Clinical evaluation of three free-thyroxine assays.

The free thyroxine (FT4) test of Clinical Assays, Corning Medical, and Damon Diagnostics were evaluated in 245 patients. Although all three assays generally correlated well with the clinical functional status, there were apparent discrepancies in the clinically euthyroid population. All patients with discordant values had either relatively high or low triiodothyronine (T3) uptakes. In these instances, the free-thyroxine index gave a more accurate assessment of the clinical thyroid status than did the FT4. In our opinion, these findings place a critical limitation on these assays as a replacement for the free-thyroxine index.