Non-Isotopic,Two-Step Free Thyroxine Immunoassay: A Better Measure of Free Thyroxine than Analogue Radioimmunoassay

Abstract

Plasma or serum free thyroxine (T₄) was measured by a novel non-isotopic, two-step immunossay in 373 consecutive patients attending a thyroid clinic, in whom thyroid status was categorized according to clinical findings, supported by routine thyroid function tests. The 95% confidence limit of free T₄ in the euthyroid patients (n = 112) was 7–20 pmol/L. Free T₄ concentrations within the reference range were found in six of 40 patients with primary hypothyroidism and nine of 182 patients with overt thyrotoxicosis, six of whom had T₃ toxicosis. Serum or plasma free T₄ measured by the two-step method showed improved diagnostic specificity over an analogue RIA in selected groups of euthyroid patients in whom abnormal binding of analogue T₄ can affect the validity of the result. Free T₄ results found by analogue RIA and the two-step method in 58 patients who were receiving thyroxine replacement therapy were similar. The between-assay precision of the two-step method was poor ranging from a coefficient of variation of 9·7% to 19·3% over a free T₄ concentration range of 5·0 to 46·0 pmol/L. We conclude that the two-step methodology offers diagnostic advantages for a laboratory which receives specimens from such patients for exclusion of thyroid disease but that improved assay precision is required before it could be used in a routine situation.