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.
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)
If a low body temperature is your only symptom, it is not something to worry about. If a low body temperature occurs with other symptoms, such as chills, shaking, breathing problems, or confusion, then this may be a sign of more serious illness.
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 dehydrationand can cause body organs to stop working.
- 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.