The FDA has evaluated the medical and radiological risks of administering potassium iodide for thyroid blocking under emergency conditions. FDA guidance states that risks of side effects, such as allergic reactions, from the short-term use of relatively low doses of potassium iodide for thyroid blocking in a radiation emergency, are outweighed by the risks of radioiodine induced thyroid nodules or cancer, if the projected dose to the thyroid gland is 25 rems or greater. These drugs are not stocked in pharmacies and must therefore be ordered directly from the companies. The directions in the package insert should be followed. Other forms of potassium iodide are available by prescription only.
Potassium iodide is effective as a thyroid blocking agent.
It reduces thyroid gland accumulation of radioiodine that has entered the body through inhalation or ingestion if the potassium iodide is administered before, or immediately after, exposure to the radioiodine. If potassium iodide is administered later than four hours after an individual has suffered an acute ingestion or inhalation of radioiodine, its effectiveness as a thyroid blocking agent is less than 50 percent. The guidelines should also address the more detailed measurements, surveys, and other follow-up procedures needed to determine whether continued treatment with potassium iodide is needed. No specific action or written response is required by this information notice. If you have any questions regarding this matter, please contact the person listed below or the appropriate NRC regional office.