Body Temperature – Normal and Abnormal (High or Low) Levels

The human body requires an internal temperature of 37°C (98°F) to function optimally. This temperature can vary slightly without compromising health and well being. However, when the internal body temperature rises too high or drops too low, then various processes within the body are affected. It can even lead to death in severe and prolonged cases. Changes in body temperature are also signs of various diseases which may assist with a final diagnosis.

What is the internal body temperature?

The internal body temperature reflects the amount of heat generated and retained within the body. It needs to be constant and usually does not fluctuate significantly beyond 37°C (98°F). This may not be the same as the surface temperature (skin temperature) which can vary significantly depending on a host of factors like environmental conditions. However, the skin plays a major role in helping maintain the internal temperature at 37°C (98°F).

The human body constantly generates heat through its metabolic activities like breaking down glucose in cells to release energy. The internal body temperature (core temperature) has to be regulated to ensure that it does not drop too low or rise too high. This is achieved through several ways  known as the thermoregulatory (temperature control) mechanism. Overall the mechanism must maintain the core temperature from 36.5 to 37.5 °C (97.7 to 99.5 °F) at all times.

Body Temperature Levels

The body temperature can vary by up to 1°C and this may be influenced by several factors. In addition, the normal human body temperature can also vary by age. However, this variation is very slight among humans of different ages.

  • Babies and children: normal body temperature ranges from 36.6°C (97.9°F) to 37.2°C (99°F).
  • Adults: normal body temperature ranges from  36.1°C (97°F) to 37.2°C (99°F).
  • Older adults (over age 65): normal body temperature is often lower than 36.2°C (98.6°F).

It is important to note that body temperature readings may also vary depending on the region where the temperature is taken. Rectal and mouth (oral) readings are usually more accurate than skin, armpit or ear readings. The device used to take the body temperature can also be a factor.

Low Body Temperature Levels

An abnormally low body temperature is known as hypothermia. This may initially be noticed as the body temperature drops below 36.1°C (97°F) although most adults do not exhibit any hypothermia symptoms immediately below this temperature. Hypothermia is defined by a body temperature below 35°C (95°F) in adults. For babies, a body temperature below 36.1°C (97°F) is considered as hypothermia.

High Body Temperature Levels

An abnormally high body temperature is known as hyperthemia and commonly referred to as a fever. This is generally accepted as a body temperature of 37.8°C (100°F) or higher among babies, children or adults when the temperature is taken orally (by mouth). When taken rectally, then a temperature of 38°C (100.4°F) or higher is considered to be a fever.

What Causes an Abnormal Temperature?

It is important to first understand how the body temperature is controlled. Most heat loss occurs across the skin. If the body temperature rises then the tiny blood vessels in the skin widen (vasodilation). It causes redness of the skin (flushing). This allows heat to pass out into the environment. It is further facilitated by perspiration. As sweat evaporates from the skin it helps to dissipate this heat.

At the other extreme, the body has mechanisms to retain heat when the body temperature drops too low. It does this by reducing perspiration. Blood vessels in the skin narrow (vasoconstriction) to prevent heat loss. These narrowed blood vessels causes paleness in moderate to severe cases. Metabolic activity increases to generate more heat internally. Shivering is one such mechanism as the increased muscle activity raises heat production.

Environmental Factors

Environmental factors is one of the most common causes of abnormal body temperatures. It is most noticeable at extremes of temperatures. Severe cold climates will eventually cause a drop in the body temperature once the thermoregulatory mechanisms (temperature control) cannot compensate. Similarly, the body temperature can rise when the environmental temperature rises as occurs during hot summers and heatwaves. However, even artificial environmental changes can affect body temperatures like with prolonged exposure to refrigerated rooms or very hot and confined spaces.

Physical Activity

The body temperature rises with increased physical activity. This is due to increased energy production with heat being generated in the process. However, the temperature usually does not rise above the normal range if a person wears suitable attire and hydrates without there being any disruption in thermoregulation or abnormal environmental conditions. Therefore strenuous exercise should not be undertaken outdoors during heat waves.


There is a slight decline in body temperature during periods of sleep. This is in line with the decreased physical activity and decline in metabolic activity. Environmental conditions can also influence body temperature during sleep as does clothing and artificial climate control. It is important to note that this slight decline in body temperature is still within the normal temperature range.


Various substances including medication and illicit drugs can alter body temperature. This occurs due to disruption of the thermoregulatory (temperature control) mechanisms and sometimes due to alteration of metabolic activity. Alcohol is one commonly used substance that can cause a decline in the body temperature as it increases heat loss from the body. Various drugs may also have a similar effect or even increase body temperature as may be seen with certain stimulants, especially when used in large quantities.


There are a host of diseases where an abnormal body temperature may occur. A raised body temperature (fever) is usually associated with an infection. However, a fever does not occur with all infections. High body temperatures may also occur with certain non-infectious diseases lie some autoimmune disorders and hormonal disorders. Sometimes a fever may even occur for unknown reasons.

Thyroid gland disorders can also cause changes in body temperature. A higher body temperature may occur with an overactive thyroid gland disorder (hyperthyroidism) whereas a lower body temperature may occur with an underactive thyroid gland (hypothyroidism). However, thyroid gland disorders usually do not cause a major change in body temperature.


1. What is normal body temperature. Health Line

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