22 sick preterm infants were enrolled in a prospective study of iodine excretion after intravenous injection of a definite amount of the radiopaque dye Conray. Measurements of TSH and T4 were performed on day 5 and 4 weeks after injection or before dismissal. For comparison iodine excretion was studied in a group of 10 pre-terms without central venous catheters. In most of the babies urinary excretion of iodine was highest on day one with an exponential decline to day three. Transient hypothyroxinemia (T4 less than 60 nmol/l) could be diagnosed in 18 infants, but no one had hypothyroidism in sick prematures after injection of iodine containing contrast agents seems to be lower than expected. Nevertheless this method of radiographic control should be avoided if possible.