Fever, a common issue in general. All of us have experienced fever at least once in our life.Though the feeling during fever can vary from person to person, but one thing is common for all- higher than normal body temperature.

As a result, other symptoms of fever arises. But when fever itself is a symptom, so there are various reasons of fever and as a result, the symptoms vary according to the cause.

Now the question is, does increased temperature always mean one has fever? The answer is definitely no. So, when we should say that someone has fever? To know all these facts, lets check below.

Normal Body Temperature

The normal body temperature is 36.5-37.5 °C (97.7–99.5°F) when we consider the temperature of environment is neutral. This temperature is caused due to our metabolic rate of body that produces heat to maintain our core body temperature.

A normal body temperature is ordinarily maintained despite environmental variations because there is thermoregulatory center in the hypothalamus in our brain that regulates our body temperature. This center balances the excess heat production that derived from metabolic activity in muscle and the liver and  dissipation of heat from the skin and lungs.

Normal Body Temperature Variations

According to various studies on healthy individuals  in the age range of 18–40 years of the mean oral temperature is [36.8 (+/-) 0.4]°C, or [98.2 (+/-) 0.7]°F, with low levels at 6 a.m. and higher levels at 4–6 p.m. The maximal normal oral temperature is 37.2°C (98.9°F) at 6 a.m. and 37.7°C (99.9°F) at 4 p.m.; these values of temperature are mostly seen in healthy individuals.

What is Fever?

An a.m. temperature of >37.2°C (>98.9°F) or a p.m. temperature of >37.7°C (>99.9°F) would define a fever.

The normal daily temperature variation, also known as the circadian rhythm, is typically 0.5°C (0.9°F). However, in some people recovering from a febrile illness, this daily variation can be as great as 1.0°C.

Fever: Points To Consider

During a febrile illness, the diurnal variation is usually maintained, but at higher, febrile levels.

The daily temperature variation appears to be fixed in early childhood; but elderly individuals can exhibit a reduced ability to develop fever, with only a modest fever even in severe infections. So, in an elderly person a slight increase of body temperature than normal can be a serious indication of underlying infection.

Rectal temperatures are generally 0.4°C (0.7°F) higher than oral temperatures. The lower oral temperature readings are probably attributable to mouth breathing, which is a factor in patients with respiratory infections and rapid breathing.

In women who menstruate or you may say who are in their reproductive age, the a.m. temperature is generally lower during the 2 weeks before ovulation; it then rises by ~0.6°C (1°F) with ovulation and stays at that level until menses occur.

During the luteal phase of menstrual cycle, the amplitude of the circadian rhythm remains the same. So, we can say, this is a physiological variation of temperature according to our menstrual cycle.

So, before diagnosing a fever in the women, one should always consider these physiological facts. This physiological change of temperature as per menstrual cycle is normal.

Fever Symptoms

Fever itself is a symptom of various diseases, be it known or unknown. There are some additional symptoms that are usually found in association with fever. The signs and symptoms are as following

  • Increase of body temperature
  • Feeling chills and shivering
  • Rapid Breathing
  • Headache
  • Muscle pain
  • Loss of taste
  • Loss of appetite
  • Fatigue
  • Feeling Sleepy
  • Irritability or mood changes
  • Dehydration
  • Anxiety

Why Fever Occur

Fever is an elevation of body temperature that exceeds the normal daily variation and occurs in conjunction with an increase in the hypothalamic set point (e.g., from 37°C to 39°C).

Now, you can ask that what is hypothalamic set point? Our regulation of bodily temperature is maintained by hypothalamus of brain, so there is a set point of hypothalamus. This set point of temperature determines what will be the core temperature of our body.

Based on this set point of temperature of hypothalamus, our body system works to either increase the temperature or decrease the temperature. If our body finds that set temperature of hypothalamus is higher than the core body temperature, then body system works in such a way so that the core body temperature increase and match with the set temperature of hypothalamus.

Similarly, the opposite is also true. When the hypothalamic set point of temperature is lower than the core body temperature, the body system works in such a way to decrease the core body temperature to match the temperature with hypothalamic set point of temperature.

Once the hypothalamic set point is raised, neurons in the vasomotor center are activated and constriction of blood vessels occur. The individual first notices constriction of the blood vessels in the hands and feet. Shunting of blood away from the periphery to the internal organs essentially decreases heat loss from the skin, and the person feels cold.

For most fevers, body temperature increases by 1–2°C. So, shivering, which increases heat production inside our body from the muscles, may begin at this time; however, shivering is not required if mechanisms of heat conservation raise blood temperature sufficiently.

So, shivering is not found in fever of all cases. Where there is need to increase the core body temperature or better to say, when bodily systems of heat conservation mechanism is not sufficient enough to elevate the core body temperature according to the set temperature of hypothalamus, only then shivering occurs as an additional mechanism to raise body temperature.

Non shivering heat production from the liver also contributes to increase core temperature of our body. Behavioral adjustments (e.g., putting on more clothing or bedding) help raise body temperature by decreasing heat loss.

The processes of heat conservation (vasoconstriction) and heat production (shivering and increased non shivering thermogenesis) continue until the temperature of the blood bathing the hypothalamic neurons matches the new “thermostat setting.”  

So, once that point is reached, the hypothalamus maintains the temperature at the febrile level by the same mechanisms of heat balance that function in the afebrile (when there is no fever) state of our body.

