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INTRODUCTION

Potassium iodide (KI) is the antidote to radioactive iodine that is released into the atmosphere after a nuclear incident. It is approved as a specific competitive inhibitor 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 would result from either under treatment or overtreatment.

HISTORY

Following the study of thyroid cancers in Pacific Islanders who were subjected to fallout from nuclear testing, scientists concluded in 1957 that KI could effectively protect the thyroid from radioactive iodine. The National Council on Radiation Protection and Measurements reported in 1977 that the sudden release of radionuclides, including radioiodine, could affect large numbers of people after a nuclear incident (Fig. A44–1). 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. Although the common terms radioiodine and radioactive iodine are used throughout this text, it is important to note that these are almost exclusively iodide salts containing a radioactive iodine atom rather than molecular iodine that is radioactive. These imprecise terms are retained here for consistency with the medical and lay literature.

FIGURE A44–1.

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. U = uranium; In = indium; Sn = tin; Sb = antimony; Te = tellurium; Xe = xenon; N = neutron.

PHARMACOLOGY

Chemistry

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. Similar to 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 KI.

Mechanism of Action

Neonates, children, and adolescents are particularly susceptible to the toxic effects of radioactive iodine. During the rapid developmental periods for these individuals, 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 human development explains ...

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