An inter-laboratory comparison of total serum triiodothyronine determination

L-triiodothyronine (T3) standards and an unknown serum were prepared and distributed to 18 different laboratories. T3 was determined on the standards and both T3 and T4 were assayed on the serum. The T3 and T4 results varied widely with a mean variance of 36 and 23%, respectively. There was, however, a positive correlation between the T3 values obtained within each laboratory and results of the unknown serum. Thus, while the data of each laboratory is internally consistent, the data between laboratories cannot be compared without an appropriate reference sera. From this fact, it is concluded that absolute ("true") T3 concentrations in human serum is not yet known. Because of the small number of laboratories involved in this comparison, it was not possible to determine which factors of the various procedures are responsible for the variance of the T3 results.