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

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 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. 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.