Alterations of binding characteristics of cytosolic thyroid hormone binding proteins (CTHBPs) were examined in livers of rats with different thyroid function. Seven days after thyroidectomy, rats were divided into three groups. Group I received no treatment. Group II was treated with 230 ng triiodothyronine (T3)/100 g body weight per day for three days, and Group III with 40 micrograms T3/100 g body weight per day for three days. On the fourth day, each rat was given 0.7 microCi of 125I-T3/100 g body weight intraperitoneally and exsanguinated two hours later. During three days' treatment, body weight in Group II increased significantly compared with that in Group I (P less than 0.05), and body weight in Group III actually decreased. The ratio of liver weight to body weight in Group II was significantly higher than that in Group I or Group III (P less than 0.01). Percent distributions of 125I-T3 in cytosol fraction per liver or concentrations of cytosolic protein did not differ significantly among these three groups. Serum T3 concentrations (mean +/- SD ng/ml: Group I; not detectable, Group II; 0.50 +/- 0.27, Group III; 7.10 +/- 2.31), cytosolic T3 concentrations (mean +/- SD ng/ml: Group I; not detectable, Group II; 0.59 +/- 0.26, Group III; 10.38 +/- 3.08) and mitochondrial alpha-glycerophosphate dehydrogenase activities (mean +/- SD delta OD500 millimicrons/min/mg: Group I; 28.0 +/- 1.5, Group II; 46.7 +/- 7.3, Group III; 267.7 +/- 9.1) suggested that Group I was in hypothyroid state, Group II in euthyroid state and Group III in thyrotoxic state. Binding characteristics of cytosolic T3 binding protein (CT3BP) were different among the three groups.(ABSTRACT TRUNCATED AT 400 WORDS)