The thyroid alternately absorbs and releases iodine, which means that the iodine in the thyroid gland is continuously substituted. Therefore, the correct timing of the intake of iodine tablets is the prerequisite for the effectiveness of iodine blockade. If the iodine tablets are taken too late, radioactive iodine may already have been absorbed by the thyroid. If the iodine tablets are taken too early, the additional non-radioactive iodine may already have been released partially or completely. In either case iodine blockade will be of no or only reduced effect.

The correct timing of the tablet intake ensures that the non-radioactive iodine as supplied by the iodine tablets is either already in the thyroid or still stored sufficiently to prevent radioactive iodine from being absorbed and stored in the thyroid.

In the case of a severe nuclear accident, the emergency response authorities are the first to have access to all necessary information on the release of radioactive iodine and its atmpspheric dispersion. Therefore, only the emergency response authorities are able to decide whether iodine blockade is necessary, and to issue recommendations as to the where and when the iodine tablets should be taken. In order for iodine blockade to be the most effective, it is vital that members of the public obey the emergency response authorities’ announcements and recommendations. In their announcements the authority may mention which group of people should take the tablets.