Chemically blocked analog assays for free thyronines. II. Use of equilibrium dialysis to optimize the displacement by chemical blockers of T4 analog and T3 analog from albumin while avoiding displacement of T4 and T3 from thyroxin-binding globulin.

Chemical blockers used to displace thyronine analog from albumin in analog kits for assay of free thyroxin (FT4) or free triiodothyronine (FT3) may also displace thyroxin (T4) or triiodothyronine (T3) from thyroxin-binding globulin (TBG), resulting in an apparent TBG dependence of results of free hormone estimates. We used equilibrium dialysis and antibody binding to assess the displacement of thyronine analogs and thyronines from albumin and TBG by use of chemical blockers. We chose a combination of two chemical blockers, which eliminated thyronine analog-albumin binding but minimized thyronine displacement from TBG for use in FT4 and FT3 assays. These blocked-analog free-hormone assays yielded accurate clinical results in euthyroid patients, hypo- and hyperthyroid patients, and in pregnant women. FT4 results were not entirely normalized in all nonthyroidally ill patients, indicating that decreased analog-albumin binding is not the only factor resulting in low FT4 results. In current Diagnostic Products Corp. (DPC) FT4 and FT3 blocked-analog kits, the blocker concentrations are the same as we used in these assays.