A Prospective Study of the Effect of Nonionic Contrast Media on Thyroid Function

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 mlU/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.