Radioactive Iodine in Health and Disaster
Radioactive iodine, an abundant by-product of nuclear fission, is used for diagnosis and treatment of thyroid diseases. Its release to the environment in nuclear accidents is a major concern.
Iodine-131 was the first radioisotope to find widespread use in medicine. In my profession, nuclear medicine, the physics, handling, and applications of radioactive iodine are fundamental. The thyroid, a small gland located in the front of the neck, controls bodily metabolism and is the only organ that uses iodine. It makes hormones in which iodine is essential. Under-activity and over-activity of the thyroid are common and serious disease conditions. Iodine-131, with a half-life of eight days, is extremely useful in both diagnosis and treatment.
In diagnosis, a tiny amount of radioiodine is given by mouth and its uptake in the thyroid measured by special instruments. In hypothyroidism (low function), the uptake is low, in hyperthyroidism it is high. A scan of the thyroid will show its size and whether the uptake is uniform or irregular. A "hot" area may be a nodule causing hyperthyroidism. A "cold" area could be cancer and may be biopsied.
Before Iodine-131 came along, hyperthyroidism was mainly treated by surgery. Several other important structures lie in the neck close to the thyroid: the parathyroid glands that control calcium metabolism, the nerves to the vocal cords, and the blood vessels to and from the brain. In a small percentage of operations, damage to these structures leads to serious complications. Treatment of hyperthyroidism with Iodine-131 (in doses much more concentrated than those used in diagnosis) is very effective and does not damage the parathyroid glands, nerves, or blood vessels. Therefore, Iodine-131 has largely replaced surgery as the standard treatment for hyperthyroidism.
But doesn't radiation cause cancer? That possibility was a major concern about Iodine-131 treatment. Hundreds of thousands of patients over many decades have showed no increase in thyroid cancer. To the contrary, the cancer rate among treated patients is lower than that in the untreated population. In some cases of thyroid cancer, there is still measurable iodine uptake, and, if surgery fails to remove all the cancer, these patients can be treated and often cured with large doses of Iodine-131.
It was a surprise, then, after the Chernobyl meltdown in Belarus in 1986, that several thousand children developed thyroid cancer. The radioactive fallout had been concentrated in the milk of grazing cows. The children, but not the adults, who drank this milk had a higher risk of cancer. These children were relatively vulnerable because their diet was low in iodine. The oceans are the major source of iodine, and those who live far from the oceans may have iodine deprivation, which sometime results in enlarged thyroids, called goiters. The children's iodine-deprived thyroids avidly took up radioiodine. Of the 18 million children exposed, about 4,000 developed thyroid cancer and 15 have died. Fortunately, thyroid cancer can be treated with surgery, drugs, and radiation therapy very successfully. Ironically, many have been treated and cured with high doses of radioactive iodine!
It's unlikely that this epidemic of thyroid cancer in children will recur as a result of the current tsunami-caused nuclear catastrophe in the Fukushima nuclear plants in Japan. These plants are on the seacoast, and seawater is rich in iodine. Therefore, the usual diet of this population is high in iodine. The thyroid uses a very small amount of iodine each day, and if children drink iodide solutions, supplied by the Japanese government, or eat seaweed, which has a lot of iodine in it, the thyroid uptake of radioiodine is reduced to very low amounts.
Like so many aspects of modern life, radioactive iodine can be a blessing or a plague, depending on how it is used.
