New Automated Nonisotopic Immunoassays for Free Thyroxin: Effect of Albumin and Thyroxin-Binding Globulin Concentrations

Abstract

Recently, nonisotopic (often automated) immunoassays for measuring serum free thyroxin (FT₄) have become available. Though more costly than radioimmunoassays, they are considerably more convenient. We studied the influence of endogenous albumin and thyroxin-binding globulin concentration on five automated, nonisotopic methods of measuring FT₄ [Enzymun on ES300 (one-step), Stratus I and II (essentially two-step), Delfia (two-step), and IM_x (two-step)] in a mixed patient population. We observed that they (a) are influenced very little by endogenous serum binding proteins and (b) seem to have sufficient within-run precision to justify performing single measurements on patients' specimens.