Subacute thyroiditis manifesting as fever of unknown origin.

Subacute thyroiditis (SAT) usually occurs in women in middle age with a viral prodrome, thyroid or neck tenderness, classic symptoms of thyrotoxicosis, and elevated erythrocyte sedimentation rate (ESR). We report a case in an 81-year-old man who initially had 2 days of fever to 101.2 degrees F, confusion, and bilateral lower extremity weakness. Extensive evaluation was remarkable only for the following laboratory values: thyrotropin (TSH) 0.02 microIU/mL, free thyroxine (FT4) 3.1 ng/dL, free triiodothyronine (FT3) 6.0 pg/mL, and ESR 98 mm/hr. One week later, the patient had persistent fevers to 102 degrees F; no source was found. The fever resolved, and 3 months later the patient had profound hypothyroidism (TSH >44.0 microIU/mL, FT4 0.4 ng/dL, ESR 13 mm/hr). A painless thyroid gland and atypