Hypothyroxinemia Detected at 4 Weeks of Life in Preterm Infants Born at Less than 30 Weeks of Gestation.

OBJECTIVE  Data on free thyroxine (FT4) concentrations beyond first 2 weeks of preterm infants are limited. This study was aimed to describe the association between perinatal characteristics and FT4 concentrations and the incidence of hypothyroxinemia at 4 weeks. STUDY DESIGN  Retrospective analysis of serum thyroid function tests at 4 weeks in preterm infants <30 weeks of gestation. Association between FT4 at 4 weeks of life and perinatal characteristics were determined by bivariate analysis and multivariable regression. Incidence of hypothyroxinemia was determined using a gestational age adjusted definition based on in utero levels at the equivalent postmenstrual age. RESULTS  The study cohort consisted of 280 infants. FT4 concentrations at 4 weeks of life were significantly associated with gestational age, birth weight, gender, and maternal history of thyroid disease. Hypothyroxinemia was found in 32.8% of the study cohort. CONCLUSION  Perinatal characteristics are associated with FT4 concentrations at 4 weeks of life. Nearly one-third of infants born <30 weeks had hypothyroxinemia at 4 weeks of life when compared with in utero levels at the equivalent postmenstrual age.