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Causes
By Mayo Clinic staffYour normal body temperature varies throughout the day according to circadian rhythm — it's lower in the morning and higher in the late afternoon and evening. In fact, your normal temperature can range from about 97 F (36.1 C) to 99 F (37.2 C). Although most people consider 98.6 F (37 C) normal, your temperature may vary by a degree or more. Other factors, such as your menstrual cycle or heavy exercise, can affect your temperature.
Here's how body temperature works:
- Your body temperature is set by your hypothalamus, an area at the base of your brain that acts as a thermostat for your whole system.
- Your temperature is the balance of the heat produced by your body tissues, particularly your liver and muscles, and the heat your body loses.
- When you're ill, your normal temperature may be set a few points higher as your body directs blood away from your skin to decrease heat loss.
- When a fever starts and your body tries to elevate its temperature, you feel chilly and may shiver to generate heat until the blood around your hypothalamus reaches the new set point.
- When your temperature begins to return to normal, you may sweat profusely to get rid of the excess heat.
- If you're very old, very young or alcoholic, your body's ability to produce a fever may be lessened.
A fever usually means your body is responding to a viral or bacterial infection. Other possible causes include:
- Heat exhaustion
- Extreme sunburn
- Certain inflammatory conditions such as rheumatoid arthritis — inflammation of the lining of your joints (synovium)
- Malignant tumor or some forms of kidney cancer (rare)
- Some medications, such as antibiotics and drugs used to treat high blood pressure or seizures
- Some immunizations, such as the diphtheria, tetanus and acellular pertussis (DTaP) or pneumococcal vaccines (in infants and children)
Sometimes it's not possible to identify the cause of a fever. If you have a temperature of 101 F (38.3 C) or higher for more than three weeks and your doctor isn't able to find the cause after extensive evaluation, the diagnosis may be fever of unknown origin.
- Manifestations of infection. The Merck Manuals: The Merck Manual for Healthcare Professionals. http://www.merck.com/mmpe/print/sec14/ch167/ch167d.html. Accessed April 13, 2009.
- Fever. The Merck Manuals: The Merck Manual for Healthcare Professionals. http://www.merck.com/mmpe/print/sec19/ch266/ch266i.html. Accessed April 13, 2009.
- Torpy J. Fever in infants. Journal of the American Medical Association. 2004;291:1284.
- Fever. National Cancer Institute. http://www.cancer.gov/cancertopics/pdq/supportivecare/fever/healthprofessional. Accessed April 13, 2009.
- Febrile seizures fact sheet. National Institute of Neurological Disorders and Stroke. http://www.ninds.nih.gov/disorders/febrile_seizures/detail_febrile_seizures.htm. Accessed April 13, 2009.
- Richardson M, et al. Assessment and initial management of feverish illness in children younger than 5 years: Summary of NICE guidance. British Medical Journal. 2007;334:1163.
- O'Grady N, et al. Guidelines for evaluation of new fever in critically ill adult patients: 2008 update from the American College of Critical Care Medicine and the Infectious Diseases Society of America. Critical Care Medicine, 2008;36:1330.
- El-Radhi A. Why is the evidence not affecting the practice of fever management? Archives of Disease in Childhood. 2009;93:918.
- Leggett J. Approach to fever or suspected infection in the normal host. Cecil Textbook of Medicine. 23nd ed. W. B. Saunders Company, 2007.
