Arousal of the FEAR system eventually leads to excessive production of cortisol. Under optimal conditions, when an animal is afraid, the secretion of cortisol mobilizes glucose as an energy supply for the skeletal muscles in case the animal decides to flee. In this way, cortisol secretion is beneficial. However, excessive secretion can begin to damage the body if elevations are sustained for too long. Normally, when cortisol has circulated through the blood back up to the brain, the paraventricular nucleus (PVN) of the hypothalamus exerts an inhibitory effect that stops further release of cortisol. If, however, a person or animal is subjected to an excessive amount of stress — when they are chronically frightened or anxious — the PVN may not be able to stop the production of cortisol. (..). all visceral organs and many areas of the brain, as well as the immune system, can be adversely affected by a prolonged excess of cortisol.

— Archaeology of Mind by Jaak Panksepp, Lucy Biven (2%)