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What Every Family Needs to Know About Thermometers and Fevers

Get an accurate temperature reading at every age to track your family's fevers.

Your body temperature is a vital sign. It’s routinely taken at almost every medical appointment. At home, it’s the first thing you check for signs of illness in you and your family.

For something so common and critical, there’s still a lot of confusion around taking a temperature and what that number really means.

Mom checking her child's temperature with a thermometerThere are many ways to take a temperature.

We’re past the unreliable methods of a hand on the forehead or a mercury thermometer in the mouth. Today, there are many types of thermometers that read your body temperature in different ways. So many that it can be a struggle to sift through all the information and select the best thermometer for your family.

“The most common types are your digital thermometers, which are the ones you can use orally, rectally or axillary,” says Allison Whitney, MD, pediatrician at The Iowa Clinic’s Ankeny campus. “In more recent years, you have the tympanic ones — the ones you stick in the ear — and temporal artery scanners where you can just point it or roll it over the forehead.”

Digital Thermometers

All modern thermometers give you a digital reading. Regular digital thermometers are the closest thing to the old mercury-in-glass thermometers that sat under your tongue. They are similarly small but much more versatile and accurate. You can use them to get a good reading in three different ways:

  • In the mouth – Place the thermometer under the tongue and hold it with a closed mouth until you get a reading.
  • Under the arm – When it’s too difficult to get an oral temperature, like with small children, you can put the thermometer in the armpit. Hold the arm down and tight until the temp is read.
  • In the rectum – This method actually gets the internal body temperature you’re looking for but is primarily used on infants where it’s harder to get a good reading in one of the other ways.

“Rectally is typically the way in the baby months. It can be used for older kids but parents typically don't feel comfortable doing that,” says Dr. Whitney. “And then axillary is under the arm. It’s hard because you have to keep a child’s arm down so it's better used as kind of a screening as opposed to the sole method of taking a temperature.”

Ear Thermometers

A tympanic thermometer is one you stick in the ear. It’s what you’ll find at a lot of clinics because it is quick, comfortable and easy. Tympanic thermometers measure the inside of your ear by measuring the infrared heat.

“The ear thermometers are popular with families but are largely unreliable in kids under six months of age just because of the size of the ears of babies. They may still sometimes be used for screening purposes, as in the clinic,” Dr. Whitney says.

Forehead Thermometers

Temporal thermometers also measure infrared heat to take body temperature. You place them on the temporal artery, a blood vessel that runs across the forehead and sits just below the skin. Depending on the temporal thermometer, you can either point it directly at one spot on the forehead or roll it from the middle of the forehead to the temple. 

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The best thermometer is the one that’s most accurate for you.

Since there’s no standard way of taking a temperature, readings vary across devices. There’s a lot of debate about whether you should add or subtract a degree, depending on the type of thermometer or where the reading was taken.

“Don’t do the math. Don’t try to add or subtract. Just take the temperature and let us take into consideration how it was taken because there’s no standard way,” Dr. Whitney advises.

Accuracy is important. And the best thermometer for you or your family is the one that gives you the most accurate reading. That has to do with a lot more than some simple math, Dr. Whitney says.

“There are ways that are more and less reliable. Under the tongue or armpit can be unreliable because if you get a kid talking or moving their arm, it disrupts the reading,” she says. “Orally, if you’ve been eating or drinking within 10 or 15 minutes before, you open your mouth or are a mouth breather, it can throw things off too.”

For older kids and adults, thermometers are on more equal footing. They can follow direction and sit still long enough to get a good reading most any way. Still, you have to follow the instructions of your specific thermometer exactly to get the most accurate reading.

“One of the other methods — in the ear or on the forehead — are probably more trustworthy just because of the way we live life, not necessarily because of the equipment itself,” Dr. Whitney says.

100.4 degrees is considered a fever.

No more. No less. No matter the method. That’s the threshold.

“98.6 degrees is normal body temperature. It’s an average so not everyone’s going to sit there every day. But a fever is always 100.4 degrees or higher. That’s our real threshold for doing further work-up,” Dr. Whitney says.

Many people think a low-grade fever is anything above 98.6. While a temperature above normal may be a sign that something’s wrong, it’s still not a fever if it falls below the 100.4 degree threshold. You can still get minor temperature elevations with viral infections, cold symptoms or other illnesses. Even teething can cause an elevated temp in infants.

“A fever on its own is not a completely harmful, bad thing. It can be a good sign because it shows your body is mounting a response,” Dr. Whitney says. “While it makes you or your children uncomfortable, it’s actually a sign that your body is doing what it needs to do to fight off that infection.”

What to Do When You Have a Fever

Stay home! Fevers are a sign of illness. Even in the absence of other symptoms, you could be contagious and infect those around you.

And it’s not just the guideline to follow during a pandemic. It’s the medical advice for any time you or your child are running a fever. Adults shouldn’t go to work or other activities and children should stay home from school or daycare until they are well again.

“You or your child normally need to be fever-free for 24 hours before returning to school, work, daycare or activities. You don’t want to spread any illness that your family may have,”  Dr. Whitney says.

“Right now, guidelines are changing frequently. Stay in good communication with your physician’s office. Keep following the latest guidelines for schools, activities and childcare going forward.”

When to Take a Fever Reducer

Fevers are uncomfortable. When you have one, it’s okay to open the medicine cabinet and grab some fever-reducing medication like acetaminophen, ibuprofen or naproxen. It’s okay to give these fever reducers to kids too.

“A lot of times, parents worry about giving kids fever reducers. If they have a fever, it’s fine to give your kids a dose of Tylenol,” Dr Whitney says. “Acetaminophen can be given at any age. With ibuprofen, you have to wait until your child is six months old or older.”

Always read the packaging to check the appropriate dosage for each age. There are a variety of liquid fever reducers designed for kids. Follow the dosage charts for infants and children to give your kids the right dose of Tylenol and to correctly administer Motrin.

When to Call Your Doctor for a Fever

A fever alone is not always a reason to worry. It’s when it comes along with other symptoms where there’s more cause for concern. If there’s nothing else going on, it’s less about the number on the thermometer and more about the duration of the fever.

“An isolated fever on its own isn’t bad, we just need to know what’s going on with it,” Dr. Whitney says. “If it’s only a fever, we may say to watch it for 24 hours or so, depending on how things sound otherwise.”

Even if it’s high. While high fevers over 102 or 103 degrees are definitely worth watching and communicating with your provider, there’s no magic number that prompts a trip to Urgent Care or the emergency room.

“Typically, for a high fever lasting more than 48 hours, we want to evaluate you or your child. And for any fever lasting for five days, we definitely want to see you because there are some diseases that cause fever as the primary symptoms but don’t have a lot of other noticeable signs,” Dr. Whitney says.

It doesn’t matter what the thermometer says, even if the temperature doesn’t meet the fever threshold, you should always call your primary care provider if you are concerned.

“We’re happy to discuss it with you over the phone or schedule an eVisit,” Dr. Whitney says. “There’s no wrong time to call when you’re concerned about your health or your child’s health.”

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