Test Overview

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 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.

Where is body temperature measured?

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.

What are Fahrenheit and 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?

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.

What is a fever?

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.