Patients with a successful renal transplant may have abnormalities in thyroid function. We evaluated serum thyroid hormone levels, serum thyrotropin (TSH) response to thyrotropin-releasing hormone (TRH), and the circadian pattern of serum TSH in 18 children aged 6.6-19.4 years (median 12.6 years), 4.0 +/- 2.9 years after renal transplantation. In 14 children, immunosuppressive therapy included methylprednisone [mean (+/-SD) 0.17 +/- 0.05 mg/ kg per day], while in 11 it included deflazacort (0.32 +/- 0.1 mg/kg per day). Seven children were studied twice, under methylprednisone and again while on deflazacort therapy. Mean total and free thyroxine (T4) values were significantly below the mean control levels (total T4 108.5 +/- 21.5 vs. 118.7 +/- 22.1 nmol/l, P < 0.05 and free T4 14.4 +/- 4.0 vs. 18 +/- 4.9 pmol/l, P < 0.001). Morning basal TSH levels were within the normal range. The mean TSH increment after TRH was 4.4 +/- 3.5 mU/l, significantly lower than that of controls (10.8 +/- 4.26, P < 0.001). Of 7 patients on methylprednisone, 4 had nocturnal TSH surges below the normal range (95% confidence limits 47%-300%); this occurred in 3 of 8 patients on deflazacort therapy. The TSH response to TRH was correlated with deflazacort dose. Patients on methylprednisone and deflazacort therapy had similar thyroid alterations. Our findings support the hypothesis that after renal transplantation some children have hypothalamic-pituitary thyroid abnormalities in which glucocorticoids may play a significant role.