Potassium iodide (KI) and potassium iodate (KIO3) are called thyroid blockers when used in radiation protection.
If a person consumes a dose of one of these chemical compounds, his or her thyroid may saturate with stable iodine, preventing accumulation of radioactive iodine found after a nuclear meltdown or explosion.
References
Thyroid blocker Wikipedia(Text) CC BY-SA