Iodine is an essential trace mineral required for proper thyroid function. iodine might also play a role in immune function. Iodine is present in some foods naturally and is added to some foods to ensure adequate intake for people who live in places where iodine levels in the environment are low. Levels of iodine vary considerably among foods.
Seaweed earns the gold standard among food sources for iodine content. A study published in the October 2004 issue of the journal "Thyroid" evaluated iodine levels in 12 species of commonly consumed seaweed and found wide variability. At the low end was nori, a popular seaweed that is used to make sushi in Japanese cooking, at 16 mcg per gram. The highest levels in the study were obtained from a commercial salt substitute made from processed kelp granules that contained 8,100 mcg per gram. Harvesting methods influenced iodine concentrations in samples of kelp from Namibia, which showed that sun-bleached kelp contained about 500 mcg per gram, while fresh samples contained more than 6,500 mcg per gram. The researcher also noted that iodine is water soluble and that cooking and storage in humid conditions decrease iodine content in foods.
Some meats and fish are good sources of iodine. Iodine content in fish varies, depending on the concentrations in the water they inhabit. Saltwater fish tend to have about six times as much iodine, on average, as freshwater fish, providing as much as 6 mcg per gram, while freshwater fish contain an average of 0.375 mcg per gram. Poultry contains 66 and beef contains 59. Processed meats, such as sausage and ham, contain 335, again due to their high salt content.
Eggs and Dairy
Eggs are an excellent source of iodine, with 1.6 mcg per gram. Milk and yogurt contain about 0.7 mcg per gram, and cheese contains about 0.47 mcg per gram, values that reflect a form of enrichment through iodine added to animal feed. A seasonal variation exists in iodine levels in milk, with lower levels in the summer months, when more animals have access to grass pastures and less supplemental feeding, and higher levels in the winter months when commercial feeds are used. Cheese is low in iodine because, being water-soluble, iodine leaves with the water that is drained off during production.
Fruits, Vegetables and Nuts
Vegetables tend to have low iodine levels. Frozen and canned vegetables provide modest amounts of iodine, at 1.2 mcg per gram, largely due to iodized salt used in processing. Leafy salad vegetables offer about 0.24 mcg per gram, and nightshade vegetables, such as tomatoes and potatoes, provide about half that amount. Mushrooms and nuts both offer about 0.2 mcg per gram.