When the hypothalamic set point is again reset downward (in response to either a reduction in the concentration of pyrogens or the use of antipyretics), the processes of heat loss through dilation of blood vessels (vasodilation) and sweating are initiated.

Loss of heat by sweating and vasodilation continues until the blood temperature at the hypothalamic level matches the lower setting.

Behavioral changes (for example, removal of clothing) facilitate heat loss.

A fever of >41.5°C (>106.7°F) is called hyperpyrexia. This extraordinarily high fever can develop in people with severe infections but most commonly occurs in people with central nervous system (CNS) hemorrhages.

In rare cases, the hypothalamic set point is elevated as a result of local trauma, hemorrhage, tumor, or intrinsic hypothalamic malfunction.

The term hypothalamic fever is sometimes used to describe elevated temperature caused by abnormal hypothalamic function.

However, most patients with hypothalamic damage have lower than normal(subnormal),  body temperatures. So, people with hypothalamus damage generally does not have greater than normal (supranormal) body temperature.

Although most patients with elevated body temperature have fever, but there are circumstances in which elevated temperature represents not fever but hyperthermia, also called as heat stroke.

How to check fever

You will be surprised to know that various body sites can be used to check fever including mouth (oral), tympanic membrane, or rectum temperature.

Electronic device or thermometer can be used to measure temperature in these sites, but the same site should be used consistently to monitor fever or a febrile disease.  For example, if oral temperature is measured to know if there is any increase of body temperature, then monitoring of body temperature should be done based on this oral temperature.

Because different body sites (oral, rectal, etc.) show variation of body temperature, so oral temperature does not match with rectal temperature. For this reason, it is highly important to maintain the same body site to measure and monitor the temperature reading.

Also, it is important to note, one should be aware that newborns, elderly patients, patients with chronic hepatic or renal failure, and patients taking glucocorticoids drugs or being treated with an anticytokine therapy may have active infection in the absence of fever because of a blunted febrile response in these patients.

So, people without fever can have active infection in the above mentioned situations. Thus, one without fever should not be considered as having no infection, there can be presence of infection. So, fever is not a reliable sign to determine if there is any infection present or not.

Fever Lab Tests

Laboratory tests are helpful to diagnose as well as to determine the cause of fever. The tests are the following

1. A complete blood count test. A differential count of WBC should also be performed. A complete blood count test and differential count test gives indication about the cause of fever.

2.  C- reactive protein and ESR- In patients with low-grade fevers or with suspected occult disease, the most valuable measurements are the C-reactive protein (CRP) level and the erythrocyte sedimentation rate (ESR). All these markers of inflammatory processes are particularly helpful to detect undiagnosed disease.

3. Measurement of circulating cytokines in patients with fever is not helpful since levels of cytokines such as IL-1 and TNF in the circulation often are below the detection limit of the assay or do not coincide with fever.

Fever coming with specific pattern

Some infections have characteristic patterns, here febrile episodes are separated by intervals of normal temperature.

For example, Plasmodium vivax causes fever every third day, whereas fever occurs every fourth day with Plasmodium malariae.

Another relapsing fever is related to Borrelia infection, with days of fever followed by a several-day afebrile (with no fever) period and then a relapse into additional days of fever.

In the Pel-Ebstein pattern, fever lasting 3–10 days is followed by afebrile or no fever periods of 3–10 days; this pattern can be classic for Hodgkin’s disease and other lymphomas.

In cyclic neutropenia, fever occurs every 21 days and accompany the neutropenia.

There is no periodicity of fever in patients with familial Mediterranean fever. However, these patterns have limited or no diagnostic value compared with specific and rapid laboratory tests.

Fever Treatment

It is vital to know the cause of fever. The objectives in treating fever are first to reduce the elevated hypothalamic set point and second to facilitate heat loss from the body. Reducing fever with antipyretics also reduces systemic symptoms of headache, muscle pain (myalgias), and joint pain (arthralgias). So, antipyretics (for example, paracetamol) is given to the patient with fever.

Treatment of fever in some patients is highly recommended and sometimes immediate management is crucial. It is also important to consider fever of unknown origin can be a potential cause of fever, in this case the treatment is based upon the laboratory test results.

 Why treatment is needed in fever?

1. Fever increases the demand for oxygen (i.e., for every increase of 1°C over 37°C, there is a 13% increase in oxygen consumption) and can aggravate or one can say worsen the condition of patients with preexisting impairment of heart (cardiac), lung (pulmonary), or brain and spinal cord (CNS) function.

2. Children with a history of febrile or nonfebrile seizure should be aggressively treated to reduce fever. Otherwise there is chance of seizure development. Though it is unclear what triggers the febrile seizure, and also there is no correlation between absolute temperature elevation and onset of a febrile seizure in susceptible children. Still, it is better to reduce fever to decrease the possibility for seizure development.

In hyperpyrexia, the use of cooling blankets facilitates the reduction of temperature; however, cooling blankets should not be used without oral antipyretics. In hyperpyretic patients with CNS disease or trauma (CNS bleeding), reducing core temperature mitigates the detrimental effects of high temperature on the brain.


  1. Goldman-Cecil Medicine (From Page 3076 to Page 3079)
  2. Harrison’s Principles of Internal Medicine 20th edition
  3. Guyton and Hall Textbook of Medical Physiology
  4. Davidson’s Principles and Practice of Medicine 23rd edition
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