Inverse log-linear relationship between thyroid-stimulating hormone and free thyroxine measured by direct analog...

Accurate measurement of free thyroxine (FT(4)) is important for diagnosing and managing thyroid disorders. Most laboratories measure FT(4) by direct analogue immunoassay methods. The validity of these methods have recently been questioned. The inverse log-linear relationship between FT(4) and thyroid-stimulating hormone (TSH) is well described and provides a physiological rationale on which to base an evaluation of FT(4) assays.

The study included 109 participants for whom FT(4) measurement was requested by their clinician. Samples were selected for inclusion to reflect a wide spectrum of TSH and albumin results. FT(4) and TSH were measured by use of the Siemens Immulite immunoassay (IA). FT(4) was also measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS) (MS-FT(4)).

The inverse log-linear correlation coefficient between TSH and FT(4) was significantly better (P < 0.0001) for MS-FT(4) (0.84, 95% CI, 0.77-0.88) than for IA-FT(4) (0.45, 95% CI, 0.29-0.59). IA-FT(4) 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, FT(4) measurement by LC-MS/MS did not show a significant correlation with albumin or TBG.

The inverse log-linear relationship between FT(4) and TSH was significantly better for FT(4) measured by LC-MS/MS than by IA. The MS-FT(4) method therefore provides FT(4) results that agree clinically with those obtained for TSH. Additionally, the significant correlation between IA-FT(4) with albumin and TBG suggests that this FT(4) method depends on binding protein concentrations and consequently does not accurately reflect FT(4).