Exposure to radioactive iodines (radioiodines such as 131I) can cause thyroid diseases because the thyroid gland concentrates and stores iodines from the blood. The main consequences of exposure are an increase in the incidence of thyroid tumors and an increase in the occurrence of loss of thyroid function (hypothyroidism). The chance of tumor development rises with increasing radiation dose up to a level that is high enough to kill all or most thyroid cells. Very high doses of radiation do not cause tumors because cells are so extensively damaged they cannot even produce the tumors. Such extensive damage can lead to hypothyroidism. Another possible consequence is autoimmune disease of the thyroid in which the body's lymphocytes become sensitized to thyroid cells and destroy them–this can also lead to hypothyroidism.
