The hypothalamic pituitary adrenal system (HPA axis) is a system that directly influences the endocrine system to active the stress response. When the brain interprets a stressor, the hypothalamus produces a hormone called CRF (corticotropin releasing hormone), which stimulates the pituitary gland to secrete ACTH (adrenocorticotropic hormone), which stimulates the adrenal cortex, part of the adrenal glands. The adrenal cortex then secretes corticosteroids, which help to increase energy levels.