title: Evaluation of a new free-thyroxin assay.
abstract: The Amerlex Free Thyroxin (T4) Radioimmunoassay Kit (Amersham International Ltd.) is a new direct equilibrium radioimmunoassay for free T4 based on an antiserum with very high affinity for T4, and a unique 125I-labeled T4 analog as tracer. It is a very simple single-tube radioimmunoassay, making use of Amerlex particles to separate antibody-bound from free species. Interassay precision (CV) is 3.7% at 13 pmol/L and 2.3% at 30 pmol/L; within-assay precision is 4.2% at 21 pmol/L. The reference interval is 11-22 pmol/L. The assay did not misclassify any patients tested who had untreated myxedema or untreated thyrotoxicosis. The free T4 assay excelled both the free T4 index and the T4/T4-binding globulin ratio in correcting for increased thyroxin-binding globulin from pregnancy, and it was better than the index but not better than the ratio in correcting for increased thyroxin-binding globulin in users of oral contraceptives.
mesh_terms: D000510:Alpha-Globulins / Q000032:analysis; D005260:Female; D006801:Humans; D011247:Pregnancy; D011933:Reagent Kits, Diagnostic; D013960:Thyroid Function Tests; D013974:Thyroxine / Q000097:blood*; D013975:Thyroxine-Binding Proteins / Q000032:analysis
chemical_list: D000510:Alpha-Globulins; D011933:Reagent Kits, Diagnostic; D013975:Thyroxine-Binding Proteins; D013974:Thyroxine