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Tables to Estimate Total Binding Capacity of Thyroxine-binding Globulin from the In vitro Thyroid Function Tests

Instituto Nacional de Cardiología, Mexico City, Mexico

Correspondence: For reprints contact: Alfredo Cuarón, Departamento de Medicina Nuclear, Cuarto B-408, Instituto Nacional de Cardiología, Juan Badiano 1, Mexico 22, D.F., Mexico.

ABSTRACT

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 estimate demonstrated a highly significant, positive correlation with TBG-TC as measured by electrophoresis in ten normal subjects, 20 patients with thyroid dysfunction (ten hypo- and ten hyperthyroid) and in 40 individuals 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 values of the in vitro tests. It is concluded that 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 herein.