Evaluation of Fever in Adults

Fever is an excessively high body temperature that occurs when the body's thermostat (located in the hypothalamus of the brain) resets to a higher temperature, usually as a result of illness. Hyperthermia is defined as elevated body temperature that is not induced by resetting the temperature set point.

Although 98.6° F (37° C) is considered normal body temperature, it fluctuates during the day. It is coldest in the early morning and warmest in the late afternoon, with temperatures occasionally exceeding 99.9° F (37.7° C). A fever, too, does not maintain a consistent temperature. Every day, the temperature peaks and then returns to normal. This is known as intermittent fever. Alternatively, the temperature fluctuates but never returns to normal. This is known as remittent fever. Doctors no longer believe that the pattern of fever rise and decline is critical in the diagnosis of some diseases.

Body temperature can be monitored in a variety of locations. The mouth (oral) and rectum are the most prevalent places. Other locations include the ear, forehead, and, much less ideally, the armpit. A digital thermometer is typically used to measure temperature. Mercury-containing glass thermometers are no longer advised because they can break and expose individuals to mercury.

When oral temperatures are considered high,

In the early morning, they reach 99° F (37° C).
They are greater than 100.4° F (38° C) after the early morning.
They are above a person's known typical daily temperature.
Rectal and ear temperatures are around one degree Fahrenheit (0.6 degree Celsius) greater than mouth temperatures.

Oral temperatures are around 1.0° F (0.6° C) lower than skin temperatures (for example, the forehead).

Many individuals use the term "fever" loosely, implying that they are overheating, underheating, or sweating but have not taken their temperature.