Determination of tetraiodothyronine in serum by HPLC and electrochemical detection

Tetraiodo-L -thyronine (T4) in human serum was determined by HPLC using a nonphysiological internal standard. For sample preparation, a cation exchange resin, RP-chromatography on C-18 or phenylphase, and electrochemical detection (Metrohm 656 or BAS-LC 4 A) were employed. The mobile phase (0.8 ml/min) was 10 mM H₃PO₄/CH₃OH (40/60 v/v) or 10 mM H₃PO₄/CH₃CN (70/30 v/v, 1.5 ml/min), the voltage applied 800 mV. The coefficient of variation (day to day) was 6.8% (N = 40); concentrations of T4 found were 11 to 20% lower than those obtained by two different commercially available immunoassays.