re: Not quite.
Iodine can be used both to protect against radiation damage of the thyroid and to introduce radiactivity into the thyroid to fight cancer:
Radioactive Iodine is given to the patient with thyroid cancer after their cancer has been removed. If there are any normal thyroid cells or thyroid cancer cells remain in the patient's body (and any thyroid cancer cells retaining this ability to absorb iodine) then these cells will absorb and concentrate the radioactive "poisonous" iodine. Since all other cells of our bodies cannot absorb the toxic iodine, they are unharmed. The thyroid cancer cells, however, will concentrate the poison within themselves and the radioactivity destroys the cell from within.
Endocrine Web