Accuracy and Standardisation of Serum Thyroxine Assays

To the Editor: In their paper, Watson et al. (Ann. din. Biochem. 11 (1974) 1) report extensive investigations in this important and difficult area of clinical biochemistry. There is, however, one important misconception in their representation of the results of the 'T-4 by column' methods. They suggest that in these methods as compared with competitive binding protein (CB.P.) methods the normal range is too low, and that 'Inherent losses (unpublished observations) make these methods unsatisfactory for clinical use'. There is some confusion in their paper between results expressed as T-4 and as T-4 iodine, but when this is allowed for the results summarised in the table below are obtained. The interesting and encouraging fact is that the normal ranges expressed as T-4 as iodine Ilg/1OO ml by both 'column' 1-4 and CB.P. methods 6-11 are very similar, showing particularly close agreement at the lower end of the normal range with two exceptions. 9 11 There is not the same agreement at the upper limit. Badman and Platten (1973) and Watson and Lees (1973) have shown that the apparent value for serum thyroxine by CB.P. methods increased with storage at 4"C and 15-20 C as compared with storage at -20 The formation of 'mimic T-4' might account in part for the higher upper limit of normal from laboratories using CB.P. methods as compared with T-4 by column methods. Woods et al. (1973) compared T-4 by column (Biorad) and Thyopac-4 (CB.P.) and showed good correlation between the two methods. The authors suggest an identical range of 3.0-6.6 Ilg/T-4 as iodine/l00 ml and although it is not stated in the paper all serum was stored at 20C prior to assay. Routinely carrying over sera from one week to the next and storing them at 4"C we have never encountered the formation of 'mimic T-4' in our T-4 by column method. It would be interesting to see the data from Watson et al. (1974) which leads them to make the statement 'that inherent losses make these methods (T-4 by column) unsatisfactory for clinical use'. Lee et al. published data showing good recovery of added thyroxine with their proposed modification of the Oxford method, and a number of authors have checked recovery of added T-4 1251 and T-4 131 1 in the column rnethods.P14 There is still a problem of satisfactory primary and secondary standards for thyroxine assay and 'the pressing need for a certified reference thyroxine' is very real. In the interim our laboratory uses Hyland control serum with careful cross-referencing of different batches and checked by using thyroxine (standardised by UV spectrometry at 325 nm) as an