In subacute thyroiditis, which laboratory finding is most characteristic?

• A. Positive thyroid antibodies
• B. Decreased RAIU
• C. Increased ESR
• D. Normal ESR

Answer: (C)

Subacute thyroiditis is an inflammatory thyroid disorder often following a viral illness, leading to release of stored thyroid hormone and a transient thyrotoxic phase. The standout laboratory finding is a markedly elevated ESR, reflecting the systemic inflammatory process driving the thyroiditis. This inflammatory marker helps distinguish it from other causes of thyrotoxicosis, such as Graves disease, where ESR is not characteristically raised. The uptake of iodine on imaging is typically decreased due to destruction of thyroid follicles, and thyroid antibodies are usually negative because this is not an autoimmune process. So, the elevated ESR is the most characteristic laboratory feature.