Why is radioactive iodine (I-131) used in radioactive iodine therapy?

I-131 is taken by mouth as a capsule or liquid. Thyroid cells naturally collect iodine to make thyroid hormones, so I-131 can concentrate in remaining thyroid tissue and some differentiated thyroid cancer cells. Its radiation then damages cells in that area. The treatment is planned according to the diagnosis, prior surgery, scans, thyroid function, and the amount of tissue that needs treatment.

Most of the iodine leaves the body through urine, with smaller amounts in saliva, sweat, and other fluids. For a period set by the treatment team, you may need distance from others, careful toilet hygiene, separate sleeping arrangements, and limits on close contact, especially with children or pregnant people. Follow the written instructions exactly.

Tell the nuclear medicine team if you are pregnant, breastfeeding, taking iodine-containing products, or have difficulty swallowing. Ask when to stop or restart thyroid medicines and whether a special diet is needed. Drink fluids if advised, wash hands carefully, and arrange practical help that does not require prolonged close contact during the restriction period.

Seek urgent help for trouble breathing, severe neck swelling, or inability to swallow fluids. Contact the treatment team for persistent vomiting, severe neck or mouth pain, marked swelling of the salivary glands, or uncertainty about radiation-safety instructions. Do not change isolation precautions without advice from the treating service.

What tissue is this treatment intended to reach? How should I prepare, including thyroid medicines and food? How long must I follow contact precautions? When can I return to work or travel? Which symptoms should prompt a same-day call?