Fever (elevated body temperature) is in fact a very complex and uncertain issue. The best description is probably a statement that fever is “a body temperature that is in a specific person higher than normal”. The temperature of our body is controlled by a brain center and it is strictly individual. Elevated temperature in a person may be entirely normal in others.


To make the situation easier we assume fever as body temperature elevated over 37.5°C when measured in an armpit. Body temperature between 37-37.5°C is in many people perceived unpleasantly and it is often referred to as subfebrile condition.

How does fever occur?

Increased body temperature can be considered a defense reaction of our organism to an external or internal stimulus that triggers inflammatory reaction. Chemical substances arising because of the inflammatory response affect the brain. There is a special area controlling body temperature as a thermostat“. The chemical substances (known as pyrogens) irritate this brain centre and cause the thermostat to increase the desired body temperature.

What is the significance of fever?

Fever is an important factor in combat against infectious diseases. Higher temperature facilitates functions of white blood cells, helps to activate the immune system and slightly inhibits bacterial proliferation. We should therefore consider fever as positive natural process that should not be aggressively decreased at all cost. Of course, this is valid for temperature between 38-39°C, higher fever can be dangerous and especially fever above 40-41°C may damage body tissues and disrupt enzymes functions.


Note: This article is focused on febrile conditions in adults; elevated body temperature in children is a special and little bit different issue.



This is by far the most common cause of elevated temperature. The doctors usually meet with upper respiratory infections, bronchitis, pneumonias and urinary tract infections. High fever accompanied by headache is typically present in bacterial meningitides and kidney infections.

Autoimmune diseases

These diseases are tightly related to immune system and inflammation and therefore they can cause elevated body temperature. The fever is very mild; it only exceptionally rises above 38°C. The patient may complaint about continuous or recurrent subfebrile condition (37-37.5°C) followed by fatigue and night sweats.

Malignant diseases

A cancer in the body may cause a general inflammatory reaction of the immune system and elevation of body temperature. It is more typical for extensive tumors with metastatic spread and for “blood tumors” such as leukemias and lymphomas. The temperature is often between 37-38°C and it is accompanied by weight loss, night sweats and overall weakness (for more details see article dedicated to the symptoms of cancer).

Overactive thyroid gland

Elevated body temperature, excessive sweating, dry skin and heat intolerance of warm are symptoms commonly associated with increased production of thyroid hormones.

Tissue damage

Extensive tissue damage leads to an inflammatory reaction and body temperature elevation, often above 38 °C. Fever may therefore accompany conditions such as the crush syndrome.

Medications and addictive drugs

A significant elevation of body temperature can be caused, for example, by an overdose of Cocaine and high fever may occur as serious side-effect of antipsychotic drugs (so-called neuroleptic malignant syndrome).

Transplantation and transfusion complications

Elevated temperature is not rare after a transfusion as a reaction to pyrogenic substances in blood transfusion products. High fever accompanied by hemolysis would follow transfusion between incompatible blood groups but this is luckily a very rare cause. In organ transplants elevated temperature follows graft versus host disease and acute organ rejection.


The symptoms depend on individual tolerance and current temperature figure. Slight temperature elevation may be asymptomatic; otherwise it manifests with fatigue, loss of appetite and chills forcing the patient to seek rest and warm environment. Drop of elevated temperature is followed by sweating. Extreme temperature increased above 41 °C may be accompanied by confusion, loss of consciousness, disruption of the inner environment, rhabdomyolysis, organ failure and death. Elevated body temperature equals increased loss of body fluid, thus leading to risk of dehydration.

Diagnostic approach

Diagnosis of fever itself is easily confirmed by a thermometer. However, correct diagnosis of its cause may be sometimes much more challenging and may include medical history, physical examination, blood tests (especially CRP level) and various imaging methods. Certain diagnostic approach differs according to individual case and our suspicion (see related articles). 


Primarily, it is necessary to treat the underlying condition (usually infection). Fever itself is treated usually by non-sterodial anti-inflammatory drugs. In infections it is generally not wise to aggressively decrease fever that is under 39°C and rather seek rest and increase fluid intake to prevent dehydration.


Note: Non-sterodial anti-inflammatory drugs containing acetylsalicylic acid should not be used in children because of risk of fatal Reye's syndrome.

Jiri Stefanek, MD  Author of texts: Jiri Stefanek, MD
 Contact: jiri.stefanek@seznam.cz
 Sources: basic text sources