Thyrotoxicosis: Unraveling the Mystery of Fever.

Subacute thyroiditis (SAT) is a transient inflammatory thyroid disorder, typically following an upper respiratory tract viral infection, and can rarely present as fever of unknown origin (FUO). Considering endocrine diseases in the differential diagnosis of fever is essential to prevent delayed diagnosis and unnecessary investigations. We report a case of a 65-year-old male who presented with a prolonged fever of two to three weeks, unresponsive to antibiotics, starting with mild upper respiratory symptoms. Extensive infectious and other workups were negative. Thyroid function tests revealed suppressed thyroid-stimulating hormone (TSH) and elevated free thyroxine (T4) and triiodothyronine (T3) levels. Thyroid ultrasound findings were consistent with thyroiditis. The patient was diagnosed with SAT and treated with corticosteroids, leading to rapid symptom resolution. SAT can be overlooked in the evaluation of FUO, particularly in the absence of classic symptoms. Thyroid function testing can be considered in the routine workup of FUO. Treatment should be based on symptoms, with an appropriate corticosteroid regimen if needed, and proper follow-up is essential to monitor thyroid recovery and potential progression.