**1. Introduction: fever vs. hyperthermia**

Elevated core body temperature is a common vital sign abnormality in the emergency department (ED) that can be caused by fever or hyperthermia. While both *fever* and *hyperthermia* describe a state of elevated core temperature, they are pathophysiologically distinct clinical entities with different underlying pathologies and treatments. Fever is defined as an elevated body temperature that occurs secondary to a normal thermoregulatory system functioning at a higher set point in response to a stimulus, most commonly infection or inflammation [1]. In contrast, hyperthermia causes elevated body temperature through primary dysfunction of the hypothalamic thermoregulatory system itself [1]. Fever accounts for the vast majority of cases of elevated core temperature presentations in the ED, while hyperthermia is much more rare [1, 2]. Distinguishing between fever and hyperthermia may be difficult on initial patient presentation to the ED but the distinction has important implications for patient treatment and outcomes.
