title: Reference range of thyroid hormones in very low birth weight infants at the time of discharge.
abstract: OBJECTIVE
  There is little information regarding the reference range for thyroid hormones in preterm babies, especially those with very low birth weight (VLBW) of less than 1500 g. The objective of our study was to evaluate the relationship between thyroid hormone levels and postmenstrual age in a cohort of stable VLBW infants.
  
  
  METHODS
  An observational cohort study of VLBW infants preparing for discharge from a high-dependency nursery in Singapore. The infants' free thyroxine (fT4) and thyrotropin (TSH) levels were assayed just before discharge and correlated with postmenstrual age, calculated as the sum of the duration of gestation at birth and chronological age in weeks.
  
  
  RESULTS
  fT4 and TSH levels were sampled in 129 ex-VLBW babies at a mean postmenstrual age of 38.5 (±4.6) weeks. The babies were born at a mean±SD gestation of 28.9±2.4 weeks (median 29.0 weeks, range 24.0-34.5 weeks) with mean±SD birth weight of 1081±268 g (median 1090 g, range 490-1490 g). Linear regression analysis revealed negative and fair correlation between fT4 and postmenstrual age (r=-0.302). The mean±SD fT4 level was 16.8±3.2 pmol/L (median 16.8 pmol/L, range 8.5-28.9 pmol/L). However, there was only a very weak negative correlation between TSH levels and postmenstrual age, both with (r=-0.116) or without logarithmic transformation. The mean±SD TSH was 4.56±2.50 mIU/L (median 4.42 mIU/L, range 1.0-13.5 mIU/L).
  
  
  CONCLUSIONS
  Our study shows a fair and inverse correlation of fT4 with postmenstrual age in a large cohort of growing ex-VLBW infants, in keeping with maturation of the hypothalamic-pituitary-thyroid axis. It suggests that fT4 levels in growing infants are best compared to postmenstrual age-specific norms instead of a single reference interval.
authors.affiliation: 1 Departments of Neonatal and Developmental Medicine, Singapore General Hospital , Singapore .
authors.forename: Daisy K L
authors.initials: DK
authors.lastname: Chan
authors.forename: Vina
authors.initials: V
authors.lastname: Tagamolila
authors.forename: Jayapriya
authors.initials: J
authors.lastname: Ardhanari
authors.forename: Xue Yan
authors.initials: XY
authors.lastname: Lim
authors.forename: Jayme
authors.initials: J
authors.lastname: Wong
authors.forename: Chin Pin
authors.initials: CP
authors.lastname: Yeo
mesh_terms: D015331:Cohort Studies; D005260:Female; D005865:Gestational Age; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007234:Infant, Premature / Q000097:blood; D019102:Infant, Very Low Birth Weight / Q000097:blood*; D008297:Male; D010351:Patient Discharge; D012016:Reference Values; D013963:Thyroid Hormones / Q000097:blood*; D013972:Thyrotropin / Q000097:blood*; D013974:Thyroxine / Q000097:blood*
chemical_list: D013963:Thyroid Hormones; D013972:Thyrotropin; D013974:Thyroxine