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Why is iodine added to salt?

I'm not sure about you, but at one point I was buying salt and wondered what the difference was between iodized salt and non-iodized... or why there was this option in the first place.

Apparently, Iodine is a micronutrient required for thyroid hormone production. Adequate levels of iodine (a trace element variably distributed on the earth and found mostly in the soil and water of coastal areas), are required for the synthesis of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3). These hormones are key in the metabolic processes of vertebrate life (yes, even the non-human ones). The major concerns regarding the global iodine deficiency are related to goiter, neurocognitive impairments, and in severe deficiency, hypothyroidism resulting in cretinism.

Early Chinese medical writings in approximately 3600 B.C. were the first to record the decreases in goiter size upon ingestion of seaweed and burnt sea sponge. Although iodine was yet to be discovered, these remedies remained effective and their use continued globally, and was documented in writings by Hippocrates, Galen, Roger, and Arnold of Villanova in later centuries.

The discovery of iodine was made incidentally during the early part of the 19th century. In 1811, while extracting sodium salts necessary for the manufacture of gunpowder, Bernard Courtois, a French chemist, observed an unusual purple vapor arising from seaweed ash treated with sulphuric acid. In 1813, the first paper presenting the new element, iodine (termed after the Greek word, ioeides, translated as violet-colored), by Gay-Lussac was read. Shortly followed by, J.F. Coindet, a physician in Switzerland, who published his observations that administration of iodine (as grains in distilled alcohol) was able to decrease the size of his patients’ goiters. In 1852, Adolphe Chatin, a French chemist, was the first to publish the hypothesis of population iodine deficiency associated with endemic goiter. This was confirmed by Eugen Baumann, who, in 1896, reported the discovery of iodine within the thyroid gland.

Prior to the 1920s, endemic iodine deficiency was prevalent in the Great Lakes, Appalachians, and Northwestern regions of the U.S., a geographic area known as the “goiter belt”, where 26%–70% of children had clinically apparent goiter. During the draft for World War I, a Michigan physician, Simon Levin, observed that 30.3% of 583 registrants had thyromegaly (including both toxic and nontoxic goiters), many of which were large enough to disqualify them from the military, in accordance with U.S. Selective Service regulations. Subsequent surveillance studies in the following year by Levin and R.M. Olin, Commissioner of the Michigan State Department of Public Health, demonstrated that the prevalence of goiter reached as high as 64.4% in some parts of Michigan.

The fortification of salt with iodine is an effective, inexpensive, and stable route of ensuring adequate iodine intake. Iodine supplementation, primarily through the fortification of table salt, did not begin until the early 1920s and occurred initially in Switzerland and the U.S. In the U.S., iodized salt first became available on grocery shelves in Michigan on May 1, 1924.

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