Owen Yi¹, Campbell Heron¹

¹Department of Chemical Pathology, LabPLUS, Auckland City Hospital, Auckland, New Zealand

Introduction: Roche Cobas measures free T4 (fT4) up to 100 pmol/L. Dilution is not an option, as it causes dissociation from binding proteins and non-linearity. We present a case of a 16yo F with amiodarone-induced thyrotoxicosis and persistent fT4 >100 pmol/L despite 10 days of treatment. A method was developed to extrapolate an estimated fT4 concentration and enable monitoring.

Methods: The principle of fT4 dissociation constant was used to determine the following relationship via serial dilution: y = n/x^k where n is the neat result, y are the results on dilution and x are the dilution factors. Log transformation into ln(n/y) = k*ln(x) produced a linear function with a slope of k. Since the neat value is unknown for samples >100 pmol/L, the process is modified to allow extrapolation of the neat result. Transformed linear relationship were consistently obtained on multiple samples tested including index patient’s samples.

Conclusion: We were able to extrapolate an estimated concentration for this patient and demonstrated improving fT4 with treatment. This method was also demonstrated to be applicable to free T3 dilution. We also briefly explored its application in identifying fT4 assay interferences, including the relationship between the neat value and the slope k.