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The Los Angeles Times reports sales of Geiger counters and potassium iodide in the United States are skyrocketing:

Sales of Geiger counters and potassium iodide supplements that can block some radiation have surged nationwide since Friday, fueled by concerns among some Americans that radiation released from Japanese nuclear plants could spread to the United States.

Yet, it seems potassium iodide doesn't really do much to help:

Despite those statements, potassium iodide supplements — which can protect the thyroid gland if taken before or shortly after a person is exposed to radiation but do not protect other body parts or prevent damage from other radioactive substances — are either sold out or selling quickly at several Chicago-area stores.

Leaving aside the question as to whether any noticeable radiation will affect the United States (most experts say it won't), is the protection of the thyroid gland offered by potassium iodide really effective? Is it useful in light of the fact that it doesn't seem to protect the remainder of the human body?

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There are several aspects to this problem:

  1. As you wrote, (potassium) iodide tablets only protect the thyroid gland. This works as follows: The thyroid gland uses iodide to synthesize its hormones. Iodide taken in with your normal diet will enrich in your thyroid gland where it is needed. Other tissues do not accumulate iodide in this way. This is the reason why one of the few "magic bullets" in medicine actually works: Using radioactive iodide to treat thyroid cancer. You inject a radioactive substance and it "magically" finds its way to the only place where it is needed, resulting in minimal side effects in other tissues.

  2. Now, since radioactive iodide isotopes are typically released in a nuclear accident, the chance of introducing radiation to your (healthy) thyroid gland can be a relevant problem if the dose is high enough. By giving iodide tablets, you effectively saturate your thyroid gland, blocking the uptake of any radioactive iodide which is then excreted (via urine, mostly).

  3. There is a risk associated with this, though: If you have a condition called hyperthyroidism, i. e. if your thyroid gland tends to produce too much of its hormones, then administration of large doses of iodide may cause an exacerbation of your hyperthyroidism or even a potentially fatal thyrotoxic crisis. There are some other illnesses where iodide may not be given. (Also, there may be some other side effects of large doses of iodide). Since there is a significant number of people who have one of these conditions without knowing it or whose treatment is inadequate, the chance of causing side effects of this kind is rather high, probably a lot higher than the risk of not taking the iodide tablets (in regions where the expected amount of radioactivity is as low as in the U.S. - it's completely different in Japan at the moment, where I'm pretty sure that those people working in and around the reactors have been given iodide tablets).

  4. Of course, there are other sources of radiation after a reactor meltdown (e. g. cesium) besides radioactive iodide that iodide tablets can't protect against. But for this specific threat, they are effective.

So, all in all, don't panic and ask your doctor if you should be taking any medication where the chance of doing harm might outweigh the chance of doing good.

Sources: Any internal medicine or nuclear medicine textbook. A good overview from a credible source (the CDC) can be found here.

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    So in a short, simplified summary: Nuclear accidents release radioactive iodide. In humans, thyroid gland collects iodide. Taking extra (non-radioactive) iodide beforehand saturates the thyroid gland so it won't collect the any more iodide. The radioactive iodide then just passes through the body, coming out with urine. Damage from radiation is dependent on the length of exposure, so since the radioactive stuff only visits and doesn't stay, the amount of damage caused is significantly less. – Ilari Kajaste Mar 16 '11 at 9:42
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    If I were in America I would keep some of these against the chance that quakes on the east side of the Pacific damaged US nuclear reactors...not take them under the off chance that airborne radiation from Japan hits high enough levels in the US to be a worry. – Rory Alsop Mar 16 '11 at 12:37
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    As a "fun" sidenote, some places here in Finland actually ran out of iodide tablets as people rushed to pharmacies. You can go check the map to see how irrelevant any radiation in Japan would be for us. Ah, the wonders of the legacy of Chernobyl. :) – Ilari Kajaste Mar 16 '11 at 18:22
  • Please add some more specific references! You should be citing "any internal medicine or nuclear medicine textbook". – jozzas Jul 1 '12 at 22:14

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