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 IMx (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.
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