Iodine is essential for proper function of the thyroid gland, which uses it to make the thyroid hormones. The thyroid is equipped with an active system or “pump” for moving iodine into its cells, where it is concentrated as iodide. The thyroid gland is the only tissue in the body that takes up and holds onto iodine.
1.) WHAT IS RADIOACTIVE IODINE (RAI)?
Iodine, in the form of iodide, is made into two radioactive isotopes that are commonly used in patients with thyroid diseases: I-123 (harmless to thyroid cells) and I-131 (destroys thyroid cells). The radiation emitted by each these isotopes can be detected from outside the patient to gain information about thyroid function and take pictures of the size and location of thyroid tissues. RAI is safe to use in individuals who have had allergic reactions to seafood or X-ray contrast agents, since the reaction is to the compound containing iodine, not the iodine itself. RAI is given by mouth in pill or liquid form.
2.) DOES RAI FOR THYROID IMAGING PROVIDE THE BEST RESULTS?
I-123 is the usual isotope used to take pictures and determine the activity of the intact thyroid gland (Thyroid Scan and Radioactive Iodine Uptake, RAIU), since it is harmless to thyroid cells. No special radiation precautions are necessary after a thyroid scan or RAIU. I-131 can also be used to take pictures of the thyroid gland, although it is rarely used due to the harmful effects it has on thyroid cells.
3.) WHEN IS RAI USED FOR TREATMENT OF THYROID DISORDERS?
NORMAL THYROID TISSUE – I-131 is given to destroy overactive thyroid tissue or to shrink thyroid glands that are functioning normally but are causing problems because of their size. Patients are asked to follow some radiation precautions after treatment in order to limit radiation exposure to others. I-131 may occasionally cause mild pain in the neck that can be treated with aspirin, ibuprofen or acetaminophen. The RAI treatment may take up to several months to have its effect. Frequently, the end result of RAI treatment of hyperthyroidism is hypothyroidism, which is treated by thyroid hormone replacement.
THYROID CANCER – Large doses of I-131 are used to destroy thyroid cancer cells. This is performed after the remaining thyroid cells (including any cancer cells) are stimulated by raising TSH levels by either withdrawing the thyroid hormone pills or by treating with recombinant human TSH. Patients are asked to follow some radiation precautions after treatment in order to limit radiation exposure to others. Depending on state regulations, patients may have to stay isolated in the hospital for about 24 hours to avoid exposing other people to radiation, especially if there are young children living in the same home.
4.) WHAT ARE THE RADIATION SAFETY PRECAUTIONS AFTER TREATMENT WITH I-131 RAI?
Although the treatments with 131-I are generally safe, RAI produces radiation so patients must do their best to avoid radiation exposure to others, particularly to pregnant women and young children. The amount of radiation exposure markedly decreases as the distance from the patient increases. Patients who need to travel in the days after I-131 RAI treatment are advised to carry a letter of explanation from their physician. This is because radiation detection devices used at airports or in federal buildings may pick up even very small radiation levels. Details should be discussed with a physician prior to, and at the time of, the RAI treatment.
5.) ARE THERE LONG TERM RISKS OF I-131 RAI?
In general, RAI is a safe and effective treatment for the thyroid disorders mentioned above. Hypothyroidism is a common side effect of RAI for hyperthyroidism and always seen after RAI for thyroid cancer. This is usually easily treated with thyroid hormone replacement. Some studies suggest a slight increase in thyroid cancers may be seen after RAI treatment for hyperthyroidism. Loss of taste and dry mouth due to salivary gland damage may be seen. The use of lemon drops, vitamin C or sour stimulation to potentially decrease the exposure of the salivary glands to RAI is controversial and should be discussed with your physician. Importantly, once you have been treated with RAI, regular medical follow-up is lifelong.
6.) WHAT ARE THE SPECIAL CONCERNS FOR WOMEN?
RAI, whether I-123 or I-131, should never be used in a patient who is pregnant or nursing. This protects the baby who would otherwise receive radioactive milk and the mother’s breasts which concentrate RAI. Breastfeeding must be stopped at least 6 weeks before administration of I-131 treatment and should not be restarted after administration of RAI, but can be safely done after future pregnancies. Also, pregnancy should be put off until at least 6 – 12 months after I-131 RAI treatment since the ovaries are exposed to radiation after the treatment and to ensure that thyroid hormone levels are normal and stable prior to pregnancy. There is no clear evidence that RAI leads to infertility.
7.) WHAT ARE THE SPECIAL CONCERNS FOR MEN?
Men who receive RAI treatment for thyroid cancer may have decreased sperm counts and temporary infertility for periods of roughly two years. Sperm banking is an option in a patient who is expected to need several doses of RAI for thyroid cancer.