Indirect estimation of thyroid hormone-binding proteins to calculate free thyroxine index: comparison of nonisotopic methods that use labeled thyroxine ("T-uptake")
There are many alternative ways of estimating free thyroxine (T4) when thyrotropin screening results are abnormal. In addition to free T4 immunoassays, the menu of most automated immunoassay instruments includes a nonisotopic version of the original triiodothyronine (T3)-uptake assay called "T-uptake." We evaluated the ability of five such assays (Access, ES-300, IMx, Magnum Opus, and Stratus) to accurately estimate the free thyroxine index (FTI) in euthyroid, hyperthyroid, and hypothyroid patients with abnormal concentrations of thyroid hormone-binding proteins, and in patients with nonthyroidal illness. For comparison, we calculated a similar FTI, using either T3-uptake or direct measurement of thyroxine-binding globulin (TBG). Euthyroid reference ranges were comparable. Of euthyroid patients with increased TBG, 12-32% and 5-20% had increased or suppressed FTI, respectively, depending on the T-uptake method used. Except for IMx, 6-35% of hypothyroid patients with increased TBG had inappropriately increased FTI. Patients with nonthyroidal illness had comparable results regardless of the method used, and T-uptake methods were variably affected by known inhibitors of thyroid hormone binding. The most reliable T-uptake method appeared to be the IMx, which, despite claims that it measures all thyroid hormone-binding proteins, correlated best with TBG concentrations.
Faix
J D
JD
Department of Pathology, Harvard Medical School, Boston, MA.
Rosen
H N
HN
Velazquez
F R
FR
D001241:Aspirin / Q000494:pharmacology; D001322:Autoanalysis; D005665:Furosemide / Q000494:pharmacology; D006801:Humans; D006980:Hyperthyroidism / Q000097:blood; D007037:Hypothyroidism / Q000097:blood; D007118:Immunoassay / Q000379:methods*; D019301:Oleic Acid; D009829:Oleic Acids / Q000494:pharmacology; D011228:Prealbumin / Q000032:analysis / Q000378:metabolism; D011786:Quality Control; D012016:Reference Values; D012044:Regression Analysis; D012680:Sensitivity and Specificity; D012709:Serum Albumin / Q000032:analysis / Q000378:metabolism; D013974:Thyroxine / Q000097:blood*; D013975:Thyroxine-Binding Proteins / Q000032:analysis* / Q000378:metabolism
D009829:Oleic Acids; D011228:Prealbumin; D012709:Serum Albumin; D013975:Thyroxine-Binding Proteins; D019301:Oleic Acid; D005665:Furosemide; D013974:Thyroxine; D001241:Aspirin