| Summary: | The purpose of this study was to define reference intervals for total thyroxine (tT<sub>4</sub>) in dried blood samples (DBSs) obtained for newborn screening. The aim of our study was to assess the possible benefit of measuring tT<sub>4</sub> concentrations directly in DBSs obtained for newborn screening in premature and term-born infants. In order to have a sufficient number of samples for the extremely premature infants (<30 weeks), we set up a retrospective study, measuring the concentrations in DBSs collected over the previous 21 weeks. This time frame was a result of the included miniature study of tT<sub>4</sub> stability in DBSs. We found that tT<sub>4</sub> strongly correlated with gestational age (GA) in premature infants, highlighting the need for age-specific reference ranges. For term-born infants, the tT<sub>4</sub> ranges did not vary significantly among different gestational ages, allowing for the use of one single reference range.
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