Everything You Need to Know About Body Temperature
What is normal, high, or low temperature—and what does it mean? Learn the significance of the digits on your thermometer, and when it’s time to call your doctor.
If the first number that pops into your head when you think about body temperature is 98.6°F, you have a study from 1851 to thank for that. Carl Reinhold August Wunderlich, a German doctor, took multiple axillary (armpit) temperatures from about 25,000 people, and based on an analysis, he established 98.6°F (or 37°Celsius) as the norm.
The reality though is that normal body temperature is on a spectrum, not an absolute. Body temperature can depend on your age, the time of day, and what you were doing before you popped the thermometer in your mouth or zapped your forehead.
If you’re wondering whether your numbers are normal (and what to do if they’re not), or even if you’re using the thermometer the right way, consider this your crash course on everything body temperature-related.
What Is Normal Body Temperature?
Though physicians continue to learn about Wunderlich’s findings in medical school, modern-day researchers have found that 98.6°F is not entirely accurate when it comes to identifying a global average body temperature.
When a group of Stanford researchers analyzed temperature measurements taken during three historical periods—1860-1940; the 1970s; and 2007-2017—they found that the average temperature for healthy people is dropping slightly. And a 2017 British study conducted on more than 35,000 patients found that the mean oral temperature among participants was 36.6°Celsius, or 97.88°F.
Why the discrepancies? It’s not clear whether people’s bodies have gotten cooler over time, or if historical temperature-taking wasn’t as accurate as it is today. Either way, experts now agree that today’s average range for a normal temperature is between 97.5°F and 97.9°F, and it’s widely accepted that the spectrum for normal extends from 97°F to 99°F. It’s even possible for you to be a little hotter or colder than that and still be completely healthy.
What Does a High Temperature Mean?
Temperature is considered a vital sign, meaning it’s an important indicator of your health. Your hypothalamus is what keeps your body’s temperature on track. The hypothalamus is a part of the brain that not only controls body temperature, but also the hormones that affect hunger, thirst, sleep, heart rate, sex drive, and mood (talk about a multitasker!). For example, when you exercise and feel hot, the hypothalamus can tell your body to sweat, and sweating helps cool you down.
When your temperature goes above your normal spectrum, it can be a sign that your body is hard at work fighting off an infection or that you may have another health issue. Infection triggers your immune system to fight back using helpers that include white blood cells and antibodies.
Fever works in two ways:
It sets your immune system in motion to respond to an infection or other issue.
Raising your body’s temperature when you have an infection makes it more difficult for the bacteria or virus, such as the flu, to reamin alive in your body (they tend to prefer more normal body temperatures over higher temperatures).
“We start to qualify something as concerning over a 100.4°F,” says Avir Mitra, M.D., an assistant professor of emergency medicine at the Icahn School of Medicine in New York, Clerkship Site Director for Emergency Medicine at Mount Sinai Beth Israel, and spokesperson for the American College of Emergency Physicians.
High Temperature in Kids
Children can be more likely to spike a high fever because their immature bodies don’t yet sweat as easily as adults to help cool them down, so even normal activity can cause them to warm up. “We don’t get too worried when a kid is 103 degrees, 104 degrees,” Dr. Mitra says. “But if you’re an adult and you’re spiking a fever like that, there’s something going on.”
Still, even a slight fever can be a sign of a serious infection in infants or toddlers and merits a call to the pediatrician because their immune systems aren’t yet strong enough to fight off germs.
In a child six months to five years of age, there’s also the possibility of a febrile seizure, a.k.a. fever-induced convulsion. This can happen when a child’s temperature changes very quickly. These are frightening for a parent to witness because the child can lose consciousness and their arms and legs can shake, but febrile seizures typically don’t cause lasting effects.
More Causes of a High Temperature
Most often, a fever is your body’s attempt to kill an infection. Remember, bacteria and viruses have a harder time surviving in a warmer body, and fever activates the immune system. But there are many other causes of fever, such as:
Endocrine disorders. “If you’re secreting too much thyroid-stimulating hormone, you’re going to have a higher temperature,” says Lisa Moreno, M.D., professor of emergency medicine at Louisiana State University Health Sciences Center in New Orleans and president of the American Academy of Emergency Medicine. “If you’re secreting too little, your body temperature is going to be lower.”
Menstruation. A woman’s body temperature warms slightly after ovulation due to the release of progesterone to prepare the body for possible pregnancy. It lowers again when menstruation starts. Women who are pregnant also will have a slightly higher temperature and white blood cell count because their bodies are reacting to what it considers a foreign object—a baby.
Certain medications. Antibiotics, some blood pressure medicines, and seizure drugs are on this list. Taking psychotropic or antidepressant medications may also cause you to become overly hot or dehydrated. The same effects can be an issue for elderly people who take diuretics.
Immunizations. Getting the pneumococcal vaccine or the DTaP (diphtheria/tetanus/pertussis) shot can cause a rise in temperature. Fevers are a normal part of the immune response to these and other vaccines.
Age. The very old and the very young can’t regulate their temperature as well as people in other age brackets. It’s even possible to bundle up a baby or an elderly person too much, which can cause a fever.
Stroke. A brain abnormality, such as a stroke, can cause difficulties with regulating temperature in rare cases.
Risks of High Body Temperature
“The reason that we have concerns about different temperatures is because the human body functions best at what we call normal temperatures or normothermia,” Dr. Moreno says. “Once you enter hyperthermia (too hot), inflammatory responses start, and inflammatory responses involve the secretion of a whole lot of different chemicals in the body that actually act to destroy cells.” Though it’s generally not a problem if you have a short-lived fever, the same fever that kills bacteria can also kill some healthy cells, she adds.
Hyperthermia can occur in a variety of situations, from the teen playing sports in hot, humid conditions to the bed-ridden senior in a very hot house without air conditioning. Both scenarios are dangerous, Dr. Mitra says, and require immediate medical care. Hyperthermia can be life-threatening. When the body becomes overwhelmed by a high temperature (typically over 104°F) and can’t regulate it on its own, it is called heat stroke.
Quick ways to cool down while waiting for medical care include:
Being sprayed with cool water
Sitting in front of a fan
Placing ice cubes or a cold pack in the armpits, groin, and around the neck, which are places that can become very warm
“You try to bring that temperature down as aggressively as you can,” Dr. Mitra says.
How to Lower a Fever
When your fever isn’t alarmingly high and doesn’t last longer than a few days, you can reduce it at home with fever-reducing medicine, such as acetaminophen or ibuprofen. These drugs stop the body from producing prostaglandins, naturally occurring hormones that cause a chain reaction leading to fever, among many other responses. Be sure to follow the instructions on the bottle.
Though children can take ibuprofen or acetaminophen with a fever, it’s important to consult your pediatrician’s office to determine proper dosage or scheduling. It's best not to give them adult aspirin, which may lead to an illness called Reye syndrome.
“It’s important to get higher fevers down because of the metabolic damage that they can cause, including to the brain,” Dr. Moreno says. “Over the long term, any kind of an inflammatory response, including fever, can cause abnormal functions like abnormal mitosis of cells, which can result in cancer. Those are things that researchers are looking at now.”
Causes for Concern
If you or another adult in your family has a fever, call your doctor if the fever reaches 103°F. Some symptoms that may accompany a fever merit medical attention, including:
Stiff neck and pain when you bend your head forward
Unusual or rapidly worsening skin rash
Pain when urinating
Unusual sensitivity to bright light
“It's never wrong to check in with your doctor and say ‘I'm having these symptoms.’ And then what the doctor will probably say is wait it out for a little while, see if it goes away, and if not, come in and we'll test you,” Dr. Mitra says.
Call your pediatrician if your child has a fever of any level and:
Is listless or irritable
Has severe headache or stomachache
Can’t maintain eye contact with you
Has had a fever for more than three days
Has been left in a hot car
If your child is still an infant or toddler, call the doctor if:
Your baby is younger than 3 months and has a rectal temperature of 100.4°F or higher.
Your baby is 3 to 6 months old and has a rectal temperature up to 102°F and seems unusually irritable or lethargic.
Your child is 6 to 24 months old and has a rectal temperature higher than 102°F that lasts longer than a day, even without other symptoms. Call sooner if the child has a cough, cold, or diarrhea.
What Low Body Temperature Means
Just as your hypothalamus tells you to sweat when your body temp rises, your brain can also tell you what to do when you’re cold. For example, it tells your body to shrink blood vessels and shiver, which helps it preserve or create heat by rapidly contracting and releasing your muscles. Goose bumps are another temperature message from the brain, which signals to the hair follicles to trap warm air, resulting in the telltale bumps on your skin.
Causes of lower body temperature can range from situational (you’re caught in a blizzard) to biological (you’re getting older). These are the most common reasons:
Age. As you get older, your body sweats less and loses heat more rapidly, resulting in lower average temperatures.
Hypothyroidism. This endocrine disorder means that your thyroid is underactive. This results in a slower metabolism, which keeps your temperature at a lower level.
Exposure to cold. There’s no escaping winter snowstorms, but if you are out in the cold for extended periods of time (an hour or more) it can lead to a very serious condition called hypothermia.
“Pay attention to a lower temperature,” Dr. Mitra says. “In older people who may not have strong immune systems, sometimes getting cold is a sign of a really bad illness.”
Hypothermia, or an extreme drop in body temperature, is dangerous because it causes your metabolism to slow. This prevents your organs, notably your heart, from working properly. If body temperature drops below 95°F, it can be fatal. You can develop hypothermia from being cold and wet for extended periods of time, and from being in cold water for too long. Infants can even become hypothermic from sleeping in a too-cold room.
Being very old or very young, chronically ill with heart or blood flow problems, and malnourished all make you more likely to develop hypothermia. Certain prescription medications or being under the influence of alcohol or drugs may also make you more susceptible to hypothermia. (Alcohol blunts your response to the cold, so you may not shiver or recognize if you become dangerously cold.)
If you or someone with you becomes hypothermic, remove any wet clothes, bundle up in dry, warm clothes, and call 911.
How to Take Your Temperature
Your best bet for the most accurate temperature is taking it internally. For an infant, that’s a rectal thermometer, because proper placement of an oral thermometer in a very young child can be difficult if not impossible to achieve.
For older children and adults, a good reading is possible using an oral thermometer placed under the tongue.
So, what about those “contactless” thermometers that the COVID-19 pandemic has popularized that zap your forehead? Thermometers that read your forehead from a distance are less accurate than those that touch your skin directly, research shows. If you’re taking a peripheral body temperature, it’s important to know that it could be off by a degree.
We’ve come a long way from the days of 98.6°F being the universal temperature standard. Knowing what’s “normal” for you when you’re feeling fine will help you know when a rise or decrease merits attention.
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Fever and Vaccine: Children’s Hospital of Philadelphia. (2021.) “Fevers and Vaccines.” https://www.chop.edu/centers-programs/vaccine-education-center/vaccine-safety/fever-and-vaccines
Prostaglandins: The Endocrine Society. (2018.) “What Is Prostaglandins?” https://www.hormone.org/your-health-and-hormones/glands-and-hormones-a-to-z/hormones/prostaglandins