Effect of albumin concentration on the assay of serum free thyroxin by equilibrium radioimmunoassay with labeled thyroxin analog (Amerlex Free T4).

Serum free thyroxin (FT4) in normal nonpregnant and pregnant subjects was measured by radioimmunoassay (RIA) with Amerlex FT4 RIA (Amersham International) and LiquiSol FT4 RIA (Damon Diagnostics) kits. Amerlex FT4 values in serum from pregnant women were lower than those in serum from nonpregnant women, but LiquiSol FT4 values were similar in serum from both groups. Amerlex FT4 values were directly correlated with the concentrations of albumin in serum and inversely correlated with those of thyroxin-binding globulin, but not with prealbumin concentrations. No significant correlations were observed between LiquiSol FT4 values and serum concentrations of thyroxin-binding proteins. Amerlex FT4 values were normal in patients with excess, deficient, or decreased thyroxin-binding globulin. Albumin added to serum samples increased Amerlex FT4 values but not LiquiSol FT4 values. Albumin inhibited the binding of labeled thyroxin analog to the solid-phase thyroxin antibody. These data indicate that the albumin concentration influences FT4 values as measured by an RIA involving a thyroxin analog and that Amerlex FT4 values should be carefully interpreted when the patient has an abnormal concentration of serum albumin.