The Chernobyl experience demonstrated that the time interval between an atomic accident and the arrival of the radioactivity in a country can be short enough to be an obstacle to preventive medication of the population.

The present study describes the effects of administering to human subjects a dose of 100 mg of potasium iodide six hours after the ingestion of a dose of iodine 131. Subjects were selected from the normal population of patients referred to our Unit for thyroid scintigraphy. Euthyroid state was assessed on the basis of radioimmunological assay of thyroid circulating hormones (free tetraiodothyronine (T4), free triiodothyronine (T3) and thyroid stimulating hormone (TSH)). The treatment (potassium iodide or a placebo) was administered 6 hours after the iodine 131 dose, and immediately after the acquisition of the thyroid scintigraphy.