Body temperature is a measure of the body's ability to generate and get rid of heat. The body is very good at keeping its temperature within a narrow, safe range in spite of large variations in temperatures outside the body.
When you are too hot, the blood vessels in your skin expand (dilate) to carry the excess heat to your skin's surface. You may begin to sweat, and as the sweat evaporates, it helps cool your body. When you are too cold, your blood vessels narrow (contract) so that blood flow to your skin is reduced to conserve body heat. You may start shivering, which is an involuntary, rapid contraction of the muscles. This extra muscle activity helps generate more heat. Under normal conditions, this keeps your body temperature within a narrow, safe range.
Your body temperature can be measured in many locations on your body. The mouth, ear, armpit, and rectum are the most commonly used places. Temperature can also be measured on your forehead.
Thermometers are calibrated in either degrees Fahrenheit (°F) or degrees Celsius (°C), depending on the custom of the region. Temperatures in the United States are often measured in degrees Fahrenheit, but the standard in most other countries is degrees Celsius.
Most people think of a "normal" body temperature as an oral temperature of 98.6°F (37°C). This is an average of normal body temperatures. Your temperature may actually be 1°F (0.6°C) or more above or below 98.6°F (37°C). Also, your normal body temperature changes by as much as 1°F (0.6°C) throughout the day, depending on how active you are and the time of day. Body temperature is very sensitive to hormone levels and may be higher or lower when a woman is ovulating or having her menstrual period.
A rectal or ear (tympanic membrane) temperature reading is slightly higher than an oral temperature reading. A temperature taken in the armpit is slightly lower than an oral temperature reading. The most accurate way to measure body temperature is to take a rectal temperature.
In most adults, an oral temperature above 100°F (37.8°C) or a rectal or ear temperature above 101°F (38.3°C) is considered a fever. A child has a fever when his or her rectal temperature is 100.4°F (38°C) or higher.
A fever may occur as a reaction to:
An abnormally low body temperature (hypothermia) can be serious, even life-threatening. Low body temperature may occur from cold exposure, shock, alcohol or drug use, or certain metabolic disorders, such as diabetes or hypothyroidism. A low body temperature may also be present with an infection, particularly in newborns, older adults, or people who are frail. An overwhelming infection, such as sepsis, may also cause an abnormally low body temperature.
Heatstroke occurs when the body fails to regulate its own temperature, and body temperature continues to rise. Symptoms of heatstroke include mental changes (such as confusion, delirium, or unconsciousness) and skin that is red, hot, and dry, even under the armpits.
Classic heatstroke can develop without exertion when a person is exposed to a hot environment and the body is unable to cool itself effectively. In this type of heatstroke, the body's ability to sweat and transfer the heat to the environment is reduced. A person with heatstroke may stop sweating. Classic heatstroke may develop over several days. Babies, older adults, and people with chronic health problems have the greatest risk of this type of heatstroke.
Exertional heatstroke may develop when a person is working or exercising in a hot environment. A person with heatstroke from exertion may sweat profusely, but the body still produces more heat than it can lose. This causes the body's temperature to rise to high levels.
Both types of heatstroke cause severe dehydration and can cause body organs to stop functioning. Heatstroke is a life-threatening medical emergency, requiring emergency medical treatment.
Body temperature is checked to:
Take your temperature several times when you are feeling well to find out what is normal for you. Check your temperature in both the morning and evening, since body temperature can vary by as much as 1°F (0.6°C) throughout the day.
Wait at least 20 to 30 minutes after smoking, eating, or drinking a hot or cold liquid before taking your temperature. Also wait at least an hour after vigorous exercise or a hot bath.
Several different types of thermometers are available:
Glass thermometers containing mercury are no longer recommended. If you have a glass thermometer, contact your local health department for instructions on how to dispose of it safely. If you break a glass thermometer, call your local poison control center immediately.
Before taking a body temperature, review the instructions for how to use your thermometer. Methods of taking a temperature are described below.
Oral is the most common method of taking a temperature. To get an accurate temperature, the person must be able to breathe through the nose. If this is impossible because of a stuffy nose or lack of cooperation, use the rectum, ear, or armpit to take the temperature.
This is the location to measure body temperature most accurately. It is recommended for babies, small children, and people who cannot hold a thermometer safely in their mouths. It is also used when getting the most accurate measurement is essential.
See a picture of how to take the rectal temperature of a baby.
Taking a temperature in the armpit may not be as accurate as taking an oral or rectal temperature.
Ear thermometers may need to be cleaned and prepared for use. These steps can be followed when using an ear thermometer, but be sure to follow the instructions for your specific model.
Taking your temperature by mouth is only mildly uncomfortable, since you must keep your mouth closed and breathe through your nose while the thermometer is in place.
Taking a rectal temperature can be slightly uncomfortable but should not be painful.
Taking your temperature with an ear thermometer causes little or no discomfort. It is not inserted very far into the ear, and it provides a reading in only a few seconds. For this reason, the ear thermometer is widely used in doctor offices and hospitals. But it may be less accurate than rectal thermometers.
Taking your temperature with a plastic strip thermometer feels like having an adhesive bandage on your forehead. Although it causes very little discomfort, it is not as reliable as other methods, so another kind of thermometer should be used if the plastic strip shows an abnormal temperature.
There is very little risk of complications from taking a temperature.
When taking a rectal temperature, do not insert the thermometer into the rectum more than 0.5 in. (1.25 cm) to 1 in. (2.5 cm). Further insertion can be painful and may damage rectal tissues.
Body temperature is a measure of the body's ability to generate and get rid of heat.
When you tell your doctor about your temperature measurement, be sure to mention whether it was taken on the forehead or in the mouth, rectum, armpit, or ear.
The average normal temperature is 98.6°F (37°C). But "normal" varies from person to person. Your temperature will also vary throughout the day, usually being lowest in the early morning and rising as much as 1°F (0.6°C) in the early evening. Your temperature may also rise by 1°F (0.6°C) or more if you exercise on a hot day. A woman's body temperature typically varies by 1°F (0.6°C) or more through her menstrual cycle, peaking around the time of ovulation.
Oral, ear (tympanic), or rectal temperature
Armpit (axillary) temperature
A rectal or ear temperature of less than 97°F (36.1°C) means a low body temperature (hypothermia).
Inaccurate temperature readings can be caused by:
Other Works Consulted
- Auwaerter PG (2007). Approach to the patient with fever. In LR Barker et al., eds., Principles of Ambulatory Medicine, 7th ed., pp. 457–465. Philadelphia: Lippincott Williams and Wilkins.
- El-Radhi AS, Barry W (2006). Thermometry in paediatric practice. Archives of Disease in Childhood, 91(4): 351–356.
|Primary Medical Reviewer||Susan C. Kim, MD - Pediatrics|
|Specialist Medical Reviewer||Thomas Emmett Francoeur, MD, MDCM, CSPQ, FRCPC - Pediatrics|
|Last Revised||January 20, 2011|
Last Revised: January 20, 2011
Author: Healthwise Staff
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