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INTRODUCTION

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Potassium iodide is the antidote to radioactive iodine that may be released into the atmosphere following a nuclear incident. It is approved as a specific blocker of thyroid uptake of radioiodine to reduce the risk of thyroid cancer in susceptible populations. The indications for its use are complex and initiation and maintenance of therapy require great attention to the details of the circumstances of the exposure to limit harm that may result from either undertreatment or overtreatment.

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HISTORY

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Following the study of thyroid cancers in Pacific Islanders who were subjected to fallout from nuclear testing, scientists concluded in 1957 that potassium iodide (KI) could effectively protect the thyroid from radioactive iodine. The National Council on Radiation Protection and Measurements (NCRP) reported in 1977 that the sudden release of radionuclides, including radioiodine, could affect large numbers of people following a nuclear incident (Fig. A43–1). The following year the US Food and Drug Administration (FDA) requested the production and storage of KI for the purpose of blocking the effects of radioiodine on the thyroid gland when needed.

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FIGURE A43–1.

The decay pathway that describes how 131I is derived from nuclear fuel (whether in a bomb or a reactor) and ultimately decays to stable xenon. s = seconds; m = months; d = days.

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PHARMACOLOGY

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Chemistry

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Iodine is a chemical element, symbol “I,” atomic number 53. Its name derives from the Greek iodes meaning violet, owing to the violet color of elemental iodine vapor. Like other halogens, iodine occurs mainly as a diatomic molecule I2. Although it is considered a relatively rare element, it is the heaviest essential element used widely in biologic functions. Of 37 iodine isotopes, only 127I is stable. The term iodide refers to the ion I, which forms inorganic compounds with iodine that is in the oxidation state –1, such as potassium iodide.

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Mechanism of Action

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Neonates, children, and adolescents are particularly susceptible to the toxic effects of radioactive iodine. During the growth periods for these groups there is increasing growth of the thyroid gland, as well as an increase in thyroglobulin and iodothyronine stores. Thyroid tissue also accumulates a larger percentage of exogenously ingested iodide and more efficiently reuses iodine from degraded hormone. This increased activity during these growth periods explains the greater risk for developing thyroid cancer in children compared to adults following exposure to radioactive iodine. During pregnancy, the maternal thyroid gland is stimulated and takes up more iodine compared to other adults, thus increasing susceptibility in pregnant women to the toxic effects of radioiodine.

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Exposure to radioactive iodine may occur via inhalation or via ingestion of contaminated food, as occurred following the Chernobyl incident. Radioiodine ...

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