What is body temperature?

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 the skin's surface. You may begin to sweat, and as the sweat evaporates it helps cool your body. When you are too cold, the blood vessels 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, these processes effectively regulate your body temperature.

What areas of the body are used to check temperature?

Your body temperature can be measured in the mouth, ear, armpit, or rectum. Temperature can also be measured on your forehead.

Why are thermometers in Fahrenheit or Celsius?

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.

What is normal body temperature?

A normal body temperature is usually referred to as an oral temperature of 98.6 °F(37 °C), but that is an average of normal body temperatures. Many healthy people have a temperature 1°F (0.6°C) or more above or below 98.6 °F(37 °C). Also, normal body temperature varies by 0.5°F (0.3°C) to 1°F (0.6°C) throughout the day, depending on a person's activity level, the time of day, and other variables, including, for instance, whether a woman is ovulating or having her menstrual period.

A rectal or ear (tympanic membrane) temperature reading is 0.5°F (0.3°C) to 1°F (0.6°C) higher than an oral temperature reading. An armpit temperature reading is 0.5°F (0.3°C) to 1°F (0.6°C) lower than an oral temperature reading.

Rectal, ear, oral, and axillary temperature comparison

To determine if a fever is present, an accurate body temperature is needed. Medical research has not determined an exact correlation between oral, rectal, ear (tympanic), and armpit (axillary) temperature measurements. Generally, the correlation of temperature results are as follows:

• The average normal oral temperature is 98.6° (37°). An oral temperature is 0.5°F (0.28°C) to 1°F (0.56°C) lower than a rectal or ear (tympanic) temperature.
• A rectal temperature is 0.5°F (0.28°C) to 1°F (0.56°C) higher than an oral temperature.
• An ear (tympanic) temperature is 0.5°F (0.28°C) to 1°F (0.56°C) higher than an oral temperature.
• An armpit (axillary) temperature is usually 0.5°F (0.28°C) to 1°F (0.56°C) lower than an oral temperature.

It is important to remember:

• Rectal temperatures are generally thought to be the most accurate for checking a young child's temperature.
• The manufacturer of the temperature device you use, such as a tympanic thermometer, provides information on how to use it. Be sure to read and follow the instructions to obtain an accurate temperature. The information may also include how the results of the device correlate with the results from other methods of taking a temperature.
• Plastic strip thermometers have some uses, but they are not recommended for general home use. Unlike oral, rectal, and ear thermometers, plastic strip thermometers measure skin temperature, not body temperature.

When you talk with your health professional about your temperature, be sure to say what method was used to take the temperature.

Temperature comparison table

The temperature comparison table below will give you the range of temperature correlation with the different methods used to take a temperature. For information about taking accurate temperatures in infants and children, see the topic Body Temperature.

The temperature comparison table lists only the temperature ranges found in the questions in the Check Your Symptoms section of the Fever topics so that you can answer the questions accurately.

Rectal temperature guidelines are used in Fever, Age 3 and Younger.

Oral temperature guidelines are used in Fever, Age 4 and Older.

To use the table:

• Find the method that you used to take a temperature.
• Find the correct temperature range.
• See the correlating rectal or oral temperature range to help you answer the Check Your Symptoms questions.
  • For example, in Fever, Age 3 and Younger: If your 2-year-old child's oral temperature is 101° (38.33°), his or her rectal or ear temperature may be about 102° (38.89°). Remember, a child has a fever when his or her temperature is 100.4° (38°) or higher, measured rectally.
  • For example, in Fever, Age 4 and Older: If your axillary temperature is 100° (37.78°), your oral temperature is about 101° (38.33°).

What is a fever?

In adults, an oral temperature above 100 °F(37.8 °C) or a rectal or ear temperature above 101 °F(38.3 °C) is sometimes considered a mild fever. A child has a fever when his or her rectal temperature is 100.4 °F(38 °C) or higher.

What can cause a fever?

A fever may occur as a reaction to:

• Infection. This is a common cause of a fever. Infections may affect the whole body or a specific body part (localized infection).
• Medications such as antibiotics, narcotics, barbiturates, antihistamines, and many others. These are called drug fevers. Some medications, such as antibiotics, raise the body temperature directly; others, including nonsteroidal anti-inflammatory drugs (NSAIDS) and phenothiazines (such as Compazine, Mellaril, or Thorazine), interfere with the body's ability to adjust its temperature when other factors cause the temperature to rise.
• Severe trauma or injury, such as a heart attack, stroke, heat exhaustion or heatstroke, or burns.
• Other medical conditions, such as arthritis, hyperthyroidism, and even some cancers (such as leukemia, Hodgkin's lymphoma, and liver and lung cancer).

Can a low body temperature be dangerous?

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. Oddly, a low body temperature may also occur in certain infections, particularly in newborns, older adults, or people who are frail. Certain overwhelming infections, such as Gram-negative sepsis, may also cause an abnormally low body temperature.

Since you are a warm-blooded animal, your body attempts to keep its internal temperature constant. Human life is only compatible with a narrow range of temperatures:

Sources:

Medical Review: May 28, 2003

Medical Review: August 26, 2004