CLINICAL-ASSESSMENT OF A RADIOIMMUNOASSAY FOR FREE-THYROXINE USING A MODIFIED TRACER

  • 1 January 1983
    • journal article
    • research article
    • Vol. 24  (6) , 498-504
Abstract
A radioimmunoassay [RIA] for measuring free thyroxine [T4] in plasma was introduced by Amersham using a I-125-labeled T4 derivative and does not bind significantly to the T4-binding proteins. This RIA was evaluated for its clinical utility in assessing 278 patients with thyroid and nonthyroidal diseases. The precision of the Amerlex free T4 assay, expressed as coefficient of variation, was 20% at 0.16 ng/dl, 6.9% at 0.55 ng/dl, 4.2% at 1.08 ng/dl, 5.3% at 2.29 ng/dl and 6.3% at 3.18 ng/dl. A reference range for free T4 was established as 0.68-1.8 ng/dl, n = 171. The correlation coefficients (r) of a dialysis method and a free T4 index were 0.871 and 0.911, respectively. Free T4 correctly classified 98% euthyroid, 92% hypothyroid, 100% hyperthyroid, 100% euthyroid with elevated T4 binding globulin and 87% of phenytoin patients. In addition, 80 patients with acute nonthyroidal illness were studied. Most of these patients have normal to low free T4, very low triiodothyronine and elevated reverse triiodothyronine. This free T4 assay was precise, easy to perform and reliable in classifying thyroid status in most patients.

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