The purpose of the study was to compare the discharge of radioactive iodine from the thyroid after po administration of perchlorate with and without supplement of 2 mg potassium iodine (Kl) in 42 euthyroid controls, 26 patients with nontoxic goiter, 39 patients with thyrotoxicosis treated with radioactive iodine, and 39 patients with Pendred's syndrome. The discharge test was found to have a satisfactory specificity with supplement of 2 mg Kl, and was more sensitive to detect an organification defect than the standard perchlorate discharge test without Kl. In all groups of patients a significant discharge occurred after addition of Kl in contrast to healthy persons. Only in the group of patients with Pendred's syndrome a discharge of iodine was found in the perchlorate test without Kl.