Iodine is a good example of a trace mineral whose deficiency creates a disease that is easily corrected by resupplying it in the diet. Goiter, an enlargement of the thyroid gland, develops when this important metabolic gland does not have enough iodine to manufacture hormones. As it increases its cell size to try to trap more iodine, the whole gland increases in size, creating a swelling in the neck. Without supplemental iodine, a hypothyroid condition results, likely leading to fatigue and sluggishness, weight gain, and coldness of the body; at this stage, the condition may be harder to treat with iodine alone and thyroid hormone supplementation may be needed.
Goiter was first noted in the Great Lakes region; the "goiter belt" included that area and the midwestern and Plains states. In the 1930s, approximately 40 percent of the people in Michigan had goiter, due mainly to iodine-deficient soil; glacier melting had washed away the iodine. Areas by oceans or in the vicinity of ocean breezes usually contain enough iodine to prevent goiters. In 1924, iodine was added to table salt, a substance that was already in wide use (our salt problem has been going on for a long time). Iodized salt was first introduced in Michigan; by 1940, it was in general use. Even today, iodine deficiency is still a problem, and many people in the United States have goiter. Cretinism, another condition caused by iodine deficiency, is characterized by mental retardation and other problems. It may be present in iodine-deficient babies or children born to women who are lacking iodine. It is a serious and nonreversible problem that should be avoided by proper iodine intake.
Iodine itself is a poisonous gas, as are the related halogens chlorine, fluorine, and bromine. However, as with chlorine, the salts or negatively charged ions of iodine (iodides) are soluble in water, and iodine is essential to life in trace amounts. Plants do not need iodine, but humans require it for the production of thyroid hormones that regulate the metabolic energy of the body and set the basal metabolic rate (BMR).
The body contains about 25 mg. of iodine. A small percentage of this is in the muscles,
20 percent is in the thyroid, and the rest is in the skin and bones. Only 1 percent is present in the blood. The concentration of iodine in the thyroid gland is very high, more than 1,000 times that in the muscles. Approximately one-fourth of thyroid iodine is in the two main thyroid hormones, T4 (thyroxine) and T3 (triiodothyronine). Thyroxine itself is nearly two-thirds iodine. The remainder is in the precursor molecules of these two important hormones.
Iodine is well absorbed from the stomach into the blood. About 30 percent goes to the thyroid gland, depending on the need. Iodine is eliminated rapidly. Most of the remaining 70 percent is filtered by the kidneys into the urine. Our bodies do not conserve iodine as they do iron, and we must obtain it regularly from the diet. There is recent concern that perhaps iodine is being overconsumed, especially in iodized salt. The incidence of goiter has been rising again, however, so there may be factors other than iodine involved in this problem.
Sources: The life from ocean waters provides the best source of iodine. Fish, shellfish, and sea vegetables (seaweed) are dependably rich sources. Cod, sea bass, haddock, and perch are a few examples of iodine-rich sea animals consumed by humans; kelp is the most common, high-iodine sea vegetable. Kelp in particular is rich in other minerals and low in sodium and thus is a good seasoning substitute for salt.