Fever and hyperthermia differ in how the body regulates temperature. Fever is a regulated rise in the body’s thermostat: pyrogens, such as cytokines or bacterial components, trigger the hypothalamus to raise the set point (via prostaglandin E2). The body then generates and conserves heat (shivering, vasoconstriction) to reach that new set point. When the pyrogens are cleared, the set point returns to normal and heat loss mechanisms bring temperature down. Hyperthermia, on the other hand, is an unregulated rise in temperature without a change in the hypothalamic set point; excessive heat gain or impaired heat dissipation overwhelms the body’s cooling mechanisms, and the temperature climbs without a deliberate shift in the thermostat. This distinction guides management: fever responds to interventions that lower the set point (like antipyretics that inhibit prostaglandin synthesis), whereas hyperthermia requires cooling and removal of the heat source.