The “official” body temperature is measured in the pulmonary artery (running from the heart to the lungs). Since that is hard to get to, body temperature is more usually measured in the armpit, under the tongue, in the rectum, or (with an infrared thermometer) on the eardrum (tympanic membrane).
The armpit technique is generally used with infants. The oral and tympanic measurements are generally used with older children and adults, although tympanic measurements are unreliable if instructions are not properly followed (reference). The rectal technique was once popular, but is less reliable than once believed, and has several obvious disadvantages.
The diagram above shows normal (blue), average (black), and fever (red) temperatures for children, as measured with the four techniques (reference). A Fahrenheit version is shown below. Clear differences exist between measurement techniques.
These temperatures vary with various factors, particularly the time of day:
Photo credits “Stilfehler,” 2009 and “Svdmolen,” 2007.
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