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Evaluation of an Immunoextraction Procedure for the Estimation of Free Thyroxine Concentration

Ochsner Clinic, and Alton Ochsner Medical Foundation, New Orleans, Louisiana

Correspondence: For reprints contact: Lynn Witherspoon, MD, Ochsner Clinic, 1514 Jefferson Highway, New Orleans, LA 70121.

ABSTRACT

We have examined the performance of a commercial free-thyroxine assay in which a radiolabeled T₄ derivative permits the competitive quantitation of extracted T₄ in the presence of serum proteins.^* After the total T₄ pool had been radiolabeled with either I-123 T₄ or I-131 T₄, the solid-phase antibody was found to be associated with 4–8% of the total T₄ present in the assay tube. Of this, 15–60% was displaceable (antibody-bound). The assay estimated free T₄ to be 0.6–1.8 ng/dl in euthyroid patients, and distinguished them from hyperthyroid (sensitivity 91%) and hypothyroid patients (sensitivity 91%) without apparent TBG dependence. In patients with severe nonthyroidal illnesses, the assay correctly quantitated a reduced extracted mass in some. In other patients, however, the assay results were inappropriately lower than the actual extracted mass, in agreement with the FTI but not with the measurements of free T₄ by dialysis. This assay appears to produce clinically appropriate results in most patients. In some nonthyroidally ill patients however, the indicated free T₄ is spuriously low.