Surgery may not remove 100% of thyroid cells.
Surgeons leave behind some thyroid tissue to minimize the risk of nerve damage affecting your voice box or damage to your parathyroid glands. Radioactive iodine removes those remaining cells which may be cancerous.
In some cases, it is possible that thyroid cancer has already spread beyond the thyroid gland to lymph nodes or other parts of the body. Radioactive iodine will destroy thyroid cancer cells regardless of their location.