Natural sea salt contains trace amounts of iodine. Iodised salt has extra iodine added to help fight iodine deficiencies.
According to the New York Times, iodine deficiency in pregnant mothers is the leading preventable cause of mental retardation. Non-iodised sea salt doesn't provide enough iodine to counter deficiencies, so many countries have initiated programs adding iodine to salt.
Non-iodised sea salt is created by harvesting ocean water and allowing it to evaporate, leaving behind salt crystals. Iodised sea salt goes through this same process, with potassium iodide added to the final product.
According to a study conducted by the World Health Organization in 1991, using iodised sea salt in place of non-iodised salt does not change the colour or taste of prepared foods.
Non-iodised sea salt naturally has 2mcg of iodine per gram. Salt that has been iodised has 77g of iodine per gram.
The World Health Organization recommends all adults consume 150g of iodine per day, which is the amount in a third of a teaspoon of iodised salt. Non-iodised salt does not provide enough iodine to reach this amount.