LABELING OF ERYTHROCYTES<i>IN VITRO</i>WITH RADIOIODINE-TAGGED<i>l</i>-TRIIODOTHYRONINE AS AN INDEX OF THYROID FUNCTION: AN IMPROVED HEMATOCRIT CORRECTION*

Abstract

The labeling of erythrocytes in plasma during incubation with I131-tagged l-triiodothyronine (T3) was studied in blood samples prepared with a widely varying hematocrit. The percentage red-cell uptake corrected to a hematocrit of 100, as employed by other investigators for an index of thyroid function, was shown to vary considerably with changes in the hematocrit. This variation is sufficient to explain the difference reported by others in the average values for euthyroid males and females. Cell labeling was shown to obey a simple equation derived from the law of mass action. The labeled T3 taken up by the red cells was proportional to the product of the red-cell mass and the concentration of labeled T3 remaining in the plasma after incubation. The constant of proportionality, or binding coefficient, was characteristic of a given plasma sample and was essentially independent of the hematocrit over a wide range. Therefore it should be a more reliable index of thyroid function than the percentage red-cell uptake per 100 hematocrit.