What is a Fever (Pyrexia)? Normal and High Body Temperature

A fever, also referred to as pyrexia, is an elevation in the body temperature above the norm. This is broadly defined as oral temperature reading above 37.5 degrees Celsius (99 degrees Farenheit) and a core body temperature (rectally) above 38 degrees Celsius (100.4 degrees Farenheit).

Variations in temperature may occur depending on where the measurement is taken (oral, rectal, axilla). A fever should not be diagnosed by touching and estimating the skin temperature or the feeling of being ‘feverish’. The body temperature varies by a maximum of 0.6 degrees Celsius (adults) and 0.5 degrees Celsius (children) in the course of a day – the lowest temperatures are usually recorded in the morning and the highest by late afternoon.

The axillary temperature (taken under the arms/armpits) is the least accurate while the temperature taken rectally (core body temperature) is the most accurate. In older patients, an temperature taken orally is unreliable.

What Causes a Fever?

A wide range of causes, particularly infections, can trigger a fever. Substances that causes a fever is known as pyrogens. The body maintains the normal body temperature (set point) between 36.5 to 37.5 degrees Celsius (97.7 to 99.5 degrees Farenheit)  which is monitored and maintained by the thermoregulatory center in the hypothalamus.

As a response to certain circumstances, the body raises this set point and a host of changes occur in the body to initiate what is termed as a fever. Blood is forced from the periphery due to vasoconstriction because this is where the a lot of heat is lost into the environment. At the same time, shivering may begin – the rapid muscle contractions causes heat generation as a result of increased metabolic activity. Since a person feels cold during this time, they may also take measures to conserve heat by covering themselves with heavy clothing or a blanket.

All of these measures serve to conserve heat, generate more heat and increase the heat around the head and torso (core body temperature). These processes continue until the blood flowing past the hypothalamus matches the new raised set point.

A temporary rise in the body temperature is not considered abnormal – hormonal changes, emotions, activity and environmental factors can affect the body temperature to a slight degree.

Normal, Mild and High Temperatures

Body Temperature Values

  • Normal
    • The normal body temperature is accepted to be around 37.5 degrees Celsius although the body may not always be recorded at such a ‘perfect’ reading.
    • The lowest but normal levels (usually in the morning) may be as low as 36.5 degrees Celsius.
    • The highest but still normal level (usually by late afternoon) may be as high as 37.8 degrees Celsius although a temperature as high as 38 degrees Celsius may be considered acceptable due to errors in thermometer readings and individual variations.
  • Moderate or Mild
    • A temperature above 37.7 degrees Celsius/99.9 degrees Farenheit (orally) or 38.2 degrees/100.7 (rectally) is considered as a fever in adults.
    • For children, a body temperature above 38 degrees Celsius/100.4 Farenheit (rectally) is considered as a fever.
  • High Fever
    • A high fever is considered as a body temperature equal to and above 40 degrees Celsius/104 degrees Farenheit.
    • This should be considered as a medical emergency if the fever does not respond to (“break”) with the use of any medication.
    • In infants younger than 8 weeks, a body temperature greater than 38.3 degree Celsius/101 degrees Farenheit should be considered as a high fever and requires prompt immediate medical attention.

Related Articles

  1. Sudden Fever – Causes of Acute Fever in Adults

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