Iodine, element 53 with a relative atomic mass of 126.90447, was first isolated by Bernard Courtois in 1811 from the ash of seaweed (by treating kelp with H2SO4). It was named by J. L. Gay Lussac in 1813, and its name derives from the Greek word iodes, meaning "violet-colored," reflecting the characteristic lustrous, deep purple color of resublimed crystalline iodine as well as the color of its vapor. Potassium iodide (KI) was used as a remedy for goiter (Derbyshire neck), an enlargement of the thyroid gland, as early as 1819. The thyroid is responsible for the production of thyroxine, a metabolism-regulating hormone. Iodine is an essential trace element for humans and plays an important role in many biological organisms. In modern times, KI is recommended for the treatment of radiation poisoning.