title: Cortisol in the first year of life: normative values and intra-individual variability.
abstract: BACKGROUND
  Many studies have incorporated cortisol measurements when studying infant development, but descriptions of normal development of basal cortisol levels in large study populations are scarce. The present study aimed to establish norm values for infant basal cortisol levels and to examine the development of intra-individual variability in the first year of life.
  
  
  METHODS
  More than 2500 cortisol samples were collected in 300 infants at three different ages. At each age four 1100h samples were collected to determine average cortisol levels and intra-individual variability. The development of basal cortisol levels and intra-individual variability was analyzed with multilevel growth curve modeling.
  
  
  RESULTS
  Norm tables with 90 and 95% intervals are presented. Basal cortisol levels decreased gradually over the year. Intra-individual variability was relatively large and stable in the first half year but decreased towards the end of the year.
  
  
  CONCLUSIONS
  The results of this study will aid researchers in evaluating cortisol data collected in early infancy. It also underscores the importance of taking intra-individual cortisol variability into account in studies involving infants.
authors.affiliation: Department of Developmental Psychology, Behavioural Science Institute, Radboud University Nijmegen, 6500 HE, Nijmegen, The Netherlands. m.tollenaar@psych.ru.nl <m.tollenaar@psych.ru.nl>
authors.forename: Marieke S
authors.initials: MS
authors.lastname: Tollenaar
authors.forename: Jarno
authors.initials: J
authors.lastname: Jansen
authors.forename: Roseriet
authors.initials: R
authors.lastname: Beijers
authors.forename: J Marianne
authors.initials: JM
authors.lastname: Riksen-Walraven
authors.forename: Carolina
authors.initials: C
authors.lastname: de Weerth
mesh_terms: D000367:Age Factors; D000704:Analysis of Variance; D002657:Child Development / Q000502:physiology*; D002940:Circadian Rhythm; D005260:Female; D006801:Humans; D006854:Hydrocortisone / Q000032:analysis*; D007223:Infant; D007231:Infant, Newborn; D008137:Longitudinal Studies; D008297:Male; D012016:Reference Values; D012463:Saliva / Q000737:chemistry*
chemical_list: D006854:Hydrocortisone