Red blood cell T4 concentrations (RBC T4) were measured in 15 normal subjects, 13 patients with hypo- or hyperthyroidism, and 10 patients with elevated or decreased serum thyroid hormone binding. In each case, RBC T4 was compared with the serum concentration of free T4 measured by equilibrium dialysis ( FT4D ). RBC T4 correlated significantly with FT4D in these subjects (r = 0.90; P less than 0.001). The normal range for RBC T4 was 0.27-0.83 ng/ml. RBC T4 was below the normal range in all 8 patients with hypothyroidism and above the normal range in all 5 patients with hyperthyroidism. It was within the normal range in all 4 subjects with absent or low T4-binding globulin (TBG) and in 5 of the 6 subjects with elevated TBG or familial dysalbuminemic hyperthyroxinemia. The sixth subject (increased TBG) had elevated RBC T4 and FT4D . RBC T4 was similarly measured in 10 patients with severe nonthyroid illness (NTI), 5 of whom had decreased serum concentrations of total T4. RBC T4 was normal in 8 of these patients, elevated in 1, and decreased in 1; in comparison, FT4D was normal in 4, elevated in 5, and decreased in 1. Eight patients receiving continuous iv infusions of heparin were also studied because of previously described similarities in the in vitro thyroid tests of heparin-treated and euthyroid sick patients. FT4D was elevated in 7 of the heparin-treated patients, whereas RBC T4 was elevated in only 2. Furthermore, for any given value of FT4D , RBC T4 was lower in heparin-treated patients than in normal subjects, indicating the presence of an inhibitor of cellular T4 binding in these patients. This putative inhibitor, demonstrated by an elevated FT4D to RBC T4 ratio, was present in 6 of the 8 heparin-treated patients and in 5 of the 10 patients with NTI. The findings of this study support the hypothesis that an inhibitor of cellular T4 binding is present in the serum of some patients with NTI and in most heparin-treated individuals.