Sex hormone-binding globulin (SHBG) and corticosteroid-binding globulin (CBG) levels were evaluated in euthyroid (N = 111), hyper- (N = 58) and hypothyroid (N = 38) men, in pre- and postmenopausal women (study 1) and in hyper- (N = 24) and hypothyroid (N = 15) patients before and after treatment with carbimazole or levothyroxine therapy (study 2). The SHBG levels are increased in hyper- and decreased in hypothyroid patients, whereas CBG levels are increased in hypo- and decreased in hyperthyroid patients. The SHBG levels are higher in women than in men with similar thyroid status. Plasma SHBG levels are correlated positively whereas CBG levels are correlated negatively with free thyroid hormone concentrations in men as well as women. In hypothyroid patients, SHBG concentrations increased (p < 0.01) and CBG concentrations decreased (p < 0.01) during levothyroxine treatment. In hyperthyroid patients, SHBG concentrations decreased (p < 0.01) and CBG concentrations increased (p < 0.01) during antithyroid treatment. The SHBG and CBG concentrations in treated hypo- and hyperthyroid patients were not significantly different from those of euthyroid controls. Our data indicate that SHBG and CBG levels depend on thyroid status. Corticosteroid-binding globulin is an index of thyroid hormone action at the liver level whose changes are opposite to those of SHBG in hyper- and hypothyroidism.