NORMAL BODY TEMPERATURE ? Normal body temperature varies over the course of the day, controlled in the thermoregulatory center located in the anterior hypothalamus. Fever is an elevation of body temperature above this normal daily variation. The body is normally able to maintain a fairly steady temperature because the hypothalamic thermoregulatory center balances the excess heat production, derived from metabolic activity in muscle and the liver, with heat dissipation from the skin and lungs. However, faced with environmental extremes, humans cannot maintain the narrow daily variation of body temperature without the aid of clothing and protective environments [1].
In healthy individuals, the mean oral temperature for those aged 18 to 40 years is 36.8 ± 0.4 ºC (98.2 ± 0.7 ºF) with low levels at 6 AM and higher levels at 4 to 6 PM [2]. The maximum normal oral temperature at 6 AM is 37.2 ºC (98.9 ºF), and the maximum level at 4 PM is 37.7 ºC (99.9 ºF), both values defining the 99th percentile for healthy subjects. From these studies, a morning reading >37.2 ºC (98.9 ºF) or an afternoon temperature of >37.7 ºC (99.9 ºF) would be considered a fever. Rectal temperatures are generally 0.6 ºC (1.0 ºF) higher than oral readings. Oral readings are lower probably because of mouth breathing, which is particularly important in patients with respiratory infections and rapid breathing. Lower esophageal temperature reflects core temperature, and tympanic membrane temperature readings are also close to core temperature.