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? At VREC we offer 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.
Just like humans, thyroid hormones are very important in regulating the body’s metabolism. Excessive amounts of thyroid hormone cause a hyper-metabolic (increased energy use) state, leading to some or all of 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 can have subtle signs, such as poor appetite and lethargy. To many owners, it is clear why hyperthyroidism should be treated – it has made their cat very ill. For others, this may be less clear, since many hyperthyroid cats, when diagnosed early, may have little evidence of the detrimental effects of the disease. If left untreated, however, hyperthyroidism is a very debilitating and eventually life-threatening disease.
How does radioactive iodine (I-131) work in the treatment of hyperthyroidism?
The thyroid gland normally uses iodine absorbed from food to produce thyroid hormones. The excessive hormone being produced by abnormal thyroid tissue also causes normal thyroid tissue to atrophy (shrink). When radioactive iodine (I-131) is administered, it is rapidly absorbed by the 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. The majority of cats, approximately 85-90%, are cured with a single treatment. Rarely, some cats may require additional treatments. Most cats return to normal thyroid function within 1-3 months after treatment. Some cats develop hypothyroidism (low thyroid) but rarely require thyroid supplementation. There are very few direct side effects – there may be transient inflammation in the tissue immediately adjacent to the thyroid gland which could cause hypersalivation (excessive drooling), or difficulty swallowing. These side effects are very mild, uncommon, and generally resolved by the time the patient is discharged from the hospital.
Radioactive iodine (I-131) is considered to be the treatment of choice for hyperthyroidism, but there are two other treatments available: medical therapy with Methimazole (Tapazole), or surgical removal of the thyroid gland. Methamazole (Tapazole) works by temporarily blocking the synthesis of thyroid hormone; it does not destroy the underlying hyper-functioning thyroid tissue itself. Although uncommon, serious side effects may occur from Methimazole (Tapazole), including abnormalities of the white blood cells, platelets, or red blood cells. It also requires the administration of 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, the parathyroid glands (important for regulating calcium levels), which are small portions of tissue embedded at the edges of the thyroid gland may be damaged or inadvertently removed causing a life-threatening low blood calcium. Further, some patients may have additional tissue which remains unidentified and therefore not removed at the time of surgery.
Are there risks or side effects of I-131?
Hyperthyroidism can 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 regarding monitoring and managing renal disease or renal failure during and after the I-131 treatment of hyperthyroidism. Please note that if your cat should become seriously ill during the treatment, either due to the effects of hyperthyroidism or some other separate disease process, we will be limited in the amount of critical care that can be provided, due to radiation safety concerns.
Your cat’s thyroid hormone levels will be checked by blood test one and three months after treatment. This may be done either by your primary care veterinarian or by VREC. Most cats have significant improvement within the first month after treatment – weight gain, normal food and water intake, etc. If you so choose, a thyroid scintigraphy study may be performed to confirm no abnormal thyroid tissue remains.
How long will my cat be hospitalized for this procedure?
Once admitted for therapy, your cat cannot, under any circumstance, be discharged until safe levels of radiation are achieved. The radioactive iodine treatment is administered in a single injection dose and your cat will need to remain hospitalized until the levels of radioactivity have lowered to safe levels. Visitors are not permitted in the therapy ward and your cat is not allowed to leave the treatment and housing area as mandated by state guidelines. Hospitalization and administration is usually scheduled for Monday and patients are discharged by Friday. There is always a possibility that patients could be discharged earlier or later depending on how they metabolize and excrete the iodine.
When your cat is discharged, we will provide detailed written discharge instructions along with a litter box kit. The litter box must be kept in a minimally occupied area like a bathroom, laundry room, etc. Pregnant women and children should not handle the soiled cat litter. In addition, it is imperative to avoid long periods in close proximity to your cat for three weeks after treatment. It is not recommended to hold your cat for extended periods or to sleep next to them. Your cat must remain confined in your home for those three weeks — no exceptions.
May I bring things from home to stay with my cat?
Unless you are comfortable not having blankets or toys returned, we do not recommend bringing any blankets because materials housed with the radioactive patient must be specially disposed of after treatment. We provide everything your cat will need while hospitalized to be comfortable and happy. If your cat is a picky eater, you may wish to bring their favorite food or snacks.
How much does this treatment cost?
We may perform initial diagnostic testing such as blood work, radiographs, a scintigraphy study and/or ultrasound prior to treatment. If medical treatment is required in preparation of I-131 therapy, an estimate for that specific component of treatment will also be discussed. In any case, a custom estimate to include all costs – treatment, and care through to discharge — will be provided during your initial consultation. Please note that additional charges may occur, unrelated to I-131 therapy.