Pathophysiology of FeverPathophysiology and Clinical Presentation (1,2)“Normal” Body Temperature and the Definition of FeverPopular lore notwithstanding, 98.6°F (37°C) is not normal body temperature. In fact, there is no single normal value; like so many other biologic phenomena, body temperature displays a circadian rhythm. In healthy individuals, mean rectal temperatures vary from a low of about 97°F (36.1°C) in early morning to a high of about 99.3°F (37.4°C) in late afternoon. P.74In children, the normal range may be even greater. Moreover, physiologic factors such as exercise and the menstrual cycle can further alter body temperature. In practical terms, understanding the diurnal rhythm of body temperature is important for two reasons. First, many patients have been unnecessarily subjected to extensive workups and even psychologically incapacitated in the erroneous quest for a cause of deviation from the mythical “normal” temperature of 98.6°F. Second, the fever of disease states is superimposed on the normal cycle so that fevers are generally highest in the evening and lowest in the morning. As a result,
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