Iodine is a grey solid that sublimes to form a purple gas.
Iodine is absorbed from food, and transported to the thyroid gland where it is stored and used for the synthesis of thyroid hormones. The hormones produced are triiodothyronine and thyroxine, which regulate the metabolic rate of adults and the growth and development of children. Iodine deficiency results in the disease "goiter", which manifests itself as a visibly enlarged thyroid gland.
The best natural food source is saltwater fish, although to prevent deficiency in inland areas iodine is now routinely added to table salt.
Fascinating Fact: Mild forms of goitre may still affect up to 5% of the population.