A Case of Painless Subacute Thyroiditis Presenting as Fever of Unknown Origin.

Subacute thyroiditis (SAT) is characterized by severe pain in the anterior aspect of the neck and tenderness is present during the thyroid gland's palpation. It is commonly caused by viruses including mumps, measles, rubella, coxsackievirus, influenza, and Epstein-Barr virus (EBV). Painless subacute thyroiditis is rare and can present as a fever of unknown origin (FUO). Our case reports an unusual case of SAT as our patient did not have any neck pain. Laboratory investigations show low thyroid-stimulating hormone (TSH), poor or no uptake of radioactive iodine by the thyroid, and elevated erythrocyte sedimentation rate (ESR). Clinicians should be aware that painless SAT can present as a fever of unknown origin.