title: Establishing a reference range for triiodothyronine levels in preterm infants.
abstract: OBJECTIVE
  Thyroid dysfunction affects clinical complications in preterm infants and older children. However, thyroid hormone replacement in preterm infants has no proven benefits, possibly owing to the lack of an appropriate reference range for thyroid hormone levels. We aimed to establish a reference range for triiodothyronine (T3) levels at 1-month postnatal age (PNA) in preterm infants.
  
  
  METHODS
  This retrospective study included preterm infants born at a tertiary referral neonatal center at gestational age (GA)<35 weeks with no apparent thyroid dysfunction, for 6 consecutive years, with follow-up from PNA 2 weeks to 16 weeks. Using thyroid function tests (TFT), the relationships between T3 levels and thyrotropin (TSH) and free thyroxine (fT4) levels, birth weight, GA, postmenstrual age (PMA), and PNA were examined. The conversion trend for fT4 to T3 was analyzed using the T3/fT4 ratio.
  
  
  RESULTS
  Overall, 464 TFTs from 266 infants were analyzed, after excluding 65 infants with thyroid dysfunction. T3 levels increased with fT4 levels, birth weight, GA, PMA, and PNA but not with TSH levels. The T3/fT4 ratio also increased with GA, PNA, and PMA. The average T3 level at 1 month PNA was 72.56 ± 27.83 ng/dL, with significant stratifications by GA.
  
  
  CONCLUSIONS
  Relatively low T3 and fT4 levels in preterm infants were considered normal, with T3 levels and conversion trends increasing with GA, PMA, and PNA. Further studies are required to confirm the role of the present reference range in thyroid hormone replacement therapy.
authors.affiliation: Department of Pediatrics, Ulsan University Hospital, University of Ulsan College of Medicine, Ulsan, South Korea.
authors.forename: Ki Won
authors.initials: KW
authors.lastname: Oh
authors.affiliation: Department of Pediatrics, Maryknoll Medical Center, Busan, South Korea.
authors.forename: Mi Sung
authors.initials: MS
authors.lastname: Koo
authors.affiliation: Department of Pediatrics, Konkuk University Medical Center, Konkuk University School of Medicine, Seoul, South Korea.
authors.forename: Hye Won
authors.initials: HW
authors.lastname: Park
authors.affiliation: Department of Pediatrics, Haeundae Paik Hospital, College of Medicine, Inje University, Busan, South Korea.
authors.forename: Mi Lim
authors.initials: ML
authors.lastname: Chung
authors.affiliation: Biomedical Research Center, Ulsan University Hospital, University of Ulsan College of Medicine, Ulsan, South Korea.
authors.forename: Min-ho
authors.initials: MH
authors.lastname: Kim
authors.affiliation: Department of Pediatrics, Ulsan University Hospital, University of Ulsan College of Medicine, Ulsan, South Korea. Electronic address: jinadmb@hanmail.net.
authors.forename: Gina
authors.initials: G
authors.lastname: Lim
mesh_terms: D000704:Analysis of Variance; D001724:Birth Weight; D005865:Gestational Age; D006801:Humans; D007231:Infant, Newborn; D007234:Infant, Premature / Q000097:blood*; D012016:Reference Values; D012189:Retrospective Studies; D013960:Thyroid Function Tests; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood*
chemical_list: D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine
keywords: Premature infants; Thyroid function test; Triiodothyronine