Inverse Log-Linear Relationship between Thyroid-Stimulating Hormone and Free Thyroxine Measured by Direct Analog Immunoassay and Tandem Mass Spectrometry

Abstract

BACKGROUNDAccurate measurement of free thyroxine (FT4) is important for diagnosing and managing thyroid disorders. Most laboratories measure FT4 by direct analogue immunoassay methods. The validity of these methods have recently been questioned. The inverse log-linear relationship between FT4 and thyroid-stimulating hormone (TSH) is well described and provides a physiological rationale on which to base an evaluation of FT4 assays.METHODSThe study included 109 participants for whom FT4 measurement was requested by their clinician. Samples were selected for inclusion to reflect a wide spectrum of TSH and albumin results. FT4 and TSH were measured by use of the Siemens Immulite immunoassay (IA). FT4 was also measured by liquid chromatography–tandem mass spectrometry (LC-MS/MS) (MS-FT4).RESULTSThe inverse log-linear correlation coefficient between TSH and FT4 was significantly better (P < 0.0001) for MS-FT4 (0.84, 95% CI, 0.77–0.88) than for IA-FT4 (0.45, 95% CI, 0.29–0.59). IA-FT4 showed a significant correlation with albumin (Spearman correlation coefficient 0.45, 95% CI, 0.29–0.5, P < 0.0001) and thyroxine-binding globulin (TBG) (Spearman correlation coefficient 0.23, 95% CI, 0.05–0.41, P = 0.02). In contrast, FT4 measurement by LC-MS/MS did not show a significant correlation with albumin or TBG.CONCLUSIONSThe inverse log-linear relationship between FT4 and TSH was significantly better for FT4 measured by LC-MS/MS than by IA. The MS-FT4 method therefore provides FT4 results that agree clinically with those obtained for TSH. Additionally, the significant correlation between IA-FT4 with albumin and TBG suggests that this FT4 method depends on binding protein concentrations and consequently does not accurately reflect FT4.