THE VALUE OF PROTEIN‐BOUND IODINE AND TRIIODOTHYRONINE RESIN UPTAKE IN ASSESSING THYROID FUNCTION

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Abstract
SUMMARY: The experiences gained in performing protein‐bound iodine (P.B.I.) and triiodothyronine resin uptake (T3 uptake) assays in a series of 332 plasma samples from human subjects are described. The subjects are classified into clinical categories on the basis of their thyroid status, and from a consideration of extrathyroid factors such as iodine contamination and the presence of thyroid‐binding abnormalities in the serum. There are categories also for patients undergoing treatment for thyroid disorders.The P.B.I. assay was performed by Acland's (1957) procedure, and the T3 uptake assay by modifications of the method of Woldring et alii (1961). The results in each category are subjected to an analysis of variance.For euthyroid subjects without goitre, the mean value for P.B.I. level is 5·65 µg. per 100 ml. of plasma, with a standard deviation of 0·72. The value for T3 uptake is 91·4%, with a standard deviation of 10·7. The values for euthyroid subjects with goitre are not significantly different from those without. Iodine contamination causes no change in the T3 uptake value. In subjects with increased binding, the P.B.I. level is elevated and the T3 uptake is depressed. With decreased binding, the P.B.I. level is depressed and the T3 uptake is increased.The “P.B.I.×T3 uptake index” of Clark and Horn (1965) is found to be very useful in assessing the thyroid status of patients with binding abnormalities. The P.B.I. assay is found to be the method of choice in the diagnosis of primary thyrotoxicosis and myxœdema. The P.B.I. assay is misleading, but the T3 uptake is accurate in the diagnosis of the thyroid status of thyroxine‐treated myxœdema patients. The P.B.I.×T3 uptake index is more useful than either the P.B.I. assay or the T3 uptake alone in assessing the response to radioiodine therapy of thyrotoxic patients.