Whether your mother taught you, or you learned it in medical school, chances are you’ve been told that 98.6° F (37° C) is the normal body temperature, and greater than 100.4° F (38° C) is a fever. It turns out it’s more complicated than that.
Those numbers came mainly from the work of Professor Carl Wunderlich. In 1870 he published an enormous study of over 1 million temperatures taken from about 25,000 patients. The thermometer he used was calibrated differently than modern thermometers, and he took temperatures in the axillae (armpit), which varies from oral (under the tongue) or tympanic (ear) measurements. He found that temperatures tended to be higher in the morning, and higher in women.A study published in JAMA in 1992 looked at patients 18 to 40-years-old who were hospitalized for a vaccine study. Prior to getting the vaccine, their temperatures were check 4 times a day for 2 ½ days. They found the upper limit of normal to be 98.9° F (37.2° C) in the early morning, and 99.9° F (37.7° C) overall.
A study that looked at 18,630 individuals from 20 to 98-years old showed women’s temperatures ran 0.3° F higher than men, and temperatures tended to decrease with age, with a 0.3° F difference between the youngest and the oldest.
An article published 13 August 2018 used an iPhone app called Feverprints. They looked at 11,458 oral temperatures recorded from 329 healthy adults and found an average normal temperature of 97.7° F, and that a fever started at 99.5° F.
I’ve often had patients tell me that a temperature we would normally consider within normal limits is high for them. That’s a good observation. There is a certain amount of normal variation in temperature by individual and time of day. It doesn’t necessarily mean it’s something that should be treated, but it may be a sign of infection and should be considered accordingly.