Kim HR, et al. - In this retrospective cohort study, researchers determined the incidence of thyroid dysfunction requiring levothyroxine treatment and identified its risk factors in preterm infants. This investigation was conducted on preterm infants who were born before 32 gestational weeks and admitted to a single tertiary academic center for more than 8 weeks between January 2008 and December 2014. Of the 220 preterm infants registered, 180 infants had thyroid function tests (TFTs) at 1, 3, and 6 weeks of postnatal age and were included. Thirty-five infants were started on levothyroxine treatment based on the results of serial TFTs of the 180 infants. In nearly one-fifth of preterm infants born before 32 gestational weeks, thyroid dysfunction requiring levothyroxine treatment occurred. At 1 week of postnatal age, approximately half of the preterm infants who were treated with levothyroxine had normal TSH and fT4 levels. Findings suggested that serial TFTs is important in identifying preterm infants that require treatment with levothyroxine.