter so it is a very calm and quiet environment.
After I131 treatment:
You will need to have limited contact with your cat for 2 weeks after treatment (ie limited time sitting on your lap and no sleeping with you). See the owner release instructions form for more details.
Monitoring post I131 treatment:
Kidney values and a T4 should be measured 1 month and 3 months after treatment. This helps monitor for any kidney disease which could be “unmasked” once the thyroid is normalized. Some cats will have a period of subclinical hypothyroidism where the T4 is low and normally does not require therapy. This typically resolves within 3 months. If T4 levels are still high at 3 months post treatment, a second treatment may be recommended.