Fever of Unknown Origin and Oncocytic Thyroid Cancer: An Unforeseen Association.

Fever of unknown origin (FUO) poses significant challenges for clinicians due to the myriad of possible etiologies, exhaustive evaluation requirements, diagnostic delays, patient and family frustration, and prolonged hospital stays. Defined as a fever lasting more than three weeks without a diagnosis despite one week of inpatient evaluation, this clinical phenomenon could result from infectious, autoimmune, neoplastic, or miscellaneous etiologies. We report the case of an elderly woman with FUO who was diagnosed with oncocytic carcinoma of thyroid origin on cervical lymph node biopsy. Interestingly, the biopsy was taken from a cervical node that was detected incidentally on neuroimaging. Oncocytic thyroid cancers, also known as Hürthle cell carcinomas, are differentiated but aggressive thyroid neoplasia. These rare tumors often present as thyroid nodules, detected incidentally during imaging or as palpable thyroid swellings. This case report highlights the clinical and diagnostic challenges in patients with FUO and reports an exceedingly rare presentation of oncocytic thyroid cancer as FUO. Additionally, it emphasizes the diagnostic utility of imaging and the need to follow even subtle diagnostic clues when evaluating patients with FUO.