BACKGROUND: Fever of unknown origin (FUO) is a diagnosis that requires a demanding workup from physicians before confirming a diagnosis. Thyroid diseases are a rare cause of FUO. Subacute thyroiditis is an inflammatory disease that can lead to a wide spectrum of presentations.

CASE REPORT: We report a case of a previously healthy male who presented with persistent fever of 4 weeks following an upper respiratory tract infection associated with constitutional symptoms. His laboratory workup included complete blood counts (CBC), complete metabolic panel (blood urea and creatinine, liver function tests, and serum electrolytes), blood cultures, abdominal and pelvic ultrasound, and computed tomography abdomen and pelvis that were inconclusive. His thyroid function tests showed a hyperthyroid state and a thyroid scan confirmed a picture of thyroiditis. The patient was treated with Ibuprofen and then with prednisolone; he showed significant improvement over a few days and was discharged with treatment of tapering doses of prednisolone over 6 weeks. Two weeks after discharge the patient had a follow-up at an outpatient clinic and was found to be in good health with resolution of his symptoms.

CONCLUSIONS: Thyroid disorders are not a common cause of FUO, and even if the clinical assessment of the patient is not suggestive of thyroid disease, we should consider it a possible cause. and thyroid function test should be performed to exclude thyroid problems.

Keywords: Fever, Fever of Unknown Origin, Thyroiditis, Subacute, Anti-Inflammatory Agents, Non-Steroidal, Diagnosis, Differential, Glucocorticoids, Ibuprofen, Prednisolone, Respiratory Tract Infections