Radioactive Iodine Treatment for Feline Hyperthyroidism

Feline hyperthyroidism is the most frequently diagnosed endocrine disorder of the cat. A small benign tumor ( thyroid adenoma ) involving one or both thyroid lobes is the most common cause of hyperthyroidism in the cat. The tumor does not respond to normal control mechanisms and produces excessive amounts of the thyroid hormones T4 and T3. This results in a variety of clinical signs including weight loss, hypertension, gastrointestinal signs and even heart disease. Malignant tumors (thyroid carcinomas) are also seen on occasion. Both of these conditions are readily treatable with radioactive iodine.
Before treatment with radioactive iodine, a thyroid scan using a low-energy, short half-life radioisotope (technetium pertechnetate) is recommended. The scan will confirm the diagnosis, identify the number of abnormal lobes and the presence of any ectopic tissue and aid in determining the radioactive dose required for effective treatment. It also helps identify those animals with thyroid carcinomas.
Radioiodine has become the treatment of choice for hyperthyroidism because it is so safe and so effective. Radioactive iodine is readily absorbed by the hyperfunctioning thyroid tissue only. Thyroid function returns to normal usually in 1-3 months.
The majority of cats (92-98%) need to be treated only once. Most cats are hospitalized from 5-7 days, depending on the dose of radioiodine administered and the excretion rate of the iodine. Cats with high thyroid levels and/or large tumors usually have to stay in the hospital longer. Once admitted, your cat cannot be discharged until his or her radiation exposure rate is at the legal, safe level. You will not be allowed to visit because of the radiation safety regulations. Toys or blankets can be left but will not be returned to you. If your cat has a favorite food, please bring some with you or let us know. Your cat is also required to be up-to-date on vaccinations for upper respiratory viruses. We will be in contact with you during your cat's stay and also encourage you to call if you are concerned. If any problems or complications arise, you will be notified immediately.
When your cat returns home, radioactive safety precautions must continue for another two weeks. You will receive detailed instructions at the time of release, but the basic recommendations are as follows: 1) your cat must be confined to your premises; 2) prolonged close contact (sleeping with you, sitting in your lap) must be kept to a minimum; 3) exposure to children or pregnant women should be avoided. The amount of radiation remaining in your cat will be extremely low. The amount of radiation any other pets will receive is negligible and they do not need to be separated.
After treatment, thyroid levels should be re-evaluated at one and three months. The one month sample may be low, normal, or high. Kidney values are also re-evaluated at that time as the hyperthyroidism often masks underlying renal disease. During the first months you may notice that your cat is sleeping more, eating less, gaining weight, and shedding excessively. All of these are normal. The three month thyroid test should be normal.

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