It’s a fact that is still taught daily to schoolchildren all over the world: Normal human body temperature is 98.6 degrees Fahrenheit. But as with most measurements, “normal” has a range. With current technology, don’t be surprised if your actual measure temperature is rarely 98.6. That’s because “normal” temperature was based on the average temperature of hundreds of people using oral, mercury thermometers. Current thermometers are not only much faster, they are much more accurate and document two things known even in the days of mercury thermometers: There is variability in body temperature over the course of the day, and there is variability between different people.
In fact, from what is now known, the case could be made to abandon 98.6 as the definition of normal body temperature and instead recognize the range of normal. Thermoregulation — the body's mechanisms to maintain body temperature within a certain range — is a key bodily function, but it does allow perfectly healthy persons to drift higher or lower than 98.6 rather often.
Based on research measuring the body temperatures of thousands of healthy people, normal oral temperatures range from 97.5 to 98.9; perhaps one in 20 people have temperatures even a bit lower or higher. It also varies over the course of the day, varying one to two degrees — from a low from 2 a.m. to 4 a.m., to a high 12 hours later. Other normal temperature variation occurs during the menstrual cycle (lower temperature for a week or two after a period, elevation by one degree or so with ovulation) or pregnancy (higher temperature during the first trimester). The elderly have lower temperatures, perhaps because metabolism slows with age.
Keep in mind that oral temperatures happen to be a convenient place to sample, but to the body, core body temperature — the actual temperature inside the body — is what counts. Core temperature, generally a degree or so higher than oral temperature, is more accurately reflected by rectal or inner-ear (tympanic) measurement. Measures from the armpit will have readings up to a degree lower than oral readings and are considered the least accurate; a trend (up or down), however, may still may be detected by repeated measurements. For infants younger than three months, an armpit measurement is a reasonable screen, but if it is high (for example, over 99 degrees), a rectal temperature should be measured. For older infants, an ear or armpit temperature can be used as the initial measure, as long as elevations in the armpit temperature are followed by more accurate ear or rectal measurement. An oral temperature should not be relied upon in children until they are four years old.
Back to top
Why Is it a Big Deal?
Body temperature is vital — in fact, it’s one of the “vital signs,” along with blood pressure, heart rate (or pulse), and breathing rate. These are indicators considered absolutely critical when evaluating whether someone is sick. If any one of these is markedly abnormal, a cause must be promptly identified and a reading closer to normal must be restored. Otherwise, the situation eventually could prove fatal. Thousands of chemical reactions occurring continuously in the body depend on a rather narrow range of temperature to occur properly. As a result, the body does not forgive wide ranges in temperature well. For example, severe hypothermia (low body temperature) or hyperthermia (high body temperature) can cause permanent organ damage.
In addition, abnormalities in temperature can suggest a cause of illness; the most common cause of elevated body temperature is infection, and almost any infection in the body can cause fever. But there are other causes, including heat stroke or a drug reaction. Doctors record the vital signs first when listing the findings on a physical examination because they are considered so essential. Although you can be sick with a normal temperature and well with an (slightly) abnormal temperature, body temperature is clearly an important and useful indicator of health. That’s why the body has such an elaborate thermoregulation system — mechanisms that keep the body’s temperature close to ideal most of the time.
An example of how important fever can be in evaluating someone is to consider back pain, an extremely common problem that affects up to 80% of adults as a significant problem at some point in their lives. The vast majority of cases do not represent any serious threat to health or longevity; in fact many defy specific diagnosis and most others are related to muscle spasm, strain, degenerative arthritis, disk disease and the like. However, when back pain is accompanied by fever (one of the “alarm” symptoms for evaluating back pain), serious conditions such as a kidney infection; bone, joint or disk infection; tumor; or bloodstream infection come to mind — not the “benign” diagnoses usually associated with back pain. Similar principles apply to a vast array of other symptoms. That’s why the presence or absence of fever is so routinely assessed.
Back to top
How Does the Body Regulate Temperature?
The body has developed its own internal thermometer and feedback system so that if a rise or fall in temperature is detected, the body reacts to return the temperature to normal. For example, if you exercise, the work of your muscles generates heat; the hypothalamus, a gland located deep within the brain, sends a message to blood vessels to dilate and to the sweat glands (to sweat) which help remove heat and cool the body. You may voluntarily remove clothing or choose looser clothing, or, eventually, body temperature will rise so high you will feel too lethargic to continue exercising (not a good sign).
Conversely, if you find yourself out in the cold, the blood vessels constrict to conserve body heat, you shiver to generate more heat or you may voluntarily bundle up or head for a warmer place. Sometimes these mechanisms don’t work well — for example, a condition called poikilothermy is the inability to keep body temperature constant when the environment changes; normal infants (and rarely adults who have had brain injury or stroke) demonstrate poikilothermy and for this reason, it is important to be sure infants are properly dressed, especially when the climate is changing.
Body temperature can rise despite these mechanisms when infectious agents, such as bacteria, invade the body and trigger the release of chemicals (called pyrogens) that stimulate the body’s temperature to rise. Although the significance is debated, fever may improve certain immune-cell function and reduce the defenses of invading organisms; for this reason, some have suggested that fever is a good thing and should not be suppressed during infection, but that remains unproven.
The real purpose of fever (if there is one) is unclear, but it does serve as a signal that something is wrong. Less common causes of elevated temperature are heat stroke, a drug reaction, certain tumors, brain injury (such as stroke) or conditions associated with inflammation (such as lupus or gout).
Low body temperature may develop after exposure to cold weather, low blood pressure (as may occur with severe infection, called septic shock), certain medications, an underactive thyroid and low blood sugar (hypoglycemia). It may be an adaptive response to severe illness — by lowering body temperature, metabolism is slowed and energy conserved until the body is healthier.
Back to top
It is time to put aside a single reading as indicative of normal body temperature because a single reading does not adequately describe body temperature. Many use 100 degrees Fahrenheit as a convenient cutoff for where fever begins, but remember that many factors go into it (including age and where you are in the menstrual cycle); and even if you are usually 97.8 degrees, 99 may not represent a significant fever. Temperature does vary between people and in an individual may vary two degrees. It may be best to recognize that body temperature is a continuum, and the higher or lower it is, the more likely it is to suggest something is amiss. But keep in mind that you can be quite sick with a normal temperature and have high or low temperature for many reasons.
Body temperature is a vital sign for a good reason. Fortunately, keeping our temperature in the proper range is something that happens automatically. It’s a helpful clue to health and illness that your doctor uses to figure out the most likely reason for symptoms. Don’t rely simply on how you feel (or how your forehead feels). When in doubt, use a thermometer. But even if you’re healthy, don’t be surprised if the result is something other than 98.6.
Back to top
Robert H. Shmerling, M.D., is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.