Please note: This information was current at the time of publication but now may be out of date. This handout provides a general overview and may not apply to everyone. For regularly updated information on a variety of health topics, please visit familydoctor.org, the AAFP patient education website.

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Am Fam Physician. 2022;105(2):online

Related article: Fever of Unknown Origin in Adults

What is a fever of unknown origin?

It's a fever of 101°F (or 38.3°C) or higher that occurs several times. Even after you see your doctor, there's not a clear reason for the fevers.

What causes it?

It can be caused by infection, cancer, or a disease that causes inflammation (when part of your body becomes hot, red, and swollen after an injury or infection). Lupus, rheumatoid arthritis, and temporal arteritis are a few examples of inflammatory diseases. Other possible causes include certain medicines, thyroid swelling, and blood clots.

What will my doctor need to know?

Your doctor will probably ask you about:

  • Your personal and family medical history

  • Places you've traveled in the United States and overseas

  • Medicines you've taken (including herbal and over-the-counter medicines)

  • Contact you've had with animals or sick people

What tests will I need?

Your doctor will test your blood and urine and order a chest x-ray. Your doctor may also order some other basic imaging to look at your abdomen, heart, thyroid gland, and leg veins. You might be tested for tuberculosis. There may be other tests based on your physical exam.

What if my doctor can't find a cause?

Most people who have a fever of unknown origin will get better or have a very mild illness. If a cause is not found, your doctor may talk with other doctors for another opinion.

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