Cortisol concentrations in 12- to 18-month-old infants: Stability over time, location, and stressor

Background: Sparse information on early development of hypothalamic pituitary adrenal (HPA) axis responsivity in human infants limits our understanding of stress hormone regulation and vulnerability to psychopathology. We considered whether infant cortisol stress response (CSR) is a suitable endocrine phenotype for developmental stress research. Methods: We assessed stability of key CSR parameters across time, location, and stressor through saliva samples taken before and then 20 and 40 min following exposure to two stressors administered 1 week apart in 27 infants aged 12 to 18 months. Time-matched home samples were collected to control for circadian rhythm and to evaluate baseline stability. Results: Baseline cortisol concentrations, peak percent change, and area under the curve (AUC) were stable across time and stressors. Following both stressors, half the infants exhibited peak cortisol concentrations at 20 min poststress; half peaked at 40 min poststress. For 56% of the infants, peak response time was inconsistent across stressors. Conclusions: In humans, baseline and CSR are stable by 12 to 18 months. Variation in CSR time course across stressors indicates that infant CSR should be sampled beyond 30 min. Results support using infant CSR, particularly as measured by AUC, as a valid endocrine phenotype for developmental stress research.