The extensive media coverage of the nuclear crisis in Japan, as well as the even more extensive reactions to it reverberating through cyberspace, suggest to me that there is fairly widespread confusion about both the risks of radiation, and the specific defense afforded by iodine supplementation. Drawing on general medical knowledge, rather than any dedicated expertise in radiation medicine, I write to clarify.
1) Thyroid hormones are manufactured by the thyroid gland using iodine. I believe it is commonly known that long-standing iodine deficiency leads to goiter, and hypothyroidism. In fact, goiter is still common in many parts of the developing world, and the World Health Organization supports an iodine distribution program to help combat that problem.
2) The thyroid gland can develop several kinds of cancer, and is more vulnerable to cancer to many other body sites because of its generally high rate of metabolic activity. Thyroid cells are especially vulnerable to the effects of radiation.
3) Nuclear fission reactions using uranium or plutonium release a variety of radioactive 'breakdown' products, and among these is radioiodine. There are several varieties of radioactive iodine, but I-131 tends to predominate.
4) The thyroid gland cannot distinguish between stable and radioactive iodine, so take up any available radioiodine just as assiduously as its stable counterpart. This is used under controlled medical conditions to treat disorders of the thyroid -- but under the uncontrolled conditions of a nuclear plant failure, I-131 exposure will potentially cause low-level radiation injury to thyroid cells -- enough to damage, but not kill them. Such injured cells are prone to develop cancers over time.
Iodine supplementation -- specifically the use of potassium iodide -- is potentially protective in two ways. First, if the the thyroid gland is saturated with stable iodine, it is less prone to take up any radioiodine because, in crude terms, there is no room at the inn.
Second, while iodine is essential for thyroid function and deficiency can cause hypothyroidism, high-dose iodine intake can actually cause hypothyroidism as well. This seemingly paradoxical response is called the Wolff-Chaikoff effect, and involves several mechanisms -- including suppression of the manufacture of thyroid hormones from the available iodine. It lasts only a short time, but for that time it will block the uptake of any more iodine into the gland.
I hasten to add that there is no reason for anyone in the U.S. to be taking iodine supplements to defend against radiation from the Japanese leak at this point; no meaningful risk currently exists here. And, there are two precautionary notes to sound. The first pertains to the 'well, it couldn't hurt' approach. Actually, it could -- if iodine is supplemented after exposure to radioiodine, there is some possibility of it slowing thyroid function, and causing the radioactive iodine to remain in the gland longer. Second, iodine supplementation protects the thyroid gland only; it does not provide any kind of total body defense against the effects of radiation.
The iodine defense works against the threat of I-131, and the Japanese in the affected area should be using it. We in the U.S. should not. But we should understand it, so it's there for us if ever we need it. I certainly hope we don't.
Dr. David L. Katz