OBJECTIVE The authors attempted to determine whether the use of nonionic contrast media causes uptake of iodine by the thyroid to be blocked and whether use of these agents could cause iodine-induced hyperthyroidism. METHODS Twenty-eight persons, including 22 with thyroid disease, were included in the study. Subjects underwent computed tomography (CT) of the thyroid after injection of 100 mL of iohexol. Thyroid function variables were measured before CT scanning and 1 week and 1 month later. In 16 subjects, uptake of iodine-131 by the thyroid was measured before and 1 week after CT. RESULTS I-131 uptake was reduced to 53.4% at 1 week after the injection of iohexol but became normal within a few weeks (followed up in one patient). An accelerated escape of I-131 from the thyroid was seen during the 1st days after the iohexol administration. Eight of 22 patients with an underlying thyroid disease had a temporary change in thyroid function. In four patients, the serum thyrotropin level was increased 1 week after the iohexol administration. In four other patients, temporary hyperthyroidism developed during the following months. CONCLUSIONS Iohexol can be used in patients with an underlying thyroid disease, but close monitoring in the following months is necessary.