Fever results when the body temperature exceeds normal limits. Daily fluctuations in temperature occur from a low at about 6AM to a peak between 4-6PM. Those with a fever typically maintain the normal circadian variation in body temperature. Even a slight elevation of body temperature in seniors represents a potentially important fever.

Fever generally represents a change in the set point of a region of the brain known as the hypothalamus. The idea of normal body temperature of 98.6 degrees Fahrenheit was conceived in 1851. More recently we accept a range of body temperature from about 97.0-99.5 degrees. On average fever represents a temperature in excess on 100.4 degrees. The body must dissipate heat since normal metabolic function of the muscles and liver produce excess amounts of heat. The skin and lungs remove the excess to keep the body thermoneutral.


With the onset of fever, the hypothamalus set point increases which shunts blood from the skin to minimize heat loss. As a result people with a fever generally feel cold. Individuals with fever add clothes or blankets and increase their metabolism. Fat cells may actively metabolize to produce heat and shivering may also occur.

Other non-specific manifestations of sickness behavior include depression, loss of appetite, sleepiness, inability to concentrate and excessive pain. As improvement occurs and the set point falls back to normal, people feel hot, flushed and begin to perspire.

Mechanisms responsible for fever include internal or external production of pyrogens. Through a series of reactions these lead to synthesis of prostaglandins that directly impact the set point. Aspirin, the NSAIDS and acetaminophen counteract this process and typically return the set point to more normal limits.

Hyperthermia represents an increase in body temperature without an alteration in the set point. This may result from the excessive production of body heat or the body’s inability to dissipate heat. Examples include heat stroke, drug reactions, response to methamphetamine and cocaine or the serotonin syndrome.

Hyperthermia differs from fever by being a medical emergency. One obvious difference is that in hyperthermia the skin tends to be hot and dry. Immediate attention is necessary with cooling the temperature to less than 102 degrees within several hours. Aspirin, NSAIDS and acetaminophen are worthless in hyperthermia. Improperly treated the mortality from hyperthermia exceeds 50%.