On the periodic table, iodine is essential for thyroid hormone production.
Iodine has atomic number 53 and symbol I. It is a halogen in group 17 and is the heaviest stable halogen. The thyroid gland uses iodine to make the hormones thyroxine, or T4, and triiodothyronine, or T3. These hormones help regulate metabolism, growth, and development.
Iodine deficiency can enlarge the thyroid gland, producing a condition called goiter, and severe deficiency can impair development. Iodized salt was introduced in many countries as a practical public-health measure to supply dietary iodine. The required amount is small, but regular intake is important.
Elemental iodine is a dark, lustrous solid that can produce a violet vapor when heated. Its name comes from a Greek word meaning violet-colored. Iodine is often confused with iodide: iodide is the negatively charged ion, I⁻, found in many dietary and chemical compounds.