Fever of Unknown Origin: A Rare Presentation of Parathyroid Carcinoma.

Fever of unknown origin (FUO) can be a common manifestation of multiple disease processes like infections, hematological & solid organ malignancies, autoimmune disorders, and autoinflammatory diseases. Endocrine causes of FUO are rare but should be considered in differential diagnosis. We present a case of a 35-year-old female with prolonged on-and-off fever and intermittent vomiting for nine months, where extensive workups for chronic infections, malignancy, and autoimmune conditions initially yielded no definitive diagnosis. Upon further investigation at our institution, the patient was found to have elevated serum calcium levels, nephrocalcinosis on ultrasound, and an extremely high parathyroid hormone level (2810 pg/ml), which directed attention toward a parathyroid disorder. A neck ultrasound revealed a right inferior parathyroid adenoma, which was confirmed by single-photon emission computed tomography (SPECT) imaging. The patient underwent parathyroidectomy, followed by postoperative hypocalcemia, which was promptly managed. Histopathological examination confirmed parathyroid carcinoma. The patient was subsequently referred for oncological care and is under follow-up. This case highlights the importance of considering endocrine causes, such as parathyroid carcinoma, in the workup of FUO, even in the absence of typical symptoms.