Radioactive Iodine (I-131) Therapy for Hyperthyroid Cats
Can you imagine medicating your cat twice a day for the rest of their life for an overactive thyroid? VREC offers radioactive iodine (I-131) therapy – an effective one-time treatment for cats with hyperthyroidism. Learn more here.
What causes hyperthyroidism?
Hyperthyroidism is an overproduction of thyroid hormone by the thyroid gland. In the vast majority of cats, this is caused by a benign hyperplasia (overgrowth) of the thyroid gland. Malignant thyroid tumors causing hyperthyroidism are very rare – approximately 2% of all hyperthyroid cats.
Thyroid hormones are very important in regulating the body’s metabolism in both cats and humans. Excessive amounts of thyroid hormones cause a hyper-metabolic (increased energy use) state. This leads to some or all the typical signs of hyperthyroidism: ravenous appetite, increased thirst and urination, weight loss, rapid heart rate, abnormal heart rhythm, heart murmurs, intermittent vomiting, diarrhea, and unkempt coat. Some cats may have more subtle signs such as poor appetite and lethargy.
For many owners, seeing the effects of hyperthyroidism on their cat is enough motivation to treat the problem. However, many cats are diagnosed early and there may be little evidence of the disease. Left untreated, hyperthyroidism is a very debilitating and eventually life-threatening disease.
How does radioactive iodine (I-131) work to treat hyperthyroidism?
A properly functioning thyroid gland uses iodine absorbed from food to produce thyroid hormones. The excessive hormone being produced by abnormal thyroid tissue causes normal thyroid tissue to atrophy (shrink.) Once administered, radioactive iodine (I-131) is rapidly absorbed by hyperthyroid tissue. Radioactive iodine selectively localizes in the abnormal tissue and destroys it.
Radioactive iodine (I-131) is considered the safest treatment available for hyperthyroidism. Approximately 85-90% are cured with a single treatment. Most cats return to normal thyroid function within 1-3 months after treatment. While rare, some cats may require additional treatments.
There are very few direct side effects with I-131. Some effects reported are transient inflammation in the tissue immediately adjacent to the thyroid gland, which can cause hypersalivation (excessive drooling) and/or difficulty swallowing. Side effects are very mild, uncommon, and generally resolved by discharge. Some cats may develop hypothyroidism (low thyroid) but rarely require thyroid supplementation.
Are other treatment options available?
While I-131 is considered the treatment of choice for hyperthyroidism, there are two other treatments available: medical therapy with Methimazole (Tapazole) and surgical removal of the thyroid gland. Methamazole works by temporarily blocking the synthesis of thyroid hormone, but it does not destroy the underlying hyper-functioning thyroid tissue itself. Although uncommon, serious side effects may occur from Methimazole including abnormalities of the white blood cells, platelets, or red blood cells. It also requires the owner to administer medications twice daily for the remainder of the cat’s life.
Surgery requires anesthesia, which due to the patient’s underlying hyperthyroidism may place them at a greater risk for anesthetic complications. When surgically removing the thyroid glands there is a risk of damaging parathyroid glands. These glands, important for regulating calcium levels, are small portions of tissue embedded at the edges of the thyroid gland. They may become damaged or inadvertently removed during surgery. This may cause life-threatening low blood calcium. Some patients may have additional thyroid tissue which remains unidentified and is not removed at the time of surgery.
Are there risks or side effects of I-131?
A cat’s thyroid hormone levels will be checked by blood tests one and three months after treatment. This may be done by your primary care veterinarian or at VREC. Most cats have significant improvement within the first month after treatment – weight gain, normal food and water intake, etc. An owner may also elect for a thyroid scintigraphy study which can confirm no abnormal thyroid tissue remains.
Hyperthyroidism may cause an increased blood flow to the kidney, masking underlying renal disease or failure. While not a direct effect of I-131, treatment of hyperthyroidism may therefore lead to worsening of renal disease or failure. This does not mean that hyperthyroidism should go untreated, as it is a life-threatening disease.
Our internal medicine clinician will discuss these issues with you and make specific recommendations on your cat regarding the monitoring and managing renal disease or renal failure during and after the I-131 treatment of hyperthyroidism. Due to safety concerns, should a cat should become seriously ill during the treatment — either due to the effects of hyperthyroidism or some other separate disease process — our staff is limited in the amount of critical care that can be provided.
How long are cats hospitalized?
Cats are typically dropped off on Mondays and are usually safe to discharge by Friday. Depending on how fast or slow a cat metabolizes and excretes the iodine, the discharge may be earlier or later. It is important to remember that once admitted for therapy, a cat cannot under any circumstance be discharged until safe levels of radiation are achieved.
No visitors are permitted in the therapy ward. As per state guidelines, cats must be deemed safe before leaving the therapy treatment/housing area.
Upon discharge, our staff provides detailed written discharge instructions along with a litter box kit. Litter boxes must be kept in a minimally occupied area, such as a bathroom, laundry room, etc. Pregnant women and children should not handle the soiled cat litter. As difficult as it may be, it is imperative to avoid long periods in close proximity to your cat for three weeks after treatment. Do not hold your cat for extended periods or to sleep next to them during this time. Your cat must also remain confined in your home for a minimum of three weeks — no exceptions.
Should I bring things from home for my cat?
Unless you are comfortable not having blankets or toys returned, we do not recommend bringing any items. Due to radioactivity, soft surfaces like blankets, clothing, etc. must be specially disposed of after treatment. VREC provides everything your cat will need while hospitalized to be comfortable and happy. If cats are picky eaters, owners can portion out meals and snacks for the duration of the hospitalization. We cannot accept raw diets.
How much does this treatment cost?
A custom estimate including all costs, treatment, and care is provided during the initial consultation. This may include diagnostic testing such as blood work, radiographs, and/or ultrasound. Additionally, the clinician may deem it necessary to perform diagnostics prior to I-131. Estimates for any additional components of treatment also will be discussed. Please note that additional charges may occur unrelated to I-131 therapy.