Iodine is a bluish-black, lustrous solid. At ambient temperatures it transforms into a blue-violet gas with an irritating odor. It forms compounds with most elements, but is less reactive than the other halogens. Iodine is primarily used in medicine, photography, and dyes. Iodine is rare in the solar system and Earth's crust. Trace amounts of iodine are required by the human body. Iodine is part of thyroxin, a hormone produced by the thyroid gland that controls the body's rate of physical and mental development. A lack of iodine can also cause a goiter, a swelling of the thyroid gland. Iodine is added to salt (iodized salt) to prevent these diseases.
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