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What is the difference between fever, low-grade fever and hyperthermia?

The body temperature is controlled by the hypothalamus . The neurons of the anterior hypothalamus and the posterior hypothalamus receive information on temperature from two sources: peripheral thermal receptors, located mainly in the skin, and thermal receptors in the blood circulation of the hypothalamus itself.

The information coming from the two sources are integrated into the thermoregulatory center and an appropriate response is made to maintain the body temperature within its normal range , between 36.5 and 37 ºC, with slight variations at different times of the day up to 0.5 – 1 ºC.

In a neutral environment, the metabolic activity of the human body produces more heat than necessary to maintain body temperature, and the hypothalamus is the organ responsible for making a balance between the heat produced and its dissipation through the skin and pulmonary ventilation.

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Fever and low-grade fever

Fever and fever are two closely related terms that refer to an increase in body temperature above normal daily variations that occurs by increasing the set point of body temperature in the hypothalamus. The fever is produced by an increase in the thermostat of the thermoregulatory center of the hypothalamus .

In most cases, fever is a normal response of our body to certain disease states. For example, fever is a normal response to microbial infections and can help fight them.

The most accurate medical term for fever is pyrexia and it is usually accepted that there is fever from these values:

  • Rectal temperature above 37.5 – 38.3 ºC
  • Oral temperature above 37.7 ºC
  • Axillary temperature or hearing temperature above 37.2 ºC

The term low- grade fever is usually used to refer to the first tenths of fever , for example, for axillary temperatures between 37.2 and 38 ° C, although it is not a medical term with an exact definition. To refer to very high fevers, above 40.5 or 41 ºC , the term hyperpyrexia can be used , which should not be confused with hyperthermia.

Some substances, called pyrogens , can produce fever by acting on the thermoregulatory center of the hypothalamus and increase the reference temperature producing the consequent febrile response.


Hyperthermia is also an increase in body temperature above normal but, unlike fever and low-grade fever, hyperthermia does not respond to a response of the hypothalamus but to the inability to dissipate the heat produced by the body effectively , which is usually due to environmental factors, for example in sunstroke or heat stroke .

That is to say, hyperthermia is characterized by an increased body temperature but it is not a febrile state . In hyperthermia the body produces too much heat, or absorbs it from the environment, and can not dissipate it effectively but without the thermoregulatory center of the hypothalamus being affected.

Hyperthermia can also cause diseases such as serotonin syndrome, neuroleptic malignant syndrome, malignant hyperthermia or the consumption of certain substances such as drugs and stimulant drugs. These substances would not be considered pyrogenic since they do not act on the hypothalamus and, therefore, would not produce fever but hyperthermia.


  1. Charles A. Dinarello and Reuven Porat. In Harrison’s Principles of Internal Medicine , 18th edition.


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