Iodine
The trace element iodine is the most effective blocker of radiodide uptake (from radioactive fallout) by the thyroid. Iodide, the mineral salt form of iodine, also protects all major glands and organs form the harmful effects of electromagnetic radiation bombardment.

Seaweeds and algaes are rich in iodine. The richest source of iodine is a seaweed called kelp. You can buy granulated kelp at the grocery store and use it to season your food. Iodized salt and iodized sea salt can also be purchased at the grocery.

Some fish (flounder, haddock, halibut, swordfish) are rich in iodine. Lobster, clams, shrimp and oysters also contain iodine.

Warning: Cruciferous foods such as cabbage, kale, kohlrabi, Brussels sprouts and cauliflower are good to eat but, along with soybeans, they tend to deplete iodine from the body if consumed to often. Asian cultures that consume large quantities of soy products generally also consume seaweeds which balance the losses of iodine.