Cat-I-131 (Radioactive Iodine Treatment)
If you answered yes to any of these questions, your cat may have Hyperthyroidism.
Hyperthyroidism is a condition that affects cats, humans and sometimes dogs. This abnormal growth of thyroid tissue results in an overproduction of thyroid hormone. Thyroid hormone (Thyroxin) controls our metabolism. The symptoms noted above (overeating with weight loss, hyperactivity) are a direct result from too much thyroid hormone.
Normal thyroid tissue is generally located in the neck but it is possible to have remnant tissue anywhere from the base of the heart to the base of the tongue. While in the womb, the thyroid tissue starts in the chest and moves during development into the neck region. Because of this, it is not always possible to feel a mass in the neck, but with most cats, a nodule (or nodules) can be felt by sliding two fingers up and down along the side of the trachea.
Bloodwork performed by your primary veterinarian will often show an elevation of the T4 (thyroid hormone). For some cases, a Free T4 will be performed as well.
Arizona Veterinary Oncology utilizes a test called a Technetium Scan to better define the extent of the disease. We are the only practice in the Phoenix Valley that has this technology available. This test requires an injection of a radioactive substance called technetium. Technetium has a half-life of only a few hours and preferentially goes to the active thyroid tissue. We have a special camera that can image the radiation. This test not only tells us the size of the mass and the number of masses present, but also gives information on whether the mass is benign or malignant (while not perfect, typically the benign lesions are smooth and round and the malignant tumors are star-like and irregular in shape). While both benign and malignant thyroid tumors in cats can be treated with radioactive iodine (I-131), the malignant masses require larger doses and often require more than one treatment. All of this information will be used to determine the appropriate dose of I-131 for your kitty, rather than just “blanket dosing” everyone with the same dose.
Radioactive Iodine (I-131) is the only non-surgical definitive treatment for hyperthyroidism. When a cat (or a person) is hyperthyroid, the normal thyroid tissue has “gone to sleep.” The normal tissue has responded to feedback signals from the body that tell it to stop making the hormone. As such, the normal tissue is not taking up iodine (radioactive or otherwise). The abnormal tissue, however, continues to absorb iodine in large quantities. As such, when we inject the radioactive iodine, it only gets absorbed by the abnormal tissue and thus, we only treat and kill the abnormal tissue. As the mass dies and stops producing thyroid hormone, the body sends a signal to the sleeping normal thyroid tissue and it once again wakes up and begins producing hormone.
Surgery was often the treatment of choice when we first started treating this disease. Surgery would remove the palpable mass in the neck of cats and people. Most of the time, this procedure would help but in a number of cases the cats did not improve or only improved for a short period of time. Besides the inherent risks of surgery (anesthesia, bleeding, infection), we found that without a technetium scan, other masses were sometimes not removed because they were not palpable. With bilateral disease (disease on both sides of the neck), removal of the thyroid glands also runs the risk of removing all of the parathyroid glands. These glands, closely associated with the thyroid glands, are responsible for controlling our calcium levels. Surgery is still the treatment of choice in most dogs as their thyroid tumors are not usually “functional” so they won’t take up the radioactive iodine.
Methimazole is a medication that prevents the complete synthesis of thyroid hormones. It is a medication that is typically given twice daily. It comes in a tablet form but is often compounded into liquids or trans-dermal (through the skin) applications. While this medication helps to eliminate the signs of hyperthyroidism, it does nothing to treat the underlying cause (the mass in the neck). Because of this, increasing doses are often required. Side effects to this medication, including allergic reactions, are not uncommon. Regular bloodwork and visits to the veterinarian are required to monitor the medication.
Iodine restricted diets like Hill’s Y/D™, prevent thyroid hormone from being produced and thus, eliminate the signs of hyperthyroidism. The only part of our body that uses iodine is our thyroid glands and as such, eliminating iodine from the diet does not have detrimental effects on other parts of the body. As with Methimazole though, this diet does not address the underlying cause- the mass growing in the neck. We also often hear stories of cats doing their darnedest to find their own source of iodine by stealing food off the counter and by digging up and consuming house plants.
With radioactive iodine treatment, we estimate that better than 90% of cats are cured of this disease with a single dose of radioactive iodine. Of the remaining 10%, a small portion become hypothyroid (not enough thyroid hormone), a small portion become normal and then again become hyperthyroid in the future and a small portion remain hyperthyroid and must be treated again.
We find that with the addition of the technetium scan, we are able to keep the number of cats that require life-long supplementation because they are hypothyroid to less than 2% of all the cats we treat. We also find that the number of cats that must be treated again is about 2% but we can generally predict which cats will require a second treatment because they have malignant appearing masses on the technetium scan.
The kidneys are the most common complication of any treatment of hyperthyroidism in cats. The reason for this relates to the nature of the kidneys in a hyperthyroid state.
The kidneys are like a big filter. When a cat is hyperthyroid, more blood than normal is being pushed through the filter. As such, the things we measure to assess kidney function (BUN and Creatinine) are falsely decreased. Kidney disease can essentially be hidden or “masked” from our blood results. When the hyperthyroid state is corrected and blood flow is returned to normal, we sometimes see those BUN and Creatinine values increase. While the hyperthyroid treatment did not directly affect the kidneys, it sometimes can reveal damage already there. In severe cases, it is possible the decreased blood flow can result in kidney failure.
As a precaution, we recommend looking at the kidney values ideally when the T4 levels are normal (through the use of Methimazole), measuring the urine specific gravity and measuring the SDMA. If these values are normal, it is much less likely that there will be any immediate problems with the kidneys.