Radioactive iodine (also called iodine-131, or I-131) is commonly used to treat thyroid cancer. It is given by mouth. The thyroid gland and the cancer cells absorb the radioactive iodine, which is toxic. It destroys the cancer cells and normal thyroid cells.
Radioactive iodine may be used:
The amount of radiation given during treatment, and when and how it is given, may be different for each person.
Iodine is an essential mineral found in food and is easily absorbed by thyroid cells. The body uses iodine to make thyroid hormone. Reducing the amount of iodine in the body prior to radioactive iodine therapy increases the level of thyroid-stimulating hormone (TSH) which stimulates the thyroid tissue and cancerous thyroid cells to take up radioactive iodine. In other words, reducing the amount of iodine in the body will make the thyroid cells hungry for iodine and will allow them to readily absorb the radioactive iodine.
Usually after thyroid surgery, the person is placed on thyroid hormone pills to replace the thyroid hormone from the missing thyroid gland. Shortly before receiving the I-131, the person will either have thyroid hormone replacement stopped, or will receive recombinant TSH (Thyrogen), an artificial form of TSH that will increase the level of TSH while the person continues to take thyroid hormone pills.
Also in preparation for I-131, the person will be instructed to follow a low-iodine diet for 1–2 weeks before treatment. This will help reduce the amount of iodine in the body. Foods that should be avoided during this time include seafood, dairy products, iodized salt and some vitamins. The treatment centre will provide complete instructions about what to eat and what not to eat during this time.
If the person has had surgery and needs I-131 to destroy any remaining thyroid tissue or cancerous cells, it is usually given 4–6 weeks or more after surgery. Before a full treatment dose of I-131 is given, a small test dose may be given to make sure the cancer cells take up the iodine. The radioactive iodine is usually given in a capsule or liquid from.
Most people are admitted to the hospital to receive the I-131 therapy however, it is becoming more common for radiation to be administered on an outpatient basis. Those people who are admitted to hospital will stay in special isolation rooms for a few days after treatment to prevent others from being exposed to radiation. Some people will be allowed to go home right after they receive I-131. This will depend on the dose of radiation given and the policy of the treatment centre.
Treatment centres will have precautions and instructions that need to be followed by people who receive I-131 therapy. These precautions may vary slightly among treatment centres. It is important that they be followed. Some precautions may include:
For approximately 3 days after going home from the isolation unit, the person treated with I-131 should:
Most of the radiation is gone from the body within a few days. Only traces of I-131 remain in the body after 3 weeks.
A whole body iodine scan is usually done 7–10 days after treatment to see if there are any remaining cancer cells. If there are, then the person may receive another treatment with I-131. The doctors will also check the person's thyroglobulin blood levels. Thyroglobulin is a protein shed by thyroid cancer cells and is considered a tumour marker. Rising levels of thyroglobulin usually indicate that the cancer has returned.