Preventive Medicine Column

Dr. David L. Katz

The threat of a nuclear calamity in Japan appears to be receding, although thus far the news reports are about as unstable as the radioactive isotopes in question.  A bit of good news is quickly followed by a bit of bad, so uncertainly will likely prevail for a while longer.

During this period of uncertainty, attention to the crisis, and anxiety about it- even among those as far away as we are in CT- are running quite high.  The extensive media coverage of the evolving story, 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.

While I readily concede I have colleagues- some of whom may read this column- with more expertise in radiation medicine, thyroid function, or both than I, general medical knowledge should be ample to clarify the basics.

1) Thyroid hormones are manufactured by the thyroid gland using iodine.  A process referred to as the “organification” of iodine is generally the rate-limiting step in thyroid hormone production.  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.  Iodine deficiency in the U.S. has been reduced to very low levels by adding iodine to salt, although there is of late some resurgent debate about whether the average intake level is truly optimal.

2) The thyroid gland can develop several kinds of cancer, some quite responsive to treatment, others less so.  In general, tissues with very high metabolic activity or rapid cell turnover are more cancer prone, and this is a factor.  But more relevant is the relationship between thyroid function and uptake of radioactive compounds.  Thyroid cells, like most cells in the body, are quite 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 it takes up any available radioiodine just as assiduously as its stable counterpart.  This tendency is used under controlled medical conditions to treat disorders of the thyroid.  Under the uncontrolled conditions of a nuclear plant failure, however, I-131 exposure will potentially cause low-level radiation injury to thyroid cells- enough to damage, but not kill them.  Dead cells may be missed, but don’t cause further harm.  Injured cells are prone to develop into cancers over time.

Iodine supplementation- specifically the use of potassium iodide- is potentially protective of the thyroid gland in two ways.  First, if the thyroid gland is saturated with stable iodine, it has less capacity to take up any radioiodine; in simplistic terms, there is no more room at the inn.

Second, while iodine is essential for thyroid function and deficiency can cause hypothyroidism, high-dose iodine intake can actually shut things down for a while, and cause transient hypothyroidism.  This seemingly paradoxical response is called the Wolff-Chaikoff effect, and seems to involve several mechanisms, including suppression of thyroid hormone manufacture.  The effect is transient, but timed right, it might further reduce uptake of radioiodine.

As things stand, there is no reason related to events in Japan for anyone in the U.S.- West Coast, let alone East- to be resorting to an iodine defense; no meaningful risk currently exists here.

Should such a risk develop, there are two precautionary notes to sound about reaching for the iodine bottle. The first pertains to: ‘well, it couldn’t hurt.’ Actually, it could — if iodine is supplemented after exposure to radiation, there is some possibility of it slowing thyroid function, and causing the radioactive iodine to remain in the gland longer. So timing and dosing are important.  Second, iodine supplementation protects the thyroid gland only; it does not provide any kind of total body defense against the effects of radiation.  So beware any false sense of security.

The iodine defense works against the threat of I-131 to the thyroid, and the Japanese in the affected area should be making use of this strategy as warranted. We in the U.S., at this point, 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;