Washington’s Blog 
March 3, 2011
Because of the fear that the radiation from the Japanese nuclear meltdowns will hit the Western United states (see this  and this ), potassium iodide has sold out in most health food and supplement stores in many California, Oregon and Washington locations.
People know that it’s good to take potassium iodide to protect against radiation, to help protect against thyroid cancer (potassium iodide does not protect any other organs).
But taking potassium iodide when there is no radiation can actually damage the thyroid gland … at least in some individuals. A doctor told me that potassium iodide is given to people with hyperthyroid disease in order to partially kill the thyroid – i.e. to lower thyroid function.
Indeed, the New York Times notes :
Experts in Japan and the United States say the country is now facing a cascade of accumulating problems that suggest that radioactive releases of steam from the crippled plants could go on for weeks or even months.
You can’t take potassium iodide for months on end.
Ideally, buy potassium iodide now, and monitor radiation levels by looking at real-time monitoring networks such as this  and this . Don’t take iodide unless and until elevated radiation levels hit your area.
- 130 milligrams for adults
- 65 milligrams for children
Each 24 hours that one is exposed to radiation.
Sees this  for further details.