In order to assess iodothyronine receptor interactions in man, we have developed a receptor assay for T3 and T4 in solubilized nuclear extracts from circulating mononuclear cells. This assay utilizes the technique of salt solubilization to isolate nuclear receptors and employs standard saturation analysis for T3 and T4 to determine maximal binding capacity (MBC) and equilibrium dissociation constants (Kd). We have determined that 11 normal subjects had a MBC for T3 of 1.20 +/- 0.20 pmol/mg DNA (+/- SE) and a Kd of 3.4 +/- 0.2 X 10(-10) M; the T4 MBC was 8.44 +/- 1.22 pmol/mg DNA and the Kd was 2.7 +/- 0.3 X 10(-10) M. Hypothyroid patients had a mean T3 MBC of 7.32 +/- 2.28 pmol/mg DNA and a mean T4 MBC of 40.04 +/- 21.36 pmol/mg DNA (P less than 0.05 compared to normal). Obese subjects (n = 12) had a basal fed MBC that was 0.66 +/- 0.13 pmol/mg DNA for T3 (P less than 0.05 compared to normal) and was 3.58 +/- 0.56 pmol/mg DNA for T4 (P less than 0.01 compared to normal). During fasting, the average T3 MBC increased to 1.43 +/- 0.31 pmol/mg DNA and the average T4 MBC increased to 9.63 +/- 2.46 pmol/mg DNA, values that are both significantly higher than those in the fed period; the dissociation constants were unaltered in obese subjects (compared to normals) in fed and fasting states. Gel filtration with 0.5 M agarose was employed to ascertain if the physicochemical properties of the solubilized mononuclear human cell receptor were similar to those previously observed in rat and human liver and kidney receptors. The elution profile obtained was similar to that reported earlier. The major binding activity has an estimated Stokes radius of 35 A and a molecular weight ratio of approximately 50,000 daltons. These studies indicate that: 1) high affinity T3 and T4 receptors exist in human mononuclear cells and have properties similar to those for T3 and T4 described previously in rat liver; 2) T3 and T4 receptor number tends to increase in hypothyroid subjects and tend to be lower in obese patients than in normal weight control subjects; 3) fasting is associated with an increase in both T3 and T4 MBC; and 4) despite their apparent physicochemical similarity, T3 receptors in rat liver and human mononuclear cells may be regulated differently, at least during fasting since hepatic T3 receptors decrease in the fasted rat. Collectively, these observations support the concept that human white cell T3 nuclear receptor binding is capable of rapid fluctuations, suggesting a mechanism for homeostatic regulation of T3 action.