Iodine

Iodine deficiency is the most common preventable cause of mental retardation in the world. Use of iodized salt or the oral administration of a single dose of iodized oil, and weekly iodine supplements are effective. Use of iodized salt should be encouraged during pregnancy, especially through the end of the second trimester. 

Iodine deficiency in the United States and many Western nations has practically been eliminated with the iodinization of salt. However, people living in many mountainous areas of the world and a few low-lying delta regions still have low iodine intakes because of the low iodine content of the soil used in cultivating crops. Others living in lowlands may have high goitrogen consumption that reduces iodine use by the thyroid gland.

The body normally contains 20 to 30 mg of iodine, with more than 75% in the thyroid gland and the rest distributed throughout the body, particulary in the lactating mammary gland, gastric mucosa and blood. Dietary iodine is needed for the synthesis of thyroid hormones.

The use of iodized salt should still be advocated in certain areas to prevent goiter. The best way to obtain an adequate intake of iodine is to use iodized salt in food preparation. Sea salt naturally contains variable amounts of iodine, and only about 1/10 the level of iodized salt. Iodized salt is not used in the processed foods.

A small subset of vegans who eat only uncooked, lacto-bacilli-rich food were tested for thyroid function and found to be within normal limits. These vegans consumed iodine in seaweed or kelp tablets. Some of the individuals in this study had iodine intakes high enough to cause potential problems, but symptoms of toxicity were not observed.

An estimated 2 billion people worldwide living in less developed nations remain at risk for iodine deficiency. These individuals may have a moderate iodine deficiency, even when obvious goiter is not evident. In schoolchildren iodine deficiency is associated with poor cognition. Goitrogens, which exist naturally in foods, can also cause goiter by blocking uptake of iodine from the blood by thyroid cells. Foods containing goitrogens include cabbage, turnips, rapeseeds (from rape plants), peanuts, cassava, sweet potatoes, kelp and soybeans. Goitrogens are inactivated by heating or cooking. 

The level of iodine in most American diets is appropriate for good health. 

Dietary reference intake

Infants 90-130 mcg/day

Children 90-120 mcg/day

Adolescents 150 mcg/day

Adults 150 mcg/day

Pregnant 220 mcg/day

Lactating 290 mcg/day

Iodine content of selected foods

Salt, iodized, 1 tsp (400 mcg)

Bread (homemade), 1 slice (140 mcg)

Haddock, 3 oz (130 mcg)

Cheese, cottage, ½ cup (50 mcg)

Bread, 1 slice (35 mcg)

Shrimp, 3 oz (30 mcg)

Egg, 1 (20 mcg)

Cheese, cheddar, 1 oz (15 mcg)

Deficiency

Goiter (thyroid gland enlargement)

Cretinism in infants (mental deficiency, deaf mutism, dysarthria, shortened stature, hypothyroidism).

Toxicity

Hypothyroidism

Goiter formation

Hyperthyroidism