Table salt fortified with small amounts of potassium iodide or potassium iodate to prevent iodine deficiency in the population. Iodine is essential for the thyroid gland’s production of hormones that regulate metabolism and brain development; deficiency causes goitre and, in pregnancy and infancy, irreversible cognitive impairment. Salt iodisation is one of the most successful public-health nutrition interventions, credited with sharply reducing iodine deficiency disorders worldwide since its introduction in the 1920s. The added iodine is stable under normal storage but can be lost with prolonged exposure to heat, light, and moisture. Many countries mandate iodisation of food-grade salt, while some allow non-iodised salt for consumers who prefer it.