Fever of unknown origin (FUO): de Quervain's subacute thyroiditis with highly elevated ferritin levels mimicking temporal arteritis (TA)

Fever of unknown origin (FUO) refers to prolonged fevers of ≥101°F and that persists for >3 weeks that remain undiagnosed after an intensive in-hospital/outpatient workup. The most common FUO categories of are infectious, neoplastic, rheumatic/inflammatory, and miscellaneous causes. Malignancies have supplanted infectious diseases as the most common cause of FUOs in the adult population. Rheumatic/inflammatory causes of FUO are relatively less common than previously because of the introduction over the years of sophisticated diagnostic tests for most rheumatic diseases. The rheumatic/inflammatory disorders that remain important causes of FUO today are those that cannot be readily diagnosed by readily available/noninvasive tests, for example, adult Still's disease and temporal arteritis (TA). In older patients with FUO, TA can be a difficult diagnosis when the characteristic findings (ie, scalp tenderness, jaw claudication) are not present. Patients with TA presenting as FUO often have only headaches that may be accompanied by bilateral jaw discomfort. Endocrine causes of FUOs are rare. The most common endocrine disorder rarely presenting as an FUO is de Quervain's subacute thyroiditis. As in TA, subacute thyroiditis may present with headache and pain at the angle of the jaw. Both TA and subacute thyroiditis may be accompanied by fatigue, weight loss, and night sweats. We present a case of 55-year-old woman who presented with an FUO with clinical and laboratory findings suggesting TA. However, the absence of thrombocytosis and a normal alkaline phosphatase argued against the diagnosis of TA. Also against the diagnosis of TA was weight loss without loss of appetite and a slightly increased pulse. After nonspecific laboratory test results suggested that TA was not the cause of her FUO, additional tests were ordered. Thyroid function test results suggested the possibility of de Quervain's subacute thyroiditis as the cause of her FUO. To the best of our knowledge, this is the first case of de Quervain's subacute thyroiditis presenting as an FUO with elevated ferritin levels.