Iodine is a metallic element belonging to the halogen group; it is atomic number 53. It is a grey metallic substance in its pure form that lets off a violet vapour when it turns to a gas. It's name is from the Greek "iodes" for "violet".
Iodine is part of the hormones triiodothyronine (T3) and thyroxine (T4) and helps the thyroid gland function normally. The daily requirement for adults is 100 to 150 mcg, but growing children, adolescents, pregnant women and people under emotional stress require more. Iodine deficiency causes goiters and hypothyroidism in people of all ages and in young children may result in cretinism. These conditions have become very rare in the past century due to the requirement that table salt be fortified with iodine.