Define the importance of Iodine in human?

Iodine derives the nutritional importance as a constituent of thyroid hormones, 3,5,3',5' tetraiodo-thyronine (thyroxine or T₄) and 3,5,3' tri iodo-thyronine (T₃).The thyroid hormones are indispensable for normal growth and development in humans and animals. Synthesis of the iodine containing thyroid hormones occurs exclusively in the thyroid gland. Goitre was known to the ancient Indians, Chinese, Greeks and Romans. Iodine as an element was discovered only in 181 1; however, its presence in the thyroid gland was discovered by Bauman et, al in 1895. The relation between iodine deficiency and enlargement of the thyroid gland or goitre was shown early in the 20Lh century when it was reported by David Marine that the thyroid gland became hyperplastic (increase in number of normal cells in an organ and therefore an increase in volume/size of the organ) with low level of iodine in the body. Subsequently in 1922, Marine and Kimball demonstrated that administration of small amounts of iodine could prevent or substantially reduce endemic goitre among school children in Ohio.

Introduction of iodized salt as a public health measure to prevent goitre was first introduced in Switzerland and Michigan. Following this, the incidence of goitre and cretinism fell rapidly in these countries. Another major development for the population at-risk of severe iodine deficiency in inaccessible mountainous areas, was the iodized oil (1 ml containing 480 mg iodine) which can be given once in three years. Oral iodized oil is also effective but the effects may last only for one year.

Iodine is a non-metallic element of the halogen group with common oxidation states of -I^-1 (iodides), I⁺⁵,KI03 (iodates), I⁺⁷ KI04 (periodates) and less common states of +I (iodine monochloride) and +3 (iodine monochloride). In humans, iodine is typically found and functions in its ionic form, iodide (I-').  About 15 -20 mg iodine is found in human body, of which 70 -80% is present in the thyroid gland. The thyroid gland weighs 15-25 grams and has a remarkable ability to concentrate iodine. In the iodine deficient individual, enlarged thyroid gland may contain only 1 mg iodine.