Effect of halofenate on serum thyroid hormone determinations in vitro

Halofenate has been shown to decrease thyroxine (T4) binding to thyroxine-binding globulin (TBG) in vitro. Several indirect serum thyroid hormone assays are dependent on thyroid hormone binding and the results might be altered by halofenate in the serum. Halofenate free acid (50-500 mug/ml) was added to serum samples in vitro, and the samples were assayed for total serum T4 by competitive protein-binding assay (CPB) and radioimmunoassay (RIA), the per cent of dialyzable T4 (%FT4), total serum triiodothyronine (T3) by RIA, and resin T3 uptake (RT3U). Halofenate increased the measured T4 (CPB), %FT4, T3 (RIA), AND RT3U, but did not alter the T4 (RIA) determination. Thus, halofenate appears to diminish T4 binding to TBG in vitro, and artifactually alters the serum determinations of T4 (CPB), %FT4, T3 (RIA), AND RT3U. Calculated values derived from these measurements for the free thyroxine index and "free" T4 will also be affected halofenate. Only the T4 (RIA) determination was unchanged by the presence of halofenate in vitro.