Iodine is a trace element that is naturally present in some foods, is added to some types of salt, and is available as a dietary supplement. Iodine is an essential component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). 

Existence

Iodine occurs in many oxidation states, including iodide (I^?), iodate (IO^?3), and the various periodate anions. It is the least abundant of the stable halogens, being the sixty-first most abundant element. As the heaviest essential mineral nutrient, iodine is required for the synthesis of thyroid hormones.Iodine deficiency affects about two billion people and is the leading preventable cause of intellectual disabilities.

Function

Iodine helps the thyroid gland, in the neck, to make the hormone thyroxine. Thyroxine controls many of the ways certain cells work.

Thyroxine is important for the growth of bones and nerves, and how proteins, fats and carbohydrates are used in the body.

Iodine is important before birth and in babies and young children. It is essential for the development of the brain and nervous system, the 5 senses, alertness and coordination.

Dangers

Elemental iodine is toxic if taken orally undiluted. The lethal dose for an adult human is 30 mg/kg, which is about 2.1–2.4 grams for a human weighing 70 to 80 kg (even if experiments on rats demonstrated that these animals could survive after eating a 14000 mg/kg dose). Excess iodine can be more cytotoxic in the presence of selenium deficiency.

Iodine supplementation in selenium-deficient populations is, in theory, problematic, partly for this reason.The toxicity derives from its oxidizing properties, through which it denaturates proteins (including enzymes).

Elemental iodine is also a skin irritant. Direct contact with skin can cause damage, and solid iodine crystals should be handled with care. Solutions with high elemental iodine concentration, such as tincture of iodine and Lugol's solution, are capable of causing tissue damage if used in prolonged cleaning or antisepsis; similarly, liquid Povidone-iodine (Betadine) trapped against the skin resulted in chemical burns in some reported cases.