The function of hypothalamus–pituitary–adrenal axis in children with ADHD

Objective:To explore the relationship between hypothalamus–pituitary–adrenal (HPA) axis and Attention Deficit Hyperactivity Disorder (ADHD) in non-stress states. Method: 128 male children with ADHD aged between 6 and 14 years old were recruited, while 30 healthy male children were chosen as a control group. The diagnostic material was based on DSM-IV. The included ADHD children were further classified into the three sub-groups: ADHD-predominantly inattention type (ADHD-I) (n = 44), ADHD-predominantly hyperactive impulsive type (ADHD-HI) (n = 32), and ADHD-combined type (ADHD-C) (n = 52). The levels of cortisol and adrenocorticotropin hormone (ACTH) were evaluated by the automatic particle enzyme immunoassay and electrochemiluminescence respectively per morning (8:00 am). Intelligence test was assessed by the Raven's Standard Progressive Matrices. Results: The children with ADHD had significantly lower intelligence quotient (IQ) (84.5 ± 11.3) in contrast to the control group (98.6 ± 12.4, P < 0.01), although the lower level of IQ in ADHD-C group (79.2 ± 10.7) was also found when compared with other two sub-groups [ADHD-I (85.6 ± 10.4) and ADHD-HI (91.3 ± 12.6)]. In addition, no significant difference between the ADHD-HI group and the control group regarding the level of IQ were revealed. The level of cortisol in the ADHD group (226.47 ± 129.12 nmol/L) was significantly lower than that of the control group (384.53 ± 141.43 nmol/L, P < 0.001). The level of cortisol of the ADHD-HI group (154.36 ± 71.62 nmol/L) was significantly lower than that of other two groups [ADHD-I group (219.42 ± 117.66 nmol/L) (P < 0.01) and ADHD-C group (258.30 ± 136.39 nmol/L) (P < 0.01)]. There were no significant differences in the ACTH level either between the ADHD and the control group (P > 0.05), or between sub-groups in ADHD (P > 0.05). Conclusion: In the non-stress states, the existence of dysfunction of the HPA axis (lower plasma cortisol) in children with ADHD might be due to the under-reactivity of the HPA axis ; the low plasma cortisol level might contribute less to the outcomes of cognitive behavior of ADHD children and instead more closely relate to the core domains of attention deficit, hyperactivity and impulsive behavior of ADHD patients.

Research Highlights
► In the non-stress state, lower plasma cortisol in ADHD might be due to the under-reactivity of HPA.
► The low plasma cortisol level might less contribute to the outcomes of cognitive behavior of ADHD children.
► Instead more closely relate to the core domains of attention deficit, hyperactivity and impulsive behavior of ADHD patients.