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GUIDE TO RADIOIODINE TREATMENT FOR FELINE HYPERTHYROIDISM
WHAT IS HYPERTHYROIDISM?
Hyperthyroidism is a clinical condition resulting from the excessive production of thyroid hormones by the thyroid gland. It is common in middle-age to older cats (>95% of the cases occur in cats over 8 years of age). It is usually due to benign changes (Hyperplasia or adenomas) in the thyroid gland. Cancer of the thyroid gland can occur but is rare.
WHAT ARE SIGNS OF HYPERTHYROIDISM?
Thyroid hormone affects every organ system, so signs can be variable. The most common clinical signs include weight loss, increased appetite, vomiting, diarrhea, drinking and urinating more, and nervousness or hyperactivity. Thyroid hormone affects the heart, causing fast heart rates, heart murmurs, abnormal heart beats and high blood pressure.
HOW CAN HYPERTHYROIDISM BE TREATED?
Three forms of therapy are available. These include:
HOW DOES RADIOACTIVE IODINE WORK?
Iodine is normally taken up by the thyroid gland. One form of iodine, I131, is radioactive. When I131 enters the thyroid gland it destroys the abnormally functioning cells. This reduces the size of the gland and its ability to produce thyroid hormone. I131 is administered to hyperthyroid cats by a subcutaneous injection. Cats treated with radioactive iodine need to be hospitalized for 2-7 days following the injection. This is when they are most radioactive. Their level of radioactivity is checked daily with a Geiger counter and only when it reaches an acceptable low level can they go home.
HOW EFFECTIVE IS RADIOIODINE THERAPY FOR FELINE HYPERTHYROIDISM?
In a recent study of 524 hyperthyroid cats treated with radioiodine1, the overall response to treatment was considered good in 94% of the cats. 8/524 (1.5%) remained hyperthyroid 6 months following treatment, requiring additional therapy. 13/524 (2.5%) had a relapse of hyperthyroidism 1-6.5 years after the initial treatment.
IS RADIOIODINE TREATMENT SAFE?
Radioiodine treatment has been used in human medicine for over 50 years and is recognized as a safe and effective method of treating human patients with hyperthyroidism. Studies in humans have shown no increased risk of developing leukemia, thyroid cancer, or other cancers after treatment with radioiodine. In a recent study of 524 hyperthyroid cats treated with radioiodine, the only adverse sign was problems swallowing in 8/524 (1.5%). This problem resolved rapidly without treatment.
WHAT ARE THE RISKS OF MY CAT BECOMING HYPOTHYROID?
Hypothyroidism is a condition resulting from a deficiency of thyroid hormone. Clinical signs include lethargy, dullness, obesity, oily skin, and matted fur. In a recent study of 524 hyperthyroid cats treated with radioiodine, only 11/524 (2.1%) developed hypothyroidism and required thyroid hormone supplementation.
IS THERE ANY INCREASED RISK IN TREATING MY CAT FOR HYPERTHYROIDISM IF THERE ARE OTHER HEALTH PROBLEMS LIKE CHRONIC KIDNEY DISEASE?
Because hyperthyroidism is seen in middle-age to older cats and may affect any organ system, it is not unusual for a hyperthyroid cat to have other health problems. Chronic kidney disease and heart disease are important concerns in these patients. Therefore, careful evaluation of a hyperthyroid cat is recommended. A complete blood workup, including tests evaluating kidney and liver function, a urinalysis, a thoracic radiograph to evaluate the heart and check for cancer, and blood pressure measurement are performed in the initial evaluation of hyperthyroid cats prior to treatment. These tests help give a better understanding of each individual hyperthyroid patient's specific needs.
WHAT ARE THE RISKS OF RADIATION EXPOSURE FROM MY CAT TO PEOPLE AND OTHER PETS?
The dose of radioiodine used to treat hyperthyroid cats is very small and people and other animals are at very low risk from the radiation. Treated cats are hospitalized until they reach an acceptable low level of radioactivity. However, they will be radioactive (to some degree) for 82 days following treatment. Radioactivity does decrease rapidly over time and distance.
Steps to minimize and unnecessary radiation exposure to others include:
Source: Peterson ME and Becker DV. Radioiodine treatment of 524 cats with hyperthyroidism. J. Am. Vet. Med. Assoc. 1995; 207: 1422-1428.
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