How Radioiodine Treatment Works

Our treatment objectives are to eliminate all hyperactive thyroid tissue and to avoid lifelong methimazole.

Radioiodine-131 selectively destroys hyperactive thyroid tissue and little of the normal tissue surrounding it. Once the abnormal tissue is destroyed, suppression of thyroid hormone production with methimazole will no longer be necessary.

How It Works

Thyroid cells require iodine to produce thyroid hormones. That is why we humans choose iodized salt for our food.

Hyperactive thyroid cells take over virtually all production of thyroid hormones. These cells uptake iodine ‘greedily’ to produce more and more hormone, and, in turn, absorb more radioactive iodine, which destroys the tumor(s) present in every hyperthyroid cat.

Normal thyroid tissue, surrounding the tumors, ‘reasonably’ shuts down production when too much hormone exists. Suppressed tissue does not absorb much, if any, iodine or radioiodine, and is thereby spared. More often than not, the residual thyroid cells reactivate and regenerate after the thyroid hormone drops to normal or below after I-131.

An injection of I-131 is given subcutaneously (under the skin) over the shoulders, just like a vaccination could be. The I-131 travels through the blood stream into the thyroid cells wherever they are (usually in the neck, but sometimes the chest). Other types of cells in the body absorb little to no iodine or radioiodine, which is why cats do not feel sick after they receive their treatment.

The vast majority (94-97%) of cats will be cured after one injection of I-131.

Even so, about 5% of hyperthyroid cats have unusually ‘stubborn’ tissue that may require a second treatment with I-131. We typically carry the cost of retreatment for the owner, unless the second dose is necessarily quite large.

At FHTC, each cat receives the lowest dose of I-131 that should cure its own level of thyroid disease. This helps assure that as little normal tissue as possible is affected, and that most will not need thyroid hormone supplementation (cats are not easy to give oral medications to). The dose is based on the size of the thyroid nodule(s), the hormone level, and the presence of other diseases, if any. The ability of the cells to uptake the I-131 varies from one individual to the next. Therefore, we do not have complete control of the outcome even with the most carefully selected dose. After the dose is determined, it is prepared by a nuclear pharmacist, and delivered to our facility the same day by a courier licensed to transport radioactive materials.

Despite low-level doses of I-131, some cats (3-5%) have insufficient normal thyroid hormone production after the abnormal tissue is destroyed, and may require a natural supplement daily for life. This statistic changes in cats with long-standing thyroid disease with more aggressive thyroid tumors, so our doctor may communicate varying outcome potentials from one individual to the next. Consider hormone supplementation a tolerable adjustment after successful cure of hyperthyroidism.

How quickly will this treatment cure my cat's hyperthyroidism?

Within 1 week to 3 months.

At 1 and 3 months after the treatment, an exam and a blood test for thyroid and kidney function should be done by your veterinarian. The 1 month thyroid level may be slightly high, low or normal. During this first month your cat generally shows no side effects as it returns to its original prehyperthyroid status, a very rare cat may be mildly sluggish, sleep more, and eat less. Excessive shedding may also occur (rarely). 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.

Follow-up Rechecks:

An exam and lab work need to be scheduled with your primary veterinarian for 1 and 3 months post I-131. The exam determines whether signs related to the thyroid disease are resolving. Blood tests assess thyroid hormone levels to determine whether cure of the hyperthyroidism has been accomplished, as well as how other organs are functioning relative to the thyroid, mainly liver and kidneys.

After the final recheck, your primary veterinarian will determine how often your senior cat should be evaluated for its general health care. A thyroid test should be included in any senior blood screens from this point on, generally every 6-12 months.


After successful cure of hyperthyroidism with I-131, most cats do not relapse, i.e., grow a new thyroid tumor in the future. The incidence of recurrence is fewer than 1 in 200 cats treated at our facility.

The reason why some cats have a recurrence is not due to inadequate treatment with I-131; rather, a gradual growth of a new tumor in some of the normal tissue which was intentionally spared after initial treatment. Cats that develop hyperthyroidism are genetically predisposed to thyroid tumors and are exposed to substances in our homes that trigger the abnormal tissue growth.

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