There's a bit of chatter on the interwebs inquiring about iodine and its protective effects against radiation. I think it's time to clear some things up about it, as a number of myths have cropped up around iodine as a micronutrient.
Iodine is an essential nutrient for humans, and many animals, but best understood function for it is in the making the hormone thyroxine in the thyroid gland. Thyroxine is one of those master regulatory hormones; with insufficient thyroxine, mental retardation, listlessness, and poor immune response are among the myriad systemic problems that emerge. Insufficient iodine causes a condition known as goiter, where the thyroid gland swells dramatically as the cells try to scavenge as much iodine as possible.
Seafood are the primary rich source of iodine for the human diet, but in the past era of poor food transport, inland areas suffered from high rates of goiter due to iodine deficiency (so much for locavorism). To combat this public health problem, iodized salt was introduced - this is salt where small amounts of iodide is added. In some countries, all commercially sold salt must be iodized. However, some cooks decry that iodizing confers some off flavors, and favor non-iodized salt.
Because of its link to seafood, many equate an allergy to seafood to an allergy to iodine. However, studies over the years have pretty much ruled out iodine as an allergen.
How does this relate to radioactivity? Iodine only has one stable isotope, I-127, the others are radioactive. The radioactive isotope I-125, for example, is commonly used as a tracer in research. Because iodine accumulates so specifically in the thyroid gland, even small amounts of radioactive iodine can cause dramatic damage by being concentrated in one spot. Thus, consuming increased amounts of non-radioactive iodine serves to dilute radioactive iodine out. However, this is pretty specific for radioactive isotopes of iodine - it has no protective effects from uranium, plutonium or any number of radioactive elements out there.