The Use of I 131 Red Cell Plasma Ratio as a Measure of Thyroid Function. ∗

Abstract

Summary

1. The ratio of red cell space for inorganic iodine to that of plasma was found to be about 0.5, in both in vitroc and in vivo studies. 2. The initial red cell-plasma ratio was the same in hyperthyroid, euthyroid, and hypothyroid patients immediately after the oral administration of a test dose of I¹³¹. 3. The ratio dropped as the inorganic iodine test dose was incorporated into larger molecules by the thyroid and returned to the blood stream. 4. The drop in red cell-plasma ratio was due to the relative impermeability of the red cell to large iodine-containing molecules. 5. The rate of drop of the red cell-plasma ratio is an expression of thyroid activity.