To prevent damage to the thyroid caused by radioactive iodine, you may be advised to take a thyroid blocking agent.

A thyroid blocking agent is a pill, typically containing potassium iodide. The thyroid blocking agent contains non-radioactive iodine which, when taken before or immediately after exposure to radioactive iodine, saturates the thyroid with non-radioactive iodine. Since additional iodine will not be absorbed by the thyroid, any radioactive iodine subsequently taken up by the body will remain spread throughout the body and will be quickly excreted.