Rapid metabolic and newborn screening of thyroxine (T4) from dried blood spots by MS/MS

BackgroundThyroxin (T4) 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 T4 or thyroid stimulating hormone (TSH). Some laboratories measure either total T4 or TSH as a means of detecting infants with hypothyroidism with a subset that analyzes both total T4 and TSH as a primary screen or a combination of primary and secondary screens.MethodsWe have developed a new MS/MS method that measures T4 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 T4 and 13C6–T4 was added to the MS/MS analysis of acylcarnitines and amino acids.ResultsAnalysis of T4 by MS/MS compares well with the immunoassay. The mean of 10,225 newborn screening specimens was 14.4 μg/dl (range 2.4–33) for MS/MS and 16.9 μg/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 μg/dl and a lower LoQ (limit of quantification) of 2 μg/dl.ConclusionThis method may provide a cost effective means of analyzing both T4 and TSH by consolidating a T4 analysis into the MS/MS panel.