Pyrexia of unknown origin due to subacute thyroiditis.

Subacute thyroiditis is an inflammatory thyroid disorder that typically presents with anterior neck pain, thyroid tenderness and transient thyrotoxicosis. Presentation as pyrexia of unknown origin (PUO) without classical thyroid symptoms is uncommon and may lead to diagnostic delay.We report a case of a patient in their late 50s who presented with persistent evening febrile spikes, weight loss and markedly elevated inflammatory markers, fulfilling the criteria for PUO. Extensive evaluation for infectious, malignant and autoimmune causes was unrevealing. Careful repeat clinical examination revealed mild thyroid enlargement, prompting thyroid function testing, which demonstrated suppressed thyroid-stimulating hormone with elevated thyroid hormone levels. Technetium-99m thyroid scintigraphy showed markedly reduced uptake, consistent with subacute thyroiditis.The patient was treated with corticosteroids, resulting in rapid defervescence of fever within 48 hours. This case highlights that subacute thyroiditis, although uncommon, should be considered in patients presenting with persistent unexplained fever and elevated inflammatory markers, even in the absence of classical thyroid symptoms.