Physiology, Fever

In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan.


Fever, or pyrexia, is the elevation of an individual's core body temperature above a 'set-point' regulated by the body's thermoregulatory center in the hypothalamus. This increase in the body's 'set-point' temperature is often due to a physiological process brought about by infectious causes or non-infectious causes such as inflammation, malignancy, or autoimmune processes. These processes involve the release of immunological mediators, which trigger the thermoregulatory center of the hypothalamus, leading to an increase in the body's core temperature.

The normal temperature of the human body is approximately 37 degrees Celsius (C), or 98.6 degrees Fahrenheit (F), and varies by about 0.5 C throughout the day. This variation in the core body temperature results from normal physiological processes throughout the human body, including metabolic changes, sleep/wake cycles, hormone variability, and changing activity levels. However, in the case of a fever, the increase in the core body temperature is often greater than 0.5 C and is attributed to a fever-inducing substance (pyrogen).

While these numbers may vary slightly based on the source, below is a summary of how to categorize fever.

  1. Low-grade: 37.3 to 38.0 C (99.1 to 100.4 F)

  2. Moderate-grade: 38.1 to 39.0 C (100.6 to 102.2 F)

  3. High-grade: 39.1 to 41 C (102.4 to 105.8 F)

  4. Hyperthermia: Greater than 41 C (105.8 F)

It is essential to understand that the definition of fever is not the same as that of hyperthermia (hyperpyrexia). In fever, there is an increase in the 'set-point' temperature brought about by the hypothalamus, enabling the body to maintain a controlled increase in the core temperature and general functionality of all organ systems. In hyperthermia, however, the rise in the body's core temperature is beyond the confines of the set-point temperature and regulation of the hypothalamus.

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