Fever
Also known as: pyrexia
A fever is a regulated rise in body temperature caused by an upward shift in the hypothalamic set point, generally defined as a temperature of 100.4°F (38.0°C) or higher. It is a common sign of infection and other inflammatory processes.
Body temperature is controlled by the anterior hypothalamus, which defends a narrow set point through shivering, vasoconstriction, sweating, and vasodilation. In fever, that set point is deliberately raised. Exogenous pyrogens such as bacterial lipopolysaccharide stimulate macrophages to release endogenous pyrogens — interleukin-1, interleukin-6, and tumor necrosis factor — which act on the hypothalamus to increase prostaglandin E2 production and reset the target temperature upward.
The clinical picture follows the mechanism. While temperature is climbing toward the new set point, the patient feels cold and may shiver and have chills, because the body is below its own target. Once the fever breaks and the set point returns to normal, the body is now too warm and responds with vasodilation and sweating. Antipyretic drugs work by inhibiting cyclooxygenase, which lowers prostaglandin E2 and returns the set point downward.
Fever must be distinguished from hyperthermia, in which the set point is normal but heat production or environmental load overwhelms heat loss — as in heat stroke, malignant hyperthermia, and serotonin syndrome. Measurement route also matters for interpretation: rectal and temporal readings run higher than oral, and axillary readings run lower, so the same patient can appear febrile or afebrile depending on how the temperature was taken. Prolonged unexplained fever meeting defined duration criteria is classified as fever of unknown origin and prompts a structured workup.
Fever is tested from several angles. The CCMA exam covers accurate measurement, route differences, and temperature management within vital signs. USMLE Step 1 links fever to specific pathogens in microbiology, including streptococcal disease and arboviral infections. The NPTE for physical therapist assistants addresses fever as a sign in infectious disease and immune disorders that affects exercise tolerance and treatment planning.
Key takeaways
- Fever is a regulated increase in the hypothalamic temperature set point, typically defined as 100.4°F (38.0°C) or higher.
- Endogenous pyrogens IL-1, IL-6, and TNF raise hypothalamic prostaglandin E2, which shifts the set point upward.
- Chills occur while temperature rises toward the new set point; sweating occurs when the set point falls back.
- Hyperthermia differs from fever because the set point remains normal while heat load exceeds heat loss.
- Rectal and temporal temperatures read higher than oral, and axillary reads lower, so route affects interpretation.