Direct separation and quantitative analysis of thyroxine and triiodothyronine enantiomers in pharmaceuticals by high-performance liquid chromatography

A rapid reversed-phase type HPLC method for the simultaneous separation and analysis of d- and l-thyroxine (d- and l-T4) and triiodothyronine (T3) was developed using a quinine-derived chiral stationary phase and applied for a quantitative assay of the enantiomeric impurity of the drugs in pharmaceutical formulations of levothyroxine. The influence of operating parameters has been studied for the optimization of the separation and also in order to gain an insight into the retention mechanism. Validation of the method included linearity, precision and accuracy which revealed R.S.D. values of <3.3% for intra-assay precision and percent error ranging from −6 to +2.1% for various defined validation samples, proving satisfactory accuracy. Quantitation was performed over the range of 0.5–500 μg ml−1 with limits of detection and quantitation lower than 0.1 and 0.5 μg ml−1, respectively, for both analytes. Further, the determination of 0.1% impurity, of d-T4 as well as l- and d-T3 in levothyroxine sodium tablets proved to be feasible.