Potassium iodide isn't much of a structure:
It's a source of iodide. Thyroid hormones contain iodine; the source of this is dietary iodide.
The human dietary requirement for iodine is vanishingly small; on the order of 100 micrograms per day. Trace iodine is present in seawater and many soils, so most people get some from seafood, fruits, or vegetables. In the absence of iodine, thyroid hormone levels are low. This induces a cascade of hormone releases; the last of which is the release of thyroid stimulating hormone. This enhances production of thyroid hormones. In excess, this induces inflammation of the thyroid gland, known as goiter.
Today, lots of salt is "iodized" by statute. The mixture sold as "table salt" usually contains the following:
- Sodium chloride - this is salt, and 98% or so of what you're getting.
- Calcium silicate and/or silicon dioxide - the first is pretty much sand, the second is just that. As odd as this seems, you're eating sand with your salt. A small amount keeps salt crystals from caking or fusing together in the presence of atmospheric water.
- Potassium iodide - it's just in there for the ol' thyroid. A lot of people think it adds an off-taste to salt, hence the popularity of "premium" sea salts, which are comparatively iodine-poor.
- Dextrose/glucose (same thing) - a certain fraction of KI in salt will be oxidized by air to iodine, or I2. This will really add an off-taste to your salt. The oxidation of glucose to gluconic acid is facile. This means that any iodine produced will quickly be reduced back to iodide (oxidizing the glucose in turn).
Potassium iodide also gets some notice for its usefulness as a prophylaxis after nuclear events. 131I is a short-lived (8 day half-life) isotope of iodine. It can be used in medicine to selectively attack the thyroid (as in thyroid cancer or hyperthyroid). Its short half-life affords it a lavish radioactivity that will set off radiation detectors for some time after treatment. Some fallout contains this radioiodine, which can target the thyroid pretty selectively. If you have some plain old iodine (as in KI), you can swamp your body with it (without much ill effect). If you've got loads of regular iodine around and only a little 131I, you're much better off exposurewise. Note that this is ALL KI administration will do for you - it provides no prophylaxis against any other radiation exposure.
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We used to give Lugol Solution (basically KI) to our young horses, once a week during their first year of being ridden and trained, approximately the age of 2-3.
The neat thing about Lugol's solution is that the KI actually makes the I2 water soluble. Normally to make a solution of iodine for disinfectant, you'd have to dissolve it in organic solvent (just about any will do, we of course use alcohol in practice). The addition of I- to I2 results in the I3- anion.
So, the question is - were we doing our horses any good, or harm, or just wasting money? About 10cc per week for a year, usually 3-4 horses at a time.
Does the K in KI reduce Cs uptake from fallout? Of do you need a lot more K than you get from KI tablets? Or can biological absorption of K (and Cs) not be reduced by high dosages?
MotD "If you have some plain old iodine (as in KI), you can swamp your body with it (without much ill effect)."
My Merk Index does not list the LD50 for KI. K+ and I- are definitely needed for life; but one can't take KI indiscriminately. Excess K+ will stop your heart.
KI has an LD50 on the order of grams. KI "radiation pills" have doses on the order of 100mg.
No, you shouldn't take KI indiscriminately. Yes, too much iodide and too much potassium are both bad. However, oral potassium isn't that likely to cause problems in most people, thanks to the presence of a functional set of kidneys.
In case it wasn't clear: "KI pills" are the kind of thing one might take orally, in measured doses, for a short while, after a nuclear event, when substantial amounts of radioiodine are present (the amount of iodine-131 decreases by half every ~8 days after generation).