What is Potassium Iodide?
Potassium iodide (KI) is a white, crystalline chemical compound with the molecular formula KI and a molecular weight of 166.0 g/mol. It is classified by the FDA as an over-the-counter (OTC) drug for use during radiation emergencies to protect the thyroid gland from radioactive iodine-131.
Potassium iodide is not the same as "iodine." Iodine (I₂) is a pure element; potassium iodide is a salt combining potassium and iodide ions. The iodide (I⁻) form is what the thyroid gland absorbs and uses. KI is also distinct from potassium iodate (KIO₃), which the FDA has explicitly rejected as inferior for emergency use.
How KI Protects the Thyroid
Your thyroid gland naturally absorbs iodine from the bloodstream to produce thyroid hormones (T3 and T4). In a nuclear event, radioactive iodine-131 can enter the environment. Your thyroid cannot distinguish between stable iodine and radioactive I-131 — it absorbs whatever is available. Once concentrated in the thyroid, I-131 emits beta radiation directly into the tissue, causing DNA damage and dramatically increasing thyroid cancer risk.
KI works as a "blocking agent." By flooding the thyroid with stable iodide before or shortly after exposure, the gland becomes saturated and physically cannot absorb any more — including radioactive I-131. Learn the complete science in our how KI protects against radiation guide.
What KI Does NOT Do
KI is NOT a general "anti-radiation pill." It protects only the thyroid gland, only against radioactive iodine (I-131). It does NOT protect against external radiation, cesium-137, strontium-90, plutonium, or any other radionuclide. It does NOT remove radioactive material already absorbed by other organs.
Who Should Have KI?
The FDA and CDC recommend KI stockpiling for populations within the 10-mile Emergency Planning Zone (EPZ) of nuclear power plants. Children under 18, pregnant women, and breastfeeding mothers are at highest risk from radioactive iodine and benefit most from KI protection. Adults over 40 have the lowest risk-benefit ratio due to decreased thyroid cancer risk but increased side effect risk.