Diagnostic value of a modified assay of free triiodothyronine in serum

The performance of a modified commercial (Diagnostic Products Corp.) assay for free triiodothyronine in serum was evaluated, and its diagnostic value was compared with two other methods for free triiodothyronine estimation in euthyroid subjects and in patients with thyroid dysfunction. No advantage was demonstrated for the modified assay as an index of thyroid status. In conditions involving thyroxin-binding globulin excess the modified assay produced misleading results more frequently than did the comparison methods.