There's a big difference between iodine dietary supplements and the potassium iodide tablets distributed during a radiation emergency:
- The Dietary Reference Intake (DRI) for iodine* for men and women over the age of 14 is 150 micrograms per day.1 This is the amount in many nutritional supplements, such as the Carlson product shown.
- The recommended dose of potassium iodide (KI) in the case of radiation exposure is 130 milligrams.2 About 3/4s of that is iodine, about 96mg. This is the amount the FDA deems necessary to effectively block the thyroid from taking up radioactive iodine.
"The protective effect of KI lasts approximately 24 hours. For optimal prophylaxis, KI should therefore be dosed daily, until a risk of significant exposure to radioiodines by either inhalation or ingestion no longer exists."2So, you'd have to take 640 of those supplement pills each day until the radiation threat subsides. Of course, you can take less. How effective that would be as a thyroid blocker is debatable and contingent on exposure.
Children are at particular risk:
"The Chernobyl data provide the most reliable information available to date on the relationship between internal thyroid radioactive dose and cancer risk. They suggest that the risk of thyroid cancer is inversely related to age, and that, especially in young children, it may accrue at very low levels of radioiodine exposure."2Taking too much iodine, consistently, can, curiously enough, lead to a type of goiter called "iodine goiter" and to hypothyroidism, with all the low-metabolism side effects that brings. (High levels of iodine can reduce the required oxidation of iodide in the thyroid gland and its subsequent use in the manufacture of thyroid hormone.) I say that's curious because taking too little iodine can lead to the same thing.
Taking stable iodine protects the thyroid from radioactive iodine. It does not protect other organs from other radioactive materials that are also released in an accident. I'm not sure how taking "a little bit more just to be safe" is beneficial in this case. It may in fact lead to a state of reduced health, making it harder for the body to stand up to other assaults an accident of this kind presents. Just an opinion.
* Iodine is not a mineral, although many refer to it as such. Nor is it a metal, even though it is a shiny solid at room temperature. Iodine is a halogen like chlorine and bromine. Iodine is often found in nature combined with a metal, though, such as potassium. This combined form, e.g. potassium iodide (KI), is called a salt. (The sodium chloride we sprinkle on food is also a salt, made up of 11 parts sodium and 17 parts chlorine.) The combined form, KI, may be called a mineral, when "mineral" is defined as a compound containing a metal.
1 USDA DRI Tables
2 FDA Guidance: Potassium Iodide as a Thyroid Blocking Agent (pdf)