Feb 18, 2021

4 min read

Pyrexia, Fever, Hyperthermia - What is the difference?

In any medical practice, the single most commonly noted abnormal finding, is increased temperature. An increase in body temperature can be a sign of something as innocuous as a cold or conversely, a critical illness, that requires intensive care (elevated body temperature is seen in more than half of all patients admitted to the ICU).

You are likely to hear terms such as Pyrexia, Fever and Hyperthermia in relation to increased body temperature. Very often, these terms are confused for one another and used interchangeably, although they refer to distinct concepts.

Fever vs Hyperthermia

The word, pyrexia, derives its origin from the Greek root, pyros, which means “fire” or “burning heat”. It is simply the medical term for fever. Fever is caused by an increase in the hypothalamic set point (the body’s thermostat, if you will), as the body’s adaptive response to a pathological state. This process is carried out through chemical mediators (cytokines), known as pyrogens.

“The hypothalamus, located in the brain, is the body’s thermoregulatory centre and is responsible for maintaining the body’s temperature within a favourable range of 36-37.5°C [97-99.5F].”

Hyperthermia (Latin for “beyond-heat”), on the other hand, is a sudden and uncontrolled increase in body temperature (even above 41 °C [105.8F] ), due to a failure of the body’s thermoregulatory mechanism. In hyperthermia, the body is unable to lose enough heat to cope with increased production and maintain a normal temperature. Unlike fever, there is no involvement of the hypothalamus, and the hypothalamic set-point remains untouched.

Hyperthermia, also known as heat-stroke, is extremely dangerous and is treated as a serious emergency.

How are they different?

Fever is a natural, adaptive response of the body, to physiological stress. Fever can be caused by a vast range of conditions, both infectious and non-infectious. Whatever the aetiology of the fever, it is associated with an increase to the “set-point”of the hypothalamus. This results in a controlled increase of the overall body temperature. This change in the set-point is mediated by the action of cellular messengers, known as pyrogens, on the hypothalamus. Pyrogens can either be endogenous (e.g.IL1, IL6 ) or exogenous (e.g. bacterial/viral toxins).

Hyperthermia, on the other hand, is caused by a failure of the body’s thermoregulatory (temperature control) apparatus - i.e The body has an impaired ability to lose heat. Hyperthermia can either caused by excessive amounts of heat from the surroundings or due to an increase in the body’s internal heat production. For instance, hyperthermia could be caused by strenuous exercise in a hot environment or due to excess heat production in diseases such as pheochromocytoma (a rare form of tumor in the adrenal gland). In either case, the heat produced overruns the body’s heat-loss capacity and causes a sharp spike in temperature, which often leads to severe complications and serious organ damage.

An important difference between fever and hyperthermia is that in the latter, the increase in temperature isn’t mediated by cytokines (cellular messengers). As a result, hyperthermias do not respond to anti-pyretic drugs, such as Aspirin or Acetaminophen, like fevers do.

Serious illnesses like sepsis, are usually accompanied by very high fevers (even up to 40.5°C) that can have a rapid onset. In such cases, it is absolutely essential to rule out hyperthermia. No matter how high the fever, administering antipyretics usually results in a slight drop in temperature. Hyperthermias can result in 41°C+ temperatures that do not respond to fever medication, in the slightest.

“Hyperthermias do not respond to anti-pyretic drugs, such as Aspirin or Acetaminophen, like fevers do. “

Because fevers are centrally controlled, there is usually some diurnal variation - i.e slight fluctuations within a day. However, there is no such variation observed in the case of hyperthermia.

Fevers are a natural, protective response of the body and are often innocuous. Hyperthermias, on the other hand are much more dangerous are always suggestive of a serious threat to the body. Hyperthermia is considered a serious medical emergency and hyperthermic patients need to be rushed into hospital for intensive care

In a Nutshell

  • Fevers, unlike hyperthermias, typically never exceed 40°C.
  • Fever is associated with a change to the hypothalamic “Set-point”; Hyperthermia is not.
  • Hyperthermias do not respond to antipyretic medication, as there is no cytokine involvement.
  • Some diurnal variation is typically seen in fevers. No such variation is seen in hyperthermia.
  • Hyperthermia is extremely dangerous and requires intensive care.

About the author

Abishek Swaminathen

Senior Content Manager

Abishek is ONiO’s senior content manager. A medical doctor by profession, he stumbled onto a writing career almost by accident, as it were. Words have enthralled Abishek since the day he first held a book and at ONiO, he channels his inner wordsmith towards providing our subscribers with regular doses of fun and informative content.

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