What is potassium iodide (KI) and what is it used for? If there is a radiation emergency at a nuclear plant, large amounts of something called radioiodine could be put into the air. This could hurt your thyroid gland, or even cause thyroid cancer later on. You could breathe in the radioiodine or eat food that has some radioiodine in it. When you take the KI pill, it protects your thyroid gland from being harmed. How does KI work? When you take the KI pill, it fills your thyroid with a kind of iodine that prevents your thyroid gland from taking in any of the radioactive kind of iodine.