How Radiodine Treatment Works
The doses of radioiodine are supplied in .5 ml quantity. This small volume typically will not even cause any sensation upon injection. The radioiodine is injected subcutaneously over the shoulders where vaccines are typically given. The radioiodine is immediately absorbed into the blood stream, which takes it throughout the body. Since no other tissue utilizes iodine, it will be taken up exclusively by thyroid tissue, even thyroid tissue in surgically inaccessible locations. Within the hyperthyroid thyroid, there are two types of cells, the overactive hyperthyroid cells and the normal thyroid cells, which are inactive. The normal inactive thyroid cells have shrunk and are not producing T4; therefore do not need any iodine. The overactive (hyperthyroid) cells need increased amounts of iodine to supply their increased production of thyroid hormone. The radioiodine that is going to bind in the thyroid binds within the first 24 hours. 90% of the unbound iodine will be excreted in the urine within the first 36 hours, hence remains in our waste storage facility.
The radiation within the abnormal cells kills those cells generally sparing normal tissue and adjacent parathyroid tissue. This decreases the serum thyroid hormone levels. Once the T4 levels drop below the normal range, the remaining normal thyroid cells are stimulated to become active again and they take over normal production. Very rarely do we need to supplement the cats after treatment .
How quickly will this treatment cure my cat's hyperthyroidism?
After the one-time injection of radioiodine, it can take one to three months for your cat's thyroid to heal and return to normal. You will see a gradual reduction in symptoms over this period of time.
75% of cats have normal serum T4 (thyroid hormone) levels within 7 days of . 90% have normal T4 levels by 1 month, approximately 95% have returned to normal by three months. (5 - 10% will have levels below normal (hypothyroid) but only about 0.25% will require oral thyroid hormone replacement medication) Approximately 5% will remain somewhat hyperthyroid after their initial dose. Cats with persistent hyperthyroidism can be re-treated 3 months after their initial therapy. Recent information suggests waiting 6 months if the cat is not showing hyperthyroid related problems; some of these cats will become euthyroid during this time. Reasons for failure to respond to the initial treatment may include prior treatment with methimazole (Tapazole/Felimazole), a very large thyroid mass, poor uptake of by some cats, certain drugs or chemicals, or a malignant thyroid tumor rather than a benign tumor. Nearly all cats will be cured by a second treatment. At 1 and 3 months after the treatment, a blood test for thyroid and kidney function should be done by your veterinarian. The 1-month level thyroid may be slightly high, low or normal. During this first month your cat shows no side effects as it returns to its original prehyperthyroid status. The signs of hyperthyroidism will diminish gradually as the thyroid hormone decreases to normal. A very rare cat(1-2%) may be mildly sluggish, sleep more and eat less. These signs are all related to correction of the thyroid hormone imbalance. The 3-month T4 test should no longer be elevated. Any signs persisting after the T4 is normal are most likely due to other medical problems that need to be identified and treated by your veterinarian. Relapse following successful radioactive iodine therapy is very rare but occurs in approximately 1 in 200 cats 2-5 years after .
Is it possible that my cat is not a good candidate for radioiodine treatment?
We require current lab work (CBC, chemistry screen, and urinalysis as well as a blood pressure when possible) done by your veterinarian within 6 weeks of the appointment date. If your cat has been on Methimazole for more than 8 weeks, you will need to have updated labs performed after your cat has been off of Methimazole for 7 days. Cats with significant heart disease should be accompanied by the appropriate diagnostics. We will contact your veterinarian to request the medical records.
If your cat has significant kidney failure, advanced heart failure or a malignant cancer, we can resolve the hyperthyroidism, but may be unable to make your cat better by treating the hyperthyroidism alone and you may not want to treat your cat due to the poor prognosis of the other disease processes.
Age is not a determining factor. We have treated multiple cats that were 21 years old. They were in good physical shape and did very well.