Diagnostic value of the absolute free thyroxine iodine test

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Several investigators1–5 have proposed that the thyroid hormone circulates in two parts—a free unbound physiologically active part and a larger protein-bound metabolically inert part. If approximately 5.0 μg/100 ml of thyroxine iodine is considered 100%, then the free thyroxine is approximately 0.05% of the total amount and is expressed in millimicrograms (nanagrams—n). The normal mean level is approximately 3.0 n/100 ml.

Several methods6–10 of measuring the amount of free thyroxine have been proposed, but the few large series7, 9, 11, 12 of patients who have been studied to determine the clinical usefulness and diagnostic accuracy of the test have depended upon the protein-bound iodine (PBI)) and the percentage of free thyroxine iodine and the product obtained thereof. Since March 1966, we have measured the absolute free thyroxine iodine based on total serum thyroxine iodine and have tested more than 1,000 patients who had a variety of thyroid and suspected thyroid diseases. Measurement is similar to the free thyroxine index reported by others,13–15 and our conclusions agree with theirs.

Materials and methods

Records of 102 patients were used for the main body of this study, and tests on an additional 43 are included in the scatter graphs (Figs. 1–3). Each diagnosis was based on clinical appraisal, standard tests and, when necessary, on results of therapy. Many tests which were previously abnormal were repeated.

The effects of iodine contamination, drugs, and estrogens were considered when making a diagnosis. The tests referred to as “standard” included BMR, PBI, . . .



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