Iodized salt is salt which has been fortified with the essential trace mineral iodine. A package of it will always be clearly marked, indicating that it contains dietary iodine. Salt which does not contain iodine may also be carefully labeled, especially when iodized salt is very common, to ensure that the consumer knows that he or she will not receive dietary iodine from that particular package of salt.
Iodine appears to have an important impact on the health of the thyroid gland. An unhealthy thyroid can lead to a number of conditions, including goiter, a swelling of the thyroid gland which manifests as a lump in the neck. A condition called cretinism, characterized by developmental and mental delays, is also caused by iodine deficiency. The role of iodine intake in both of these conditions was recognized in the 20th century, and since they are fully preventable through diet, public health advocates hope to entirely eliminate them at some point.
Many things are natural sources of iodine. Saltwater fish and sea vegetables, for example, both contain abundant amounts of iodine. The material can also be found in plants grown on soil which is rich in oceanic materials, and in animals grazed on such soils. In some regions of the world, however, access to iodine is limited, and conditions like goiter are epidemic.
In the United States, this began to be recognized during the First World War, when many young men from the Midwest were declared unfit for duty due to iodine deficiency. This led to a widespread movement to add iodine to salt. Some salt producers were initially reluctant, but by the mid-1920s, iodized salt was extremely common in many American markets. Given the success of American iodized salt, other nations began to include the additive in their salt as well, protecting their populations from fully preventable conditions caused by iodine deficiency.
Salt with iodine does not generally have a noticeably different flavor, according to taste tests. Therefore, most people are encouraged to use iodized salt, to ensure that they receive plentiful amounts of this vital element. You don't need much—about 150 micrograms a day is the recommended daily allowance, and one teaspoon of iodized salt typically offers around 400 micrograms. Some countries also make flourinated salt to promote dental health, and salt may be supplemented with other dietary minerals as well.