Evaluation of free thyroxin measurements in patients with non-thyroidal illnesses.

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Abstract
Sera from 51 euthyroid patients with various non-thyroidal illnesses were used for the evaluation of free thyroxin measurements by use of two commercially available radioimmunoassay kits. A derived value for the percentage of free thyroxin was then calculated as the ratio of the free and total thyroxin concentrations. Values for the percentage of free thyroxin as measured directly via equilibrium dialysis were compared with those derived from both the Corning Medical (r = 0.56) and the Clinical Assays (r = 0.85) kit procedures. The Clinical Assays procedure, in which a solid-phase "immunoextraction" is used, demonstrated a large dynamic range of response for this patient population, in contrast to the narrow response seen with the Corning Medical procedure. Results for euthyroid patients with significant systemic illness by the Clinical Assays procedure compared favorably with those by equilibrium dialysis.