Effects of irradiation on the thyroid gland.

External irradiation to the thyroid, even at low doses, induces subsequent development of thyroid nodules and carcinomas, with an estimated incidence of five per one million people per year if they have been exposed to 1 rad. Larger doses are associated with hypothyroidism. External irradiation also induces thyroid hyperplasia and is associated with an increased incidence of thyroid autoimmunity and the development of thyroiditis and Graves' disease. 131I, used in the treatment of Graves' disease and thyroid cancer for nearly 50 years, has not been associated with an increased incidence of thyroid cancer. Nevertheless, caution is advisable in use of radioiodide therapy in adolescents. Low levels of fallout from nuclear explosions probably have minimal, if any, effect on the thyroid; but recent high-level exposure in individuals near Chernobyl is reported to be associated with the development of thyroid malignancies in children.