Courtois' discovery of iodine in seaweed ash in 1811 apparently constitutes the first reference to iodine as a plant substance. The amount present in land varies from 10-100mg per 100g of dry matter. The seaweeds richest in iodine contain 10 000 times more iodine.
Further studies of the iodine content of plant and animal foods arose from the discovery in 1895 that iodine is a normal constituent of the thyroid gland. The major cause of goiter is iodine deficiency in soil, food and water. In endemic goiter areas which have existed in many parts of the world the addition of one part of NaI or KI to 100 000 parts of NaC used as tablesalt appears quite satisfactory in the prevention of the disease.
The isotopeI-131, with an eight day period, is a useful radioactive tracer in particular on the study and treatment of thyroid cancer.