Tables to estimate total binding capacity of thyroxine-binding globulin from the in vitro thyroid function tests

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 (T3U). This extimate demonstrated a highly significant, positive correlation with TBG-TC as measured by electrophoresis in ten normal subjectdividuals with altered TBG-TC but without thyroid dysfunction (ten normal pregnant women, ten healthy women receiving anovulatories, ten nephrotics, and ten 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 calues of the invitro tests. It is concluded that this estimates 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 herein.