Radioactive Iodine (I-131)
Therapy for Hyperthyroid Cats
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.
Why should hyperthyroidism be treated?
Thyroid hormone is very important in regulating the body’s metabolism. Excessive amounts of thyroid hormone causes 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.
What other treatment options are available?
Radioactive iodine (I-131) is considered to be the treatment of choice for hyperthyroidism. There are two other treatments available: either medical therapy with a medication called 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 radioactive iodine (I-131)
Radioactive iodine (I-131) is considered the safest treatment available for hyperthyroidism. 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.
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 necessarily mean that the hyperthyroidism should go untreated as it is a debilitating and life threatening disease itself. As an Internal Medicine Specialist, Dr. Dionne Ferguson 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.
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.
Will I-131 treatment cure my cat’s 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) and rarely require thyroid supplementation.
How long will my cat be hospitalized?
The radioactive iodine treatment is administered in a single injection dose. Your cat will need to remain hospitalized until the levels of radioactivity have lowered to that deemed acceptable for discharge to your home. 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. Once admitted for therapy, your cat cannot, under any circumstance, be discharged until current radiation levels are achieved.
May I visit my cat while hospitalized?
Visitors are not permitted in the in the therapy ward and your cat is not allowed to leave the treatment and housing area as mandated by state guidelines.
May I bring things from home to stay with my cat?
We provide everything your cat will need while hospitalized to be comfortable and happy. Any materials housed with the patient must be disposed of after treatment so we don’t recommend you bring toys or blankets unless you are prepared to not have them returned. You may bring your cat’s favorite food or snacks if you wish, especially if they are a fussy eater.
How will I be informed of my cat’s condition while hospitalized?
You may call for updates at any time. We will discuss the status of treatment and how your cat is doing with you on a regular basis.
What precautions must I take when my cat comes home?
We will give you detailed written discharge instructions along with a litter box kit. In general, it will be important to avoid long periods in close proximity to your cat for three weeks after treatment. It is safe to pick up your cat for short periods of time. It is not recommended to hold your cat for extended periods or to sleep next to them.
The litter box must be kept in a in a minimally occupied area like a bathroom, laundry room, etc. Pregnant women and children should not handle the soiled cat litter.
Cats must remain confined in your home. Other individuals are not aware of the necessary precautions.
How do we know if the treatment worked?
You will have your cat’s thyroid hormone levels checked by a blood test one and three months after treatment. This may be done either by your primary veterinarian or Dr. Ferguson. 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 much does this treatment cost?
An individualized estimate will be provided during your initial consultation. During the consultation Dr. Ferguson may perform initial diagnostic testing such as blood work, radiographs, a scintigraphy study and/or ultrasound. If medical treatment is required in preparation of I-131 therapy, an estimate for that specific component of treatment will also be discussed. Overall, we will provide an estimate to include everything including I-131 treatment, hospitalization and care through to discharge. Additional charges may be incurred for medical management unrelated to hyperthyroidism.