Element of the Day - Friday June 4th

Who am I? Under regular conditions I am a dark-purple/dark-brown solid. I am primarily used in medicines to clean and disinfect wounds, photography, and in dyes. Table salt is often fortified with me. I am rare in the solar system and Earth's crust, however, compounds with me in them are very soluble in water and I am concentrated in seawater. I am necessary for the proper functioning of the thyroid gland in the human body.

E-mail me with your answer at dwilliams@reading.k12.ma.us
Include the element's chemical symbol in your answer.
Image taken from: http://www.bridgat.com/food_additives-b803_8.html