When a nuclear reactor experiences a catastrophic failure, an uncontrolled release of nuclear fission products such as radioactive iodine (eg, 131I) may occur (Fig. A42–1). Monitoring of the exposed population of Belarus, Russia, following the Chernobyl meltdown of the reactor in 1986 revealed almost a 100-fold increase in the incidence of thyroid cancer among children, echoing similar long-term effects of the nuclear destruction of Hiroshima and Nagasaki in 1945.4 Iodine is a solid, bimolecular halogen that sublimates under standard conditions. Potassium iodide (KI), the most commonly available iodide salt, has been "generally recognized as safe" by the Food and Drug Administration (FDA) for nearly 40 years. Potassium iodide is recommended to prevent the uptake of radioactive iodine into the thyroid in order to reduce the future risk of thyroid cancer.
The decay pathway that describes how 131I derives from nuclear fuel, whether in a bomb or a reactor, ultimately decaying to stable xenon.
Iodine, or its ionic form iodide (I–), is an essential nutrient present in humans in minute amounts of 15 to 25 mg. Iodine is required for the synthesis of the thyroid hormones L-triiodothyronine (T3) and L-thyroxine (T4), which in turn regulate metabolic processes and determine early growth of most organs, especially the brain. Radioactive tracer iodine is distributed in the neck only 3 minutes after ingestion in a fasting subject. Iodide is actively transported with sodium into thyroid follicular cells where it is concentrated 20- to 40-fold compared with its serum concentration. It is then transported into the follicular lumen where it iodinates thyroglobulin to form T3 and T4 (see Chap. 49). Thyroid hormones are metabolized in hepatic and other peripheral, extrathyroid tissues by sequential deiodination. Iodide is then excreted in sweat, feces, and urine, and the presence of iodine in the urine is considered a reliable indicator of adequate iodine intake.
Iodine deficiency is a worldwide health problem with large geographic areas deficient in iodine in the foods; this occurs predominantly in mountainous areas and regions far from the world's oceans. In 2003 the World Health Organization (WHO) estimated there were 1.9 billion people with insufficient iodine intake despite universal salt iodization.22 Iodine deficiency disorders include spontaneous abortions, congenital anomalies, endemic cretinism, goiter, subclinical or overt hypothyroidism, mental retardation, retarded physical development, decreased fertility, and increased susceptibility of the thyroid gland to radiation.
During the critical, immediate postnatal period and the prepubertal and pubertal growth periods, there is a progressive growth of the thyroid gland as well as an increase in thyroglobulin and iodothyronine stores. Insufficient iodine supply in the diet results in increased iodine trapping by the thyroid gland. That is, the thyroid gland accumulates a larger percentage of exogenous ingested iodide and more efficiently reuses iodine that ...