Accuracy and Standardisation of Serum Thyroxine Assays

Abstract
Serum thyroxine was assayed concurrently by at least two of three different radio-displacement analysis kits—‘Tetralute’ (Ames), ‘Tetrasorb’ (Abbott) and ‘Thyopac-4’ (Radiochemical Centre). Repeated and careful cross checking by each of the methods was carried out on the ‘standard’ sera supplied with the kits and on other commercially available control sera. Differences of −3 to + 32% in apparent T-4 concentration were found. An unacceptable variability in T-4 content amongst batches of the same control sera was also evident which can be explained only by a lack of constancy in the commercial serum T-4 ‘standards’. The molar extinction coefficients, water and tri-iodothyronine contents of commercial ‘pure’ thyroxine preparations indicate purities of 89–99 %. An examination of published ‘euthyroid’ ranges and those found by the ‘kit’ radiometric methods suggests that many normal serum T-4 values are based on the use of an impure primary or doubtful secondary standard. A primary standardisation of the assay with thyroxine of known purity and of known concentration, as determined from its molar extinction at 324 nm is advocated. Using this procedure, the normal euthyroid range for serum T-4 is 55–148 nmol T4/1 (2.8–7.5 μg T-4I/100 ml) or close to it—in agreement with the earlier findings of Howarth and Maclagan (1969) and Ekins et al. (1969). During the period studied, ‘Tetralute’ produced the most accurate results.