Iodine is a trace element that is naturally present in some foods, added to others, and available as a dietary supplement. Iodine is an essential component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Thyroid hormones regulate many important biochemical reactions, including protein synthesis and enzymatic activity, and are critical determinants of metabolic activity [1,2]. They are also required for proper skeletal and central nervous system development in fetuses and infants .
Thyroid function is primarily regulated by thyroid-stimulating hormone (TSH), also known as thyrotropin. It is secreted by the pituitary gland to control thyroid hormone production and secretion, thereby protecting the body from hypothyroidism and hyperthyroidism . TSH secretion increases thyroidal uptake of iodine and stimulates the synthesis and release of T3 and T4. In the absence of sufficient iodine, TSH levels remain elevated, leading to goiter, an enlargement of the thyroid gland that reflects the body’s attempt to trap more iodine from the circulation and produce thyroid hormones.
Iodine may have other physiological functions in the body as well. For example, it appears to play a role in immune response and might have a beneficial effect on mammary dysplasia and fibrocystic breast disease .
The earth’s soils contain varying amounts of iodine, which in turn affects the iodine content of crops. In some regions of the world, iodine-deficient soils are common, increasing the risk of iodine deficiency among people who consume foods primarily from those areas. Salt iodization programs, which many countries have implemented, have dramatically reduced the prevalence of iodine deficiency worldwide [2,3].
Iodine in food and iodized salt is present in several chemical forms including sodium and potassium salts, inorganic iodine (I2), iodate, and iodide, the reduced form of iodine . Iodine rarely occurs as the element, but rather as a salt; for this reason, it is referred to as iodide and not iodine. Iodide is quickly and almost completely absorbed in the stomach and duodenum. Iodate is reduced in the gastrointestinal tract and absorbed as iodide [2,5]. When iodide enters the circulation, the thyroid gland concentrates it in appropriate amounts for thyroid hormone synthesis and most of the remaining amount is excreted in the urine . The iodine-replete healthy adult has about 15–20 mg of iodine, 70%–80% of which is contained in the thyroid .
Median urinary iodine concentrations of 100–199 mcg/L in children and adults, 150–249 mcg/L in pregnant women and >100 mcg/L in lactating women indicate iodine intakes are adequate . Values lower than 100 mcg/L in children and non-pregnant adults indicate insufficient iodine intake, although iodine deficiency is not classified as severe until urinary iodine levels are lower than 20 mcg/L.
Sources of Iodine Food
Seaweed (such as kelp, nori, kombu, and wakame) is one of the best food sources of iodine, but it is highly variable in its content (Table 2) . Other good sources include seafood, dairy products (partly due to the use of iodine feed supplements and iodophor sanitizing agents in the dairy industry ), grain products, and eggs. Dairy products, especially milk, and grain products are the major contributors of iodine to the American diet . Iodine is also present in human breast milk [2,5] and infant formulas.
Fruits and vegetables contain iodine, but the amount varies depending on the iodine content of the soil, fertilizer use and irrigation practices . Iodine concentrations in plant foods can range from as little as 10 mcg/kg to 1 mg/kg dry weight . This variability in turn affects the iodine content of meat and animal products because it affects the iodine content of foods that the animals consume . The iodine content of different seaweed species also varies greatly . For these reasons, the values in Table 2 are approximate.
|Table 2: Selected Food Sources of Iodine [10,11,12]|
|Seaweed, whole or sheet, 1 g||16 to 2,984||11% to 1,989%|
|Cod, baked, 3 ounces||99||66%|
|Yogurt, plain, low-fat, 1 cup||75||50%|
|Iodized salt, 1.5 g (approx. 1/4 teaspoon)||71||47%|
|Milk, reduced fat, 1 cup||56||37%|
|Fish sticks, 3 ounces||54||36%|
|Bread, white, enriched, 2 slices||45||30%|
|Fruit cocktail in heavy syrup, canned, 1/2 cup||42||28%|
|Shrimp, 3 ounces||35||23%|
|Ice cream, chocolate, 1/2 cup||30||20%|
|Macaroni, enriched, boiled, 1 cup||27||18%|
|Egg, 1 large||24||16%|
|Tuna, canned in oil, drained, 3 ounces||17||11%|
|Corn, cream style, canned, 1/2 cup||14||9%|
|Prunes, dried, 5 prunes||13||9%|
|Cheese, cheddar, 1 ounce||12||8%|
|Raisin bran cereal, 1 cup||11||7%|
|Lima beans, mature, boiled, 1/2 cup||8||5%|
|Apple juice, 1 cup||7||5%|
|Green peas, frozen, boiled, 1/2 cup||3||2%|
|Banana, 1 medium||3||2%|
*DV = Daily Value. DVs were developed by the U.S. Food and Drug Administration (FDA) to help consumers compare the nutrient contents of products within the context of a total diet. The DV for iodine is 150 mcg for adults and children aged 4 and older. However, the FDA does not require food labels to list iodine content unless a food has been fortified with this nutrient. Foods providing 20% or more of the DV are considered to be high sources of a nutrient.
More than 70 countries, including the United States and Canada, have salt iodization programs. As a result, approximately 70% of households worldwide use iodized salt, ranging from almost 90% of households in North and South America to less than 50% in Europe and the Eastern Mediterranean regions .
In the United States, salt manufacturers have been adding iodine to table salt since the 1920s, although it is still a voluntary program . The U.S. Food and Drug Administration (FDA) has approved potassium iodide and cuprous iodide for salt iodization  while the WHO recommends the use of potassium iodate due to its greater stability, particularly in tropical climates . According to its label, iodized salt in the United States contains 45 mcg iodine/g salt (between 1/8 and 1/4 teaspoon); measured salt samples have an average level of 47.5 mcg iodine/g salt . However, the majority of salt intake in the United States comes from processed foods, and food manufacturers almost always use non-iodized salt in processed foods. If they do use iodized salt, they must list the salt as iodized in the ingredient list on the food label .