Subacute Thyroiditis Causing Fever of Unknown Origin: A Reminder of the Uncommon Cause

Subacute thyroiditis (SAT) is a transient inflammation of the thyroid gland that often occurs following a viral infection. It is an infrequent cause of fever of unknown origin (FUO). We present a 46-year-old gentleman who presented with two weeks of fever and some non-specific left-sided neck pain. His initial investigations and microbiological workup were unremarkable. He did not report any hyperthyroid symptoms. A computed tomography of the neck, chest, abdomen, and pelvis showed a heterogeneous appearance of his thyroid gland. Thyroid function was then performed, and it showed primary hyperthyroidism. His thyroid autoantibodies were negative. Ultrasonography of his thyroid showed features consistent with thyroiditis. He was treated with a course of oral steroids. His fever lysed. His thyroid function turned from a primary hyperthyroid pattern to subclinical hypothyroidism. His anti-thyroglobulin antibody level remained elevated after the steroid treatment. Our case highlights that SAT is an uncommon cause of FUO in patients without specific localizing symptoms. It can present without overt hyperthyroid clinical features. Steroid treatment is useful. There may be value in monitoring the anti-thyroid antibodies in SAT's management.