Subacute thyroiditis (de Quervain's) due to influenza A: presenting as fever of unknown origin (FUO).

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.