Iodine is an element found naturally in the Earth, but is not produced by the human body. For this reason, iodine intake from foods becomes necessary to replenish the body's supply and the trace amount of the element that was in the body at birth. There are a select few foods which contain extremely high levels of iodine and others which contain lesser, but still beneficial, levels.
There is much iodine in the Earth's oceans and foods produced in or near these bodies of water will have higher levels of iodine. They include seaweed, especially kelp, and salt water fish such as haddock, cod, sea bass and perch.
Foods that contain lower levels of iodine include breads, cheese, yoghurt, soy sauce, soy and cow's milk, iodine-based vitamins, eggs, shellfish and ice cream.
Cow's milk does not inherently contain iodine, but the nipples of the animals are cleaned with iodine solutions during milking, which bleeds into the milk as a result of the process. This accounts for the amount of iodine found in milk products.
Iodine's main purpose in the body is to produce thyroxine, a hormone used by the thyroid. Most iodine is stored in the thyroid and deficiency of the element causes these thyroid-related disorders: goitre, hypothyroidism and mental retardation in babies of mothers who suffered from iodine deficiency while pregnant.
Iodised table salt is another food source containing high levels of iodine. In 1922, Switzerland began adding the element to the country's salt supply to combat excessive iodine deficiency in the population. The same strategy was implemented in the United States during the same decade and the process of adding iodine to table salt has continued since then.
Kelp is potentially the best source of iodine, but over-consumption of the seaweed can lead to poisoning, which can cause thyroid-related disorders.