Stress Response System
Unlike chronic stress in which cortisol levels will invariably rise, cortisol levels in people with PTSD tend to be low. Because cortisol is meant to restore balance to the body after a stressful event, the lack of cortisol can potentially prolong and worsen PTSD. Even during a panic attack, epinephrine levels will shoot up in people with PTSD; cortisol levels will not.
Hyperarousal is believed to be caused when the neurological pathways to the hypothalamus-pituitary-adrenal (HPA) axis—which regulates the stress response—become overly sensitized. When confronted with certain sensory stimuli, the HPA axis will overreact, triggering the secretion of excessive amounts of epinephrine which, in turn, stimulate the fear centers of the brain.