Equations for an estimate of the total binding capacity of serum thyroxine-binding globulin (TBG-TC) were developed relating this parameter to serum thyroxine concentration (T4) and in vitro uptake (T3[triiodothyronine]U). This estimate demonstrated a highly significant, positive correlation with TBG-TC as measured by electrophoresis in 10 normal subjects, 20 patients with thyroid dysfunction (10 hypo- and 10 hyperthyroid) and in 40 individuals with altered TBG-TC but without thyroid dysfunction (10 normal pregnant women, 10 healthy women receiving anovulatories, 10 nephrotics and 10 patients with severe malnutrition). True-positive (sensitivity) and true-negative (specificity) ratios were calculated for total and free T4 in serum, T3U, Free T4 Index and both measured and calculated TBG-TC. False-positive results for free T4 index (12%) were due to altered TBG-TC. In such cases, 93% were recognized by the calculated TBG-TC from the values of the in vitro tests. This estimate should be added to the in vitro thyroid tests for their proper interpretation in cases where altered TBG-TC could be misleading. This estimate applies only to the particular in vitro testing system used.