Iodine's primary importance to the human body is related to the thyroid gland, which uses it to produce thyroxine and triiodothyronine, (two hormones that control the metabolism). A diet deficient in iodine can lead to a swelling of the thyroid, known as a 'goiter'.

Taking iodine pills, (in the form of Potassium Iodide), was also recommended during times of nuclear fallout, such as after a nuclear power station accident or detonation of a nuclear bomb. There is a high concentration of several radioactive isotopes of iodine, (predominantly Iodine-131), in nuclear fallout material and the theory is that the taking of Potassium Iodide pills would saturate the thyroid gland with non-radioactive iodine to such an extent that the excess, including the radioactive material, would be expelled from the body through the kidneys.