A non-metallic element which is found largely in seaweed. The body contains about 30 mg, largely concentrated in the THYROID GLAND where it is used to synthesise thyroid hormones. Iodine has a highly irritating action and, when applied to the skin, stains the latter dark brown and causes it to peel off in flakes, while internally it is a violent irritant poison in large doses.

Externally iodine is used as an antiseptic. Its drawback is that it is fixed by protein, which reduces its antiseptic efficiency in open wounds. Its main use in this sphere therefore is for sterilising the unbroken skin, as before an operation. Radioactive iodine is used for diagnosing and treating disease of the thyroid gland.