Establishment of reference intervals for thyroid hormones in premature infants beyond the first week of life using Beckman Coulter Unicel DxI 800

•Thyroid hormone reference intervals were established for preterm infants.•Free triiodothyronine and thyroxine levels were correlated with gestational age.•Thyrotropin levels were slightly inversely correlated with gestational age.•Establishing instrument-specific reference intervals for preterm infants is vital. This 4-year retrospective cohort study aimed to establish reference intervals for free triiodothyronine (FT3), free thyroxine (FT4), and thyrotropin (TSH) in premature infants using the Beckman Coulter Unicel DxI 800 automated immunoassay system. Study subjects included 605 preterm infants with a gestational age of 26–36 weeks (corrected: 29–38 weeks). Pearson correlation was used to evaluate the association between hormone levels and gestational and corrected gestational ages. A nonparametric method was used to establish reference intervals based on corrected gestational age. FT3 and FT4 levels were positively correlated with gestational and corrected gestational ages, respectively. TSH levels were slightly negatively correlated with gestational and corrected gestational ages. FT3 significantly differed according to corrected gestational age (29–33 weeks vs 34–38 weeks); however, the difference was smaller than the reference change value (RCV) for the FT3 test. Thus, we combined the FT3 reference intervals into a single reference interval: 2.65–4.93 pmol/L (29–38 weeks). The reference intervals of FT4 and TSH were 11.20–24.97 pmol/L (29–38 weeks) and 1.01–10.14 mIU/L (29–38 weeks), respectively. Unlike those of full-term infants or adults, the reference intervals established in this study are applicable in premature infants. These results highlight the importance and complexity of establishing instrument-specific thyroid hormone reference intervals for preterm infants.