A complex set of interactions between the hypothalamus, the pituitary gland, and the adrenal or suprarenal glands. The HPA axis is involved in the regulation of temperature, digestion, immune response, mood, sexuality, and energy usage. The HPA axis is also involved in anxiety disorders, bipolar disorder, posttraumatic stress disorder (PTSD), and depression.
A primary system of the fight-or-flight response that involves the hypothalamus influencing the pituitary gland to secrete ACTH, which, in turn, stimulates the adrenal cortex to release glucocorticoids.