Direct determination of serum T-4 by isotopic exchange: comparison with T-3 and PBI in 1700 cases.

We have studied in detail the procedure for the direct determination of serum thyroxine based on the liberation of (125)I T-4 from a tagged euthyroid serum reagent, the thyroxine being liberated by alcohol denaturation from 0.5 ml. of patient serum.We have established that there is proceeding simultaneously a second mechanism of T-4 liberation which is in no way associated with patient thyroxine. A simple technique for determining the extent of, and correcting for this second reaction is described.The procedure employs the same equipment used for the (125)I T-3 and involves only one additional step. No calibration curve is required. Results are not influenced by iodine in any form.The T-4 content is best expressed as a ratio to that of a standard mid-euthyroid serum. The ratio values for the hypothyroid, hyperthyroid and euthyroid states are characteristic and free from overlap. Separation between low normals and hypothyroids is very sharp. The relation between the serum T-4 ratio and the assayed thyroxine content has a correlation coefficient of 0.91. The product of the (125)I T-3 ratio and the T-4 ratio makes the procedure applicable in pregnancy and during steroid use, and provides a good indication of the free thyroxine present.The values of T-4, the (125)I T-3 and the PBI have been applied to 1084 patient sera and the T-4 and the (125)I T-3 to an additional 616. In this series of 1700 patients the serum T-4 was found to indicate the most probable clinical classification with a reliability of at least 95%.