Low hair cortisol concentration and emerging attention‐deficit/hyperactivity symptoms in preschool age

Previous research demonstrated hypoactivity of the HPA axis in children with attention‐deficit/hyperactivity disorder (ADHD) or externalizing symptoms. We analyzed the predictive association between the long‐term HPA axis activity and increasing symptoms of ADHD in the preschool period. The sample consisted of n  = 125 4‐year‐old children and their families (including n  = 64 children with elevated ADHD symptoms). ADHD symptoms were assessed by a structured clinical interview with the mother and by parent‐ and teacher‐report questionnaires. The long‐term HPA axis activity was assessed by the hair cortisol concentration (HCC) (over a 3‐month period). After controlling for potential confounders, low HCC predicted an increase in ADHD symptoms between the age of 4 and 5 years. Exploration of gender effects revealed that cross‐sectional and predictive associations were significant in boys but not in girls. Low HCC might thus be regarded as an early marker of a possibly gender‐related developmental pathway to ADHD.