Let me give you a bit of background before I answer your question directly. Humans require trace amounts of iodine, a non-metallic mineral, for proper development and growth. It exists in most soils, and is taken up by plants which are in turn ingested by humans and animals. Most of the body's iodine stores are in the thyroid gland, which requires the mineral for synthesis of the hormones it secretes. An iodine deficiency leads to an enlarged thyroid gland (goiter), slowed metabolism, and weight gain, as well as other symptoms of hypothyroidism such as fatigue and intolerance of cold, as well as neurological, gastrointestinal, and skin abnormalities. Iodine deficiency in pregnant or nursing mothers can result in thyroid problems during fetal and child development, and is the most common cause of preventable brain damage in the world.
Iodine was first added to salt in the United States in 1924 in order to eliminate goiter, which was common in a sweeping area once called the "goiter belt" extending from the Great Lakes region across the northern states and encompassing parts of the Pacific Northwest. Goiter due to iodine deficiency is still common in parts of central Asia and central Africa - areas where iodine-rich foods are not available.
We can get iodine naturally by eating saltwater fish and seafood, kelp and other sea vegetables as well as vegetables grown in soils that contain iodine. Dairy products also provide iodine if the animals graze on plants growing in iodine-rich soils.
However, don't depend on processed foods for iodine - the salt they contain is not iodized, and probably because people are cooking less at home and substituting take-out food or processed foods, iodine intakes in the United States have declined from about 250 micrograms (mcg) per day to 157 mcg daily. We need 150 mcg or more (pregnant women need to make sure they're getting adequate iodine for the health of their babies). Daily intakes of up to 1,100 mcg daily for adults and children over four are considered safe.
If you are eating a healthy, balanced, varied diet, you're probably getting enough iodine and don't need to use iodized salt. I myself use both unrefined (gray) and refined (white) sea salt, which I prefer to commercial salts that often have additives I don't like, such as aluminum compounds to prevent caking. Sea salt contains trace amounts of iodine.
Andrew Weil, M.D.