Background: Thyroxin (T(4)) plays an important role in the regulation of the rate of metabolism of proteins, fats, and carbohydrates. Congenital hypothyroidism, an inherited deficiency of thyroid hormones, is screened in newborns using a variety of methods that include total T(4) or thyroid stimulating hormone (TSH). Some laboratories measure either total T(4) or TSH as a means of detecting infants with hypothyroidism with a subset that analyzes both total T(4) and TSH as a primary screen or a combination of primary and secondary screens.
Methods: We have developed a new MS/MS method that measures T(4) from a filter paper blood spot following methanol extraction using the essentially the same method as MS/MS analysis of amino acids. Product ion spectra show a transition of 833.8-->731.8 (102 Da). An SRM for T(4) and (13)C(6)-T(4) was added to the MS/MS analysis of acylcarnitines and amino acids.
Results: Analysis of T(4) by MS/MS compares well with the immunoassay. The mean of 10,225 newborn screening specimens was 14.4 microg/dl (range 2.4-33) for MS/MS and 16.9 microg/dl (range 1.4-71.8) for AutoDELFIA. The MS/MS assay was linear with a correlation coefficient of 0.998. Regression analysis of MS/MS with the AutoDELFIA had a slope of 0.7, a y-intercept of 3.25, and a correlation coefficient of 0.727. We determined a MS/MS detection limit of 0.97 microg/dl and a lower LoQ (limit of quantification) of 2 microg/dl.
Conclusion: This method may provide a cost effective means of analyzing both T(4) and TSH by consolidating a T(4) analysis into the MS/MS panel.