Saturday Evening Rant, Iodine Edition

Mar 12 2011 Published by under Medicine

Hey, folks.  Thanks for sticking around despite my relative blogopenia.  Real life pulls at me relentlessly, and needs must when the Devil drives.   But for a few moments, at least, I have some time to myself.  PalKid's having a sleepover and is playing some sort of "tsunami magic pony land" game with her friend, and MrsPal is out having coffee with a friend.  I figure I have about a half an hour before some sort of fight erupts downstairs, likely having to due with said magic pony land and the oncoming tsunami.

Speaking of tsunamis, I'm pretty much glued to the set, watching the news from the nuclear plants in Japan.  There's a lot of talk about "iodine" in this context.  Nuclear fission has many toxic products, one of which is iodine-135.  It isn't the most dangerous fission product, but it has specific dangers that can be prevented (that is, prevented in ways other than preventing a nuclear disaster in the first place).

The thyroid gland is responsible for the production of thyroid hormone, a chemical that modulates many critical physiologic functions.  A key component of this hormone is iodine, an element naturally present in the diet to a greater or lesser extent.  Like many hormones, thyroid hormone production is regulated by a feedback loop.  When the brain senses a drop in thyroid levels, it sends a chemical signal to the gland to ramp up production.  When thyroid levels rise, the brain sends a signal to turn things down.

Iodine is not evenly distributed in the world, and in regions with less iodine in the environment, the thyroid cannot produce enough hormone.  The brain senses this, and sends the signal to the gland to ramp up production.  The gland responds by growing.  And growing.


The results can be quite dramatic, and so can treatment.  When iodine is consumed, it collects in the thyroid, so taking in iodine can both prevent and treat goiters.  This avidity for iodine is also exploited in the diagnosis and treatment of other thyroid diseases.  Radioactive isotopes of iodine can be used, for instance, to image the thyroid gland.  A patient can be given a small amount of a radioisotope of iodine and a gamma camera can look for thyroid tissue that "lights up".  It can also be used to treat thyroid cancers.

Radioactive iodine is toxic to cells, and since iodine-135 is taken up mainly in the thyroid, and has a relatively short half-life, a patient can be given a dose high enough to kill off a diseased or cancerous thyroid gland.  If a thyroid cancer has spread, the iodine will "seek out" metastases and kill them.

Radioactive iodine is used very, very carefully.  When I was a medical resident, we would round on I-135 patients from the doorway, and the nutrition service would leave food at the door.  I-135 from, say, a nuclear accident can be taken up by the thyroid, causing genetic damage to thyroid cells which lead to cancer.  This continues to happen in the areas affected by the Chernobyl disaster.

(Speaking of disaster, someone just yelled, "Pillow fight!"  Ten bucks this ends poorly.)

Unlike many other types of radiation poisoning, I-135 poisoning is preventable.  People in danger of exposure, especially kids, can be given iodine tablets or liquid.  The non-radioactive iodine is taken up quickly by the thyroid, blocking the entrance of radioactive iodine, preventing future cancers.  The thyroid can only take so much iodine, and the rest is excreted, radioactive or otherwise.

I imagine that many Japanese children will get unneeded doses of iodine in the next several days.  But needs must when the Devil drives.

6 responses so far

  • GoatRider says:

    Thanks, that's good to know! Let us know how the pillow fight turned out.

  • Interesting. I was wondering if it is the radioactivity itself of I135 that causes cell death. But a little googling says that I135 decays to Xenon 135, which is a poison. So I wonder if it is mostly the radioactivity, mostly the xenon, or a combination of both that drive cytotoxicity.

  • daedalus2u says:

    Candid, it is the radioactivity. Xenon is an inert gas and is non-toxic. Xenon is an anesthetic, like N2O, but that takes gigantic amounts to have an effect ~20's of percent in air.

    I135 is a beta emitter (high energy electron) with an energy of 2.8 MeV. In tissue, there is about one ion pair produced per 50 eV of energy deposited. That is about 50,000 positive radicals and 50,000 negative radicals per I135 decay.

  • JJM says:

    @daedalus2u Xenon-135 is also a beta emitter. Depending on its half-life it may contribute to the destruction. However, since it is chemically inert it may migrate and spread the destruction; or it may mostly leave the body before decaying.

  • William Wallace says:

    You started the iodide panic. I should have bought some to sell on ebay.

  • daedalus2u says:

    JJM, migrating and "spreading" the damage would be a good thing. The reason I135 is so bad is because it concentrates in the thyroid. The same total dose spread out over the whole body would be less harmful.

    In looking at it a little more, I think the isotope used for treating thyroid cancer is I131 and not I135. I135 would work and might be a bit better because of the shorter half life (6.7 hours vs 8 days). The beta from I135 is higher energy which is not as good, and it makes Xe135 which is also radioactive which is bad. I think the main reason I131 is used is that it is easier to make.

    I131 can be made by irradiating tellurium 130 (which is stable) in a reactor. I135 is pretty much only made as a fission product which makes it quite cumbersome to make. You need to irradiate U235 (but you can irradiate natural uranium) and then separate the fission products. I135 has such a short half life that you have to do the chemistry and the delivery to the hospital pretty quickly. A week is too late. With I131 you could wait a week and the chemistry is a lot easier.

    I131 does give off a gamma which is bad (because it penetrates tissue farther). The reason that rounds are done from the door is that they use a lot of I131, they want to saturate all tissues that will take it up. That high a dose means that some I131 will be excreted, and sweating is a normal excretion pathway. Anyone with a non-saturated thyroid would take that iodine up and could get thyroid damage. Anyone with a saturated thyroid could transport some of that iodine to someone else.