The concentrations of triiodothyronine (T3), thyroxine (T4) and thyroxine binding globulin (TBG) have been measured in serum of 20 patients with ulcerative colitis or Crohn's disease. The patient group was compared with 20 healthy control subjects matched for sex and age. The concentration of T3 and T4 were similar in the two groups but TBG in serum was higher in the patient group, mainly due to the high TBG levels in the female patients. The concentration of T3 in serum was lower in the severely ill patients than in those who were mildly-moderately ill, while T4 and TBG were not affected by the severity of the disease. The concentration of T3 was lower in the corticosteroid-treated patients than in those who did not have such treatment, just like the TBG level. However, TBG was not subnormal in the corticosteroid-treated patients, whereas the serum concentration of T3 was. T4 in serum was not affected by treatment with corticosteroids. These findings indicate that the metabolism of T4 and T3 are influenced differently by corticosteroids.