Subacute (de Quervain's) thyroiditis is a rare but important cause of fever of 
unknown origin. Most cases of subacute thyroiditis are caused by a variety of 
viruses, for example, Coxsackie, cytomegalovirus, Epstein-Barr virus, and 
adenovirus. Influenza immunization or infection may cause subacute thyroiditis. 
We present the first reported case of a fever of unknown origin due to seasonal 
influenza A in a 67-year-old woman.