To observe the effect of iodine in nonionic contrast media on thyroid function, we measured free thyroxine (FT4) and thyroid stimulating hormone (TSH) following nonionic contrast radiography in 73 patients (49 males; 24 females) aged 50 to 84 years, mean 65.7 years. FT4 was significantly (p < 0.01) raised above baseline at 8 weeks but not 4 weeks following contrast injection (mean +/- standard deviation, 17.1 +/- 5.9 and 14.3 +/- 4.0 vs 13.3 +/- 2.7 pmol/L at baseline); however, TSH was significantly (p < 0.03) depressed at both 4 and 8 weeks (1.09 +/- 0.68 and 1.21 +/- 1.56 vs 1.40 +/- 0.90 mIU/L). T3 did not change significantly. FT4 rose by more than 20% in 15/73 and TSH fell by more than 20% in 41/73 compared to a fall of FT4 in 3/73 and a rise in TSH of 8/73 (p < 0.005 and < 0.001, respectively). Two patients became hyperthyroid and in four others either FT4 was elevated or TSH suppressed, one of whom developed atrial fibrillation. Although frank hyperthyroidism following contrast radiography was uncommon, there was a significant trend towards thyroid stimulation rather than suppression after iodine exposure. This may be related to the age of the patients studied.