Introduction: 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.