The healthy thyroid can adapt to an iodine excess in many different ways. The autoregulation mechanisms may, however, fail in a diseased thyroid and severe hyperthyroidism may result. There must be autonomous portions of the thyroid functionally present to cause an iodine-induced hyperthyroidism. Retrospective and prospective studies, as well as animal experiments, were performed to establish further these pathophysiologic mechanisms. It was demonstrated that 15% of all cases of hyperthyroidism can be attributed to the use of iodine-containing contrast media. Furthermore, all contrast media exhibit a deiodinization during their intracorporeal retention time. The most frequent thyroid disorders, usually of a temporary nature, occur after choledochal contrast media. To some extent, these pathophysiologic processes can be reproduced in the animal model. Beyond this, the animal experimental studies show that it is possible to determine functionally autonomous portions of the thyroid by means of a dynamic thyroid scintigraph.