Iodine is a chemical element with symbol I and atomic number 53 and is a non-metal. The name is from Greek ἰοειδής ioeidēs, meaning violet or purple, due to the color of elemental iodine vapor.
Iodine, a trace mineral, is required by the body for the synthesis of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3). T4 contains 4 iodine atoms. When one of the iodine atoms is stripped off of T4, it becomes T3, with 3 iodine atoms remaining.
In the universe and on Earth, iodine's high atomic number makes it a relatively rare element. Iodine's rarity in many soils, due to initial low abundance as a crust-element, and also leaching of soluble iodide by rainwater, has led to many deficiency problems in land animals and inland human populations. Iodine deficiency affects about two billion people and is the leading preventable cause of intellectual disabilities.
Besides severe conditions of goiter and cretinism from iodine deficiency, more widespread is the chronic issue of hypothyroidism and even hyperthyroidism characterized by lack of dietary iodine.
Foods richest in iodine include sea vegetables, milk, yogurt, eggs and strawberries. While it is unusual to overdose on iodine when getting it from whole food sources, one must be aware that excessive consumption can be toxic, and that those with autoimmune thyroid diseases such as Hashimoto's or Grave's disease must pay particular attention to iodine consumption or supplementation.
sources:
http://en.wikipedia.org/wiki/Iodine
http://www.whfoods.com/genpage.php?tname=nutrient&dbid=69
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