Clinical problems with a free thyroxine assay.

THE measurement of serum free thyroxine (FT4), which forms a small fraction (0.01-0.05 per cent) of the circulating total thyroxine (TT4), might be expected to provide the most accurate single biochemical index of thyroid status, on the basis that only free hormone is bioavailable. The conventional estimation of FT4 by equilibrium dialysis' is time consuming and the arrival of radioimmunoassay kits suitable for routine clinical use has been eagerly awaited. We have assessed one such kit (Amerlex free T4) incorporating an 1125 labelled thyroxine analogue and an antithyroxine antibody which binds both the analogue and free, but not protein bound, thyroxine. The competition for antibody binding sites between the known amount of analogue and the unknown amount of free thyroxine allows measurement of the free thyroxine. We have compared the clinical usefulness of FT4 measured by this technique with that of the more traditional thyroid function tests (TT4 and a derived free thyroxine index, FT4I).