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Clinical Evaluation of a Thyroxine-Binding Globulin Assay in Calculating a Free-Thyroxine Index

Associated Endocrinologists, Southfield, Michigan

Correspondence: For reprints contact: Walter E. Szpunar, PhD, Associated Endocrinologists, 4400 Town Center, Suite 275, Southfield, MI 48075.

ABSTRACT

A free-thyroxine index (FTI) using the thyroxine-binding globulin (TBG) assay of Corning Medical (FTI-TBG) was compared with a FTI using a tri-iodothyronine (T₃) uptake (FTI-T₃U) in 173 patients. Markedly elevated FTI-TBG values were obtained in clinically euthyroid patients with a relatively low T₃ uptake and an elevated thyroxine level. In contrast, the FTI-T₃U values for these same patients agreed with the clinical evaluation.Under these circumstances, the T₃ uptake is clearly superior to the TBG assay in calculating a free-thyroxine index.