Iodine thyroid blocking: Guidelines for use in planning and responding to radiological and nuclear emergencies

Overview

These guidelines are an update of the 1999 World Health Organization (WHO) guidance on the use of iodine thyroid blocking (ITB) with a special focus on public health considerations of ITB implementation.

During a nuclear accident, radioactive iodine may be released in a plume, or cloud, contaminating the environment, thus resulting in external exposure. Inhalation of contaminated air and ingestion of contaminated food and drinking water may lead to internal radiation exposure and uptake of radioactive iodine mainly by the thyroid. The thyroid gland uses iodine to produce thyroid hormones and does not differentiate between radioactive and stable iodine. Hence after a nuclear accident, if radioactive iodine is inhaled or ingested, the thyroid gland absorbs it in the same way as stable iodine. If stable iodine is administered prior to, or at the onset of the exposure to radioactive iodine, the uptake of the latter will be blocked by saturation of the thyroid gland with stable iodine, thus effectively reducing internal exposure of the thyroid. Overall, oral administration of stable iodine (together with control of food and drinking water) is considered an appropriate strategy for reducing the risk of adverse health outcomes in people exposed to an accidental release of radioactive iodine and is included in many countries preparedness plans.

The technical guidance provided in these guidelines aims to support public health preparedness for radiation emergencies in Member States, as required by the International Health Regulations (IHR). It is confined to planning and implementation of ITB before and during a radiation emergency.

These guidelines do not address the radiation protection basis set for ITB planning and implementation, but rather complements the relevant international safety standards and technical guides published by the International Atomic Energy Agency (IAEA) and co-sponsored by WHO and other international organizations. The main objectives of these guidelines are to:

• assess the evidence base and provide guidance on the implementation of ITB in case of radiological or nuclear emergencies, including advice on timing and repeated administration during continuing release of radioactive iodine;
• identify most vulnerable groups and specify the applicability and modalities of ITB implementation for those groups;
• considering the side effects and associated risks of ITB use; and
• identify research gaps in relation to ITB evidence base.

These guidelines supersede the 1999 WHO Guidelines for Iodine Prophylaxis following Nuclear Accidents