Body Temperature

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Test Overview

Body temperature is a measure of your body's ability to make and get rid of heat. The body is very good at keeping its temperature within a safe range, even when temperatures outside the body change a lot.

  • When you are too hot, the blood vessels in your skin widen to carry the excess heat to your skin's surface. You may start to sweat. As the sweat evaporates, it helps cool your body.
  • When you are too cold, your blood vessels narrow. This reduces blood flow to your skin to save body heat. You may start to shiver. When the muscles tremble this way, it helps to make more heat.

Your body temperature can be measured in many places on your body. The most common ones are the mouth, the ear, the armpit, and the rectum. Temperature can also be measured on your forehead.

Thermometers show body temperature in either degrees Fahrenheit (°F) or degrees Celsius (°C). In the United States, temperatures are often measured in degrees Fahrenheit. The standard in most other countries is degrees Celsius.

Normal body temperature

Most people think a normal body temperature is an oral temperature (by mouth) of 98.6°F (37°C). This is an average of normal body temperatures. Your normal temperature may actually be 1°F (0.6°C) or more above or below this. Also, your normal temperature changes by as much as 1°F (0.6°C) during the day, depending on how active you are and the time of day. Body temperature is very sensitive to hormone levels. So a woman's temperature may be higher or lower when she is ovulating or having her menstrual period.

A rectal or ear temperature reading will be a little higher than an oral reading. A temperature taken in the armpit will be a little lower than an oral reading. The most accurate way to measure temperature is to take a rectal reading.

Fever

In most adults, a fever is an oral temperature above 100.4°F (38°C) or a rectal or ear temperature above 101°F (38.3°C). 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:

  • Infection. This is the most common cause of a fever. Infections may affect the whole body or one body part.
  • Medicines. These include antibiotics, opioids, antihistamines, and many others. This is called a "drug fever." Medicines like antibiotics raise the body temperature directly. Other medicines keep the body from resetting its temperature when other things cause the temperature to rise.
  • Severe trauma or injury. This may include heart attack, stroke, heatstroke, or burns.
  • Other medical conditions. These include arthritis, hyperthyroidism, and even some cancers, such as leukemia and lung cancer.

Low body temperature (hypothermia)

A very low body temperature (hypothermia) can be serious or even deadly. Low body temperature usually happens from being out in cold weather. But it may also be caused by alcohol or drug use, going into shock, or certain disorders such as diabetes or low thyroid.

A low body temperature may occur with an infection. This is most common in newborns, older adults, or people who are frail. A very bad infection, such as sepsis, may also cause an abnormal low body temperature.

High body temperature (heatstroke)

Heatstroke occurs when the body fails to control its own temperature and body temperature keeps rising. Symptoms of heatstroke include mental changes (such as confusion, delirium, or unconsciousness) and skin that is red, hot, and dry, even under the armpits.

Heatstroke can be deadly. It needs emergency medical treatment. It causes severe dehydration and can cause body organs to stop working.

There are two types of heatstroke.

  • Classic heatstroke can happen even when a person isn't doing much, as long as it's hot and the body isn't able cool itself well enough by sweating. The person may even stop sweating. Classic heatstroke may develop over several days. Babies, older adults, and people who have chronic health problems have the greatest risk of this type of heatstroke.
  • Exertional heatstroke may happen when a person is working or exercising in a hot place. The person may sweat a lot, but the body still makes more heat than it can lose. This causes temperature to rise to high levels.

Why It Is Done

Body temperature is measured to:

  • Check for fever.
  • Check for a very low body temperature in people who have been exposed to cold.
  • Check for a very high body temperature in people who have been exposed to heat.
  • Find out how well a fever-reducing medicine is working.
  • Help a woman plan for pregnancy by finding out if she is ovulating.

How To Prepare

Take your temperature a few times when you are well. This will help you find out what is normal for you. Check your temperature in both the morning and evening. Body temperature can vary by as much as 1°F (0.6°C) during the day.

Before you take your temperature:

  • Wait at least 20 to 30 minutes after you smoke, you eat, or you drink a hot or cold liquid.
  • Wait at least an hour after hard exercise or a hot bath.

There are different types of thermometers.

  • Electronic thermometers are plastic and shaped like a pencil. They have a display window at one end and the temperature probe at the other end. These thermometers can be used in the mouth, rectum, or armpit. They are easy to use and easy to read. If you buy this type of thermometer, check the package for information about its accuracy.
  • Ear thermometers are plastic and come in different shapes. The small cone-shaped end of the thermometer is placed in the ear. Body temperature is shown on a digital display. The results appear in seconds. Some models also show what the oral and rectal readings would be.
  • Temporal artery thermometers have a small "cup" that is moved across the skin over the artery in the forehead. When used correctly, these thermometers are accurate.
  • Disposable thermometers are thin, flat pieces of plastic with colored dots and temperature markings on one end. The color of the dots shows the temperature. These thermometers can be used in the mouth or rectum. A patch form can be used on a baby's skin to measure temperature for 48 hours straight. These thermometers are not as accurate as electronic or ear thermometers.
  • Forehead thermometers are thin pieces of plastic with numbers on them. You press the strip against a person's forehead. The temperature makes some numbers change colors or light up. These thermometers are not very accurate.
  • Pacifier thermometers are shaped like a baby's pacifier. They have a display that shows the temperature. You place the pacifier in your child's mouth to measure temperature. These thermometers may take longer to get a reading and are not as accurate as other types.

Glass thermometers that contain mercury are not recommended. If you have a glass thermometer, contact your local health department to find out how to dispose of it safely. If you break a glass thermometer, call your local poison control center right away.

How It Is Done

Before you take a temperature, read the instructions for how to use your type of thermometer. Some common ways to take a temperature are described below.

How to take an oral temperature

Oral (by mouth) is the most common method of taking a temperature. For you to get an accurate reading, the person must be able to breathe through his or her nose. If this is not possible, use the rectum, ear, or armpit to take the temperature.

  1. Place the thermometer under the tongue, just to one side of the center. Ask the person to close his or her lips tightly around it.
  2. Leave the thermometer in place for the required amount of time. Time yourself with a clock or watch. Some digital thermometers give a series of short beeps when the reading is done.
  3. Remove the thermometer and read it.
  4. Clean a digital thermometer with cool, soapy water and rinse it off before you put it away.

How to take a rectal temperature

This is the most accurate way to measure body temperature. It is recommended for babies, small children, and people who can't hold a thermometer safely in their mouths. It is also used when it is very important to get the most accurate reading.

  1. Apply a lubricant jelly or petroleum jelly, such as Vaseline, on the bulb of the thermometer. This will make it easy to insert.
  2. With a baby or small child, turn the child facedown on your lap or on a flat covered or padded surface, such as a bed. Choose a quiet place so that the child won't be distracted or move around too much.
  3. Spread the child's buttocks with one hand. With the other hand, gently insert the bulb end of the thermometer into the anus. Push it in about 0.5 in. (1.25 cm) to 1 in. (2.5 cm). Don't force it into the rectum. Hold the thermometer in place with two fingers close to the anus (not near the end of the thermometer). Pressing the child's buttocks together will help keep the thermometer in place.
  4. Leave the thermometer in place for the required amount of time. Time yourself with a watch or clock. Some digital thermometers give a series of short beeps when the reading is done.
  5. Remove the thermometer and read it.
  6. Clean a digital thermometer with cool, soapy water and rinse it off before you put it away.

Do not use a thermometer to take an oral temperature after it has been used to take a rectal temperature.

How to take an armpit (axillary) temperature

Taking a temperature in the armpit may not be as accurate as taking an oral or rectal temperature.

  1. Place the thermometer under the arm with the bulb in the center of the armpit.
  2. Press the arm against the body, and leave the thermometer in place for the required amount of time. Time yourself with a watch or clock.
  3. Remove the thermometer and read it. An armpit temperature reading may be as much as 1°F (0.6°C) lower than an oral temperature reading.
  4. Clean a digital thermometer with cool, soapy water and rinse it off before you put it away.

How to take an ear (tympanic) temperature

Ear thermometers may need to be cleaned before they are used.

  1. Check that the probe is clean and free of debris. If dirty, wipe it gently with a clean cloth. Do not put the thermometer underwater.
  2. To keep the probe clean, use a disposable probe cover. Use a new cover each time you take an ear temperature.
  3. Turn on the thermometer.
  4. For babies younger than 12 months, gently pull the earlobe down and back. For children older than 12 months and for adults, pull the earlobe up and back. This will help you place the probe in the ear canal.
  5. Center the probe tip in the ear, and push gently inward toward the eardrum. Do not force it in.
  6. Press the "on" button to display the temperature reading.
  7. Remove the thermometer, and throw away the used cover.

How to take a temporal artery temperature

  1. Remove the cap over the cup part of the thermometer, if it has a cap.
  2. Turn on the thermometer.
  3. Place the thermometer cup on the skin in the center of the forehead. Make sure nothing is between the thermometer cup and the skin.
  4. Press the button for making a measurement.
  5. Slide the thermometer across the forehead to one side (not up or down).
  6. Listen for a sound. Most of these thermometers make a beep or other sound when they are ready to read.
  7. Remove the thermometer from the forehead, and read the temperature.

How to take a forehead temperature

Forehead thermometers are not as accurate as electronic and ear thermometers. If your baby is younger than age 3 months or your child's fever rises higher than 102°F (39°C), check the temperature again using a better method.

  1. Press the entire plastic strip firmly against a dry forehead.
  2. Hold the strip in place for the required amount of time. Time yourself with a watch or clock.
  3. Read the temperature before removing the thermometer.
  4. Clean the thermometer with cool soapy water and rinse it off before you put it away.

How to use a pacifier thermometer

Pacifier thermometers are not as accurate as electronic and ear thermometers. If your baby is younger than age 3 months or your child's fever rises higher than 102°F (39°C), check the temperature again using a better method.

  1. Some pacifier thermometers can be used as regular pacifiers. Attach the temperature part if you need to.
  2. Let your child suck on the nipple for the required amount of time. Time yourself with a watch or clock.
  3. Remove the pacifier, and read the temperature.
  4. Clean the pacifier with cool, soapy water and rinse it off before you put it away.

How It Feels

Taking an oral temperature causes only mild discomfort. You have to keep the thermometer under your tongue and hold it in place with your lips.

Taking a rectal temperature can cause a little discomfort, but it should not be painful.

Taking an ear temperature causes little or no discomfort. The probe is not inserted very far into the ear, and it gives a reading in only a few seconds.

Taking a temporal artery, forehead, or armpit temperature does not cause any discomfort.

Risks

There is very little chance of a problem from taking a temperature.

When taking a rectal temperature, do not push the thermometer in more than 0.5 in. (1.25 cm) to 1 in. (2.5 cm). Pushing it farther can be painful and may damage the rectum.

Results

Body temperature is a measure of your body's ability to make and get rid of heat.

If you tell your doctor about your temperature reading, be sure to say where it was taken: on the forehead or in the mouth, rectum, armpit, or ear.

Body temperature
Normal:

The average normal temperature is 98.6°F (37°C). But that may not be normal for you. Your temperature also changes during the day. It is usually lowest in the early morning. It may rise 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 often changes by 1°F (0.6°C) or more through her menstrual cycle. It peaks around the time she ovulates.

Abnormal:

Oral, ear, rectal, or temporal artery temperature

  • Fever: 100.4°F (38°C) to 103.9°F (39.9°C)
  • High fever: 104°F (40°C) and higher

Armpit temperature

  • Fever: 99.4°F (37.4°C) to 102.9°F (39.4°C)
  • High fever: 103°F (39.5°C) and higher

A rectal or ear temperature of less than 97°F (36.1°C) is a low body temperature (hypothermia).

What Affects the Test

A temperature reading may not be accurate if:

  • You don't keep your mouth closed around the thermometer when taking an oral temperature.
  • You don't leave the thermometer in place long enough before you read it.
  • You don't put the thermometer in the right place.
  • You don't follow the instructions that come with the thermometer.
  • The thermometer has a weak or dead battery.
  • You take your oral temperature within 20 minutes after smoking or after drinking a hot or cold liquid.
  • You take your temperature within an hour of exercising hard or taking a hot bath.

What To Think About

  • Thermometers with a digital display usually have a battery. If your thermometer uses a battery, make sure it is working before you take the temperature.
  • Glass thermometers that contain mercury are not recommended. If you have a glass thermometer, contact your local health department to find out how to dispose of it safely. If you break a glass thermometer, call your local poison control center right away.
  • A fever can make you feel uncomfortable. To reduce discomfort, wear light clothing and use light bedding. A lukewarm (not cool) bath or shower can lower body temperature. A fever can also lead to dehydration, so it is important to drink plenty of fluids.

To learn more about fever, see:

References

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.

Credits

ByHealthwise Staff

Primary Medical ReviewerSusan C. Kim, MD - Pediatrics

Adam Husney, MD - Family Medicine

Martin J. Gabica, MD - Family Medicine

Kathleen Romito, MD - Family Medicine

Specialist Medical ReviewerJohn Pope, MD - Pediatrics

David Messenger, MD

Current as ofMarch 20, 2017

Current as of: March 20, 2017

Author: Healthwise Staff

Medical Review: Susan C. Kim, MD - Pediatrics & Adam Husney, MD - Family Medicine & Martin J. Gabica, MD - Family Medicine & Kathleen Romito, MD - Family Medicine & John Pope, MD - Pediatrics & David Messenger, MD