98.6 - What's a normal body temperature?
by Michael Melgar, MD on December 4th, 2014

Winter is coming and with it colds and flu. When trying to figure out whether you have the flu or a cold its often important to know whether you have a fever but what qualifies as a fever? Recent coverage in the news of ebola and enteroviral infections have also raised this issue. When is a temperature abnormal? When is it a fever?

What is a normal temperature?

From the time we are young we are tought that a normal temperature is 98.6. Most people would assume that something like 101 is therefor a fever and they would be correct, but when does the temperature become a fever as it rises from normal to 101?

First we need to discuss what a normal temperature is. While 98.6 is the often quoted number that is not completely correct. If we were to take the temperatures of 100 healthy people we would find that most would not be 98.6.  A 1992 study of 148 adults showed that oral temperatures in healthy individuals can range from 96.0ºF to 100.8ºF with a average of about 98.2. Normal rectal temperatures are about 1.0ºF higher.
 

Figure 1



The study also found that the lowest temperatures were in the mornings around 6am with peak temperatures generally occuring around 6pm in the evening.

 

Figure 2



How high does the temperature have to be to be considered a fever?


From these studies, a morning reading >37.2ºC (98.9ºF) or an afternoon temperature of >37.7ºC (99.9ºF) would be above average, but as you can see from Figure 1 above, there is a lot of variation from person to person so even normal healthy individuals can sometimes have a temperature of just under 101 and sometimes a sick individual will have a lower temperature. For this reason its important to take the temperature several times over the course of the day. Normally an individual's temperature varies by about 1ºF during the course of a day but when ill the temperature might vary by twice as much.

The standard medical definition of a fever is a temperature of 101 or more but obviously this has to be used as only a rough guide. When we say that someone has a fever what we are really saying is that their temperature is higher than normal, but what is normal varies depending on the time of the day and the person who's temperature we are taking.. The reason we want to know if someone has a fever is because it can be a clue to whether the person is ill and also because a temperature that is too high can be dangerous at times.

What causes a fever?

Fevers can have several causes. The great majority of fevers are caused by infections but there are occasional other causes.
  • Infections: Viral and Bacterial infections can both cause fever
  • Heat exhaustion/ dehydration
  • Malignant tumors
  • Medications -Sometimes antibiotics and certain seizure medications can cause a fever
  • Vaccines - Some people will develop a mild fever for a day or so after vaccination
  • Inflammatory conditions such as rheumatoid arthritis
  • Burns
  • Damage to the hypothalamus - very rare

Why do we get a fever?

Thats a good question actually. Why do we get a fever? The best answer at the moment is that we don't really know. We know how it occurs, but not exactly why. Some experts believe that fever is part of the body's defense mechanism against infection since viruses and bacteria may not be able to function and reproduce as well if the body is too warm. This seems logical since many enzymes work best in only a narrow temperature range and enzymes are important for many cellular functions that viruses and bacteria depend on in order to reproduce and metabolize. If this were true we would expect people to be sicker longer if they took measures to lower their body temperature but this is not the case. Taking Tylenol or Ibuprofen does not seem to prolong or worsen the outcome for most illnesses. For the moment its not clear whether a fever is an adaptation by the body to deal with the infection or just a necessary side effect that arises when our immune system goes to battle an infection, similar to the way our body heats up when we run or do other physical work. The heating up serves no purpose and can even be harmful. Its just the natural result when our muscles burn fuel and do work.

Interesting Fact

Sometimes I will hear a patient say. "It doesn't make any sense. I feel hot but when I take my temperature its normal" or "I've been freezing but now I have a fever".  It sounds odd but this actually is exactly what you would expect when you undertstand what's going on.

Lets say the body is trying to raise its temperature. How does it accomplish that? Well it does it the same way you would raise the temperature if you were trying to warm your house. You can either turn up the furnace or you can close the windows and insulate it better to keep the heat in.  The same thing happens in your body. The only difference is that the body has to convince you to do these things even though it can't actually talk to you. It communicates with you by the only method it has. It makes you feel cold or hot. It does this by changing the internal thermostat in a gland called the hypothalamus. When you feel cold you naturally do things to try and get warm. You involuntarily start to shiver. When you shiver your muscles contract and relax vigorously generating heat (this is like turning up the furnace in your house). In addition you may put on a sweatshirt or dive under the blankets (This is the same as adding insulation to your home or closing the windows to keep the heat inside). So in response to feeling cold you generate more heat and do everything you can to hold onto the heat you are generating. In a little while the temperture begins to soar and you have a fever. You're freezing but your temperature is 101!

When its time to get your temperature down your body makes you feel hot. You stop shivering, peel off all the layers and maybe you even drink something cold. A little while later your temperature goes away. So now you feel hot but your temperature is normal.

This up and down fluctation in the temperature may happen several times during the day when you are sick especially if you are taking ibuprofen or tylenol to knock the fever down, but most of the time it follows the same cycle as when we are healthy. It tends to be lower in the morning and spikes in the evening hours. Sometimes people think their illness has gone away because the temperature is normal in the morning when they wake up only to be disappointed when the fever is back later that evening. This isn't a sign that anything is seriously wrong, its just the normal pattern we expect to see with any illness that causes fever. As the days go by and the illness begins to resolve the temperature will be less and less each evening until eventually there's no more fever even at night.

Temperature pattern during the course of an illness

References:

  1. A critical appraisal of 98.6 degrees F, the upper limit of the normal body temperature, and other legacies of Carl Reinhold August Wunderlich.AUMackowiak PA, Wasserman SS, Levine MMSOJAMA. 1992;268(12):1578.


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