INTRODUCTION
Fever, an elevation in core body temperature above the daily range for an individual, is a characteristic feature of most infections but is also found in a number of noninfectious diseases such as autoimmune and autoinflammatory diseases. Definitions of normal body temperature, the pathophysiology of fever, the role of cytokines, and the treatment of fever in adults will be reviewed here. Fever of unknown origin in adults, drug fever, and the treatment of fever in infants and children are discussed separately. (See "Fever of unknown origin in adults: Evaluation and management" and "Fever of unknown origin in adults: Etiologies" and "Drug fever" and "Fever in infants and children: Pathophysiology and management".)NORMAL BODY TEMPERATURE
Range of normal temperature — Normal body temperature ranges from approximately 35.3 to 37.7°C (95.5 to 99.9°F), with an average of 36.7°C (98.0°F) when measured orally, as suggested by studies in both outpatients and hospitalized individuals:
●In a study that included 35,488 individuals who underwent 243,506 oral temperature measurements during routine outpatient visits, the mean temperature was 36.6°C (97.9°F), with a 99 percent range 35.3 to 37.7°C (95.5 to 99.9°F) [1]. The mean age of participants was 52.9 years, 64 percent were female, and 41 were percent non-White.
●In another study that included 42,622 medical inpatients without known infection, malignancy, or immunocompromising condition who underwent 495,866 oral temperature measurements during the first week of hospitalization, the mean temperature was 36.7°C (98.0°F), with a 99 percent range 35.4 to 37.7°C (95.8 to 99.9°F). The mean age of participants was 61 years, 50 percent were female, 25 were percent Black, and mean body mass index was 30 [2].
In both studies, older age was associated with lower temperatures, as was lower body mass index. Higher temperatures were recorded in females compared with males. Temperatures also vary by underlying conditions. Hypothyroidism has been associated with lower temperatures and cancer with higher temperatures [1]. Pregnancy and endocrinologic dysfunction also affect body temperature.