Radioactive Iodine (I-131)

Radioactive iodine I-131 is a medicine that thyroid cells can absorb and that delivers radiation.

The thyroid normally uses iodine to make hormones. This natural uptake allows radioactive iodine to concentrate in thyroid tissue and in some thyroid cancer cells that retain iodine uptake. The radiation is delivered from within the body rather than by an external radiation machine.

After it is taken, radioactive iodine moves through the body and is taken up mainly by iodine-absorbing cells. Radiation released near those cells can damage them. Iodine that is not taken up leaves the body over time, chiefly in urine. Care teams give specific instructions because the medicine can temporarily expose people nearby to radiation.

Radioactive iodine may be part of care for certain thyroid conditions and selected thyroid cancers. In cancer care, it may be considered after thyroid surgery when the planned aim is to treat remaining thyroid tissue or cancer cells that absorb iodine. Whether it is suitable depends on the diagnosis, previous treatment, thyroid hormone plan, and how the cancer behaves.

The alternative to radioactive iodine depends on why treatment is being discussed. A plan may instead involve surgery, thyroid hormone treatment, external radiation treatment, another systemic medicine, or monitoring. Some thyroid cancer cells do not absorb enough iodine for this treatment to be useful, so the care team will explain whether a different approach better matches the situation.

Preparation can include changes to thyroid hormone treatment, dietary guidance, blood tests, or imaging, depending on the plan. The treatment team will tell you how to take the medicine and when you can go home or need a protected care setting. Dry mouth, taste changes, nausea, or neck discomfort can occur. Contact the treatment unit if you have symptoms that concern you or cannot follow the preparation or safety instructions.

Your radiation safety instructions are personal to the dose and your circumstances. They may cover close contact, sleeping arrangements, bathroom hygiene, handwashing, laundry, travel, work, and contact with children or pregnant people. Follow the written instructions from your nuclear medicine or cancer team, and ask before making changes to them.