Comparison of Serum Thyroxine, Protein-bonnd Iodine and Thyroxine Iodine Methods

A PBI and thyroxine iodine manual method (Oxford Laboratories) have been compared with a serum thyroxine method based on competitive protein-binding. The thyroxine iodine technique appeared to be more reliable and practicable than the PBI method, except that the latter has the advantage that it can be automated.The closecorrelation betweenresults from the serum thyroxine-iodine and thyroxine methods makes the former a useful addition to the range of tests employed in the investigation of thyroid function. The value of serum thyroxine and protein-bound iodine (PBI) estimations in the investigation of thyroid function is undoubted. A technique for thyroxine iodine (T- 4 Manual Method, Oxford Laboratories) was recently introduced. In this test, thyroxine is separated from other serum components by chromatography and its iodine content then measured by its catalytic activity in the arsenious acid-eerie ammonium sulphate reaction. The method should therefore have certain advantages over the PBI technique. The purpose of the present investigation was to compare the Oxford T- 4 and PBI methods with a serum thyroxine technique based on com-petitive protein-binding. MATERIALS AND METHODS The analyses were performed on 123 unselected serum samples. Serum thyroxine, PBI and thyroxine iodine were determined on 33 of these while duplicate determinations, using only one of the three tech-niques, were carried out on 90 specimens (30 specimens for each method). Serum thyroxine was determined by the method of Murphy and Jachan (1965) and PBI by that of Bird and Jackson (1962). The manufacturer's instructions were followed in the Oxford T- 4 technique for thyroxine iodine. All the analyses were performed by the same individual, the dupli-cate determinations being carried out at different times. The precision of the three methods was determined by calculating the standard deviation of the mean differences between duplicate estimations (Snedecor, 1952). The results from each ofthe methods involving iodine determination were correlated with those obtained when serum thyroxine was determined. Finally, the results from each of the three methods were compared with respect to the normal range for each test