Critical assessment of the Amerlite free T4 assay

The measurement of serum free T4 (FT4) by analogue methods has been severely criticised because the T4 analogue binds to albumin. Amersham have recently introduced a method utilising horseradish peroxidase-labelled-T4 (HRP-T4) designed to overcome this problem and have incorporated it into the Amerlite enhanced luminescence immunoassay system. We have critically evaluated this method for its analytical and clinical validity. Experiments in which anti-albumin was added to normal serum suggested that the HRP-T4 label did not bind to endogenous albumin while the addition of albumin caused no significant change in FT4 concentration. Adding oleic acid up to 5 mmol/L to simulate increased non-esterified fatty acid concentration did not increase the apparent FT4. Serum sampled from subjects independently allocated to clinical groups were compared with an euthyroid group. The untreated hyperthyroid group values were distinctly elevated while the untreated hypothyroid group were appropriately low. Oestrogen therapy, low TBG, familial dysalbuminaemic hyperthyroxinaemia and non-thyroidal illness groups all reflected their euthyroid status, as did pregnancy samples which also showed a tendency to lower values in late pregnancy, consistent with previous observations. In conclusion, the Amerlite FT4 method appears to overcome some of the problems associated with analogue methods. A small survey showed it to be diagnostically valid in a wide variety of clinical states.