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Radioimmunoassay of Free Thyroxine with Prebound Anti-T₄ Microcapsules

University of Miami School of Medicine, Miami, Florida
Damon Diagnostics, Needham Heights, Massachusetts

Correspondence: For reprints contact: Robert J. Buehler, Director, R & D, Damon Diagnostics, 111 Fourth Ave., Needham Heights, MA 02194.

ABSTRACT

Free thyroxine (FT₄) may be one of the active thyroid hormones in contact with target end organs. It is unaffected by alterations in serum protein levels. In most cases, measurement of FT₄ reflects an individual's true thyroid function or dysfunction. Previous FT₄ assay techniques have been difficult, tedious, indirect, and inaccurate. A rapid, simple, and accurate radioimmunoassay for FT₄ has been developed using microencapsulated rabbit anti-T₄ antiserum to which I-125 T₄ tracer of high specific activity has been complexed. Addition of FT₄ standards or unknown samples displaces a proportional amount of I-125 T₄ from antibody. Protein-bound T₄ is excluded from the reaction by short incubation time and spatial configurations. Specimens representing known thyroid dysfunction were tested using the above procedure. The normal range of FT₄ was 0.8–2.4 ng/dl. The mean FT₄ for the hyperthyroid group was 6.92 ± 1.38 (range 4.4–9.6) ng/dl. The mean FT₄ for the hypothyroid group was 0.43 ± 0.37 (range 0.1–1.3) ng/dl, and in pregnancy the mean FT₄ was 1.64 ± 0.44 (range of 1.0–2.2) ng/dl (1).

This article has been cited by other articles:

				J. E.M. Midgley Direct and Indirect Free Thyroxine Assay Methods: Theory and Practice Clin. Chem., August 1, 2001; 47(8): 1353 - 1363. [Abstract] [Full Text] [PDF]