What is fever and what mechanism increases the hypothalamic set point?

• A. Fever is an elevated set point for core temperature driven by pyrogen-induced prostaglandin E2 production in the hypothalamus.
• B. Fever is a direct result of environmental heat exposure raising the environment temperature.
• C. Fever results from reduced hypothalamic activity causing a drop in the set point.
• D. Fever is caused by dehydration decreasing blood volume.

Answer: (A)

Fever is a regulated rise in body temperature caused by an elevated hypothalamic set point. Pyrogens—such as cytokines from an infection (endogenous) or components from pathogens (exogenous)—trigger production of prostaglandin E2 in the hypothalamus. PGE2 then acts on receptors in the hypothalamic heat-regulating centers, shifting the set point upward. In response, the body activates heat-producing and heat-conserving mechanisms—shivering, increased metabolism, and constriction of skin blood vessels—to raise core temperature to the new set point. Once the pyrogenic signal diminishes, PGE2 levels fall, the set point returns to normal, and heat-dissipating responses (vasodilation and sweating) lower temperature back to baseline. This is distinct from hyperthermia driven by environmental heat or volume loss, which do not involve a deliberate reset of the hypothalamic thermostat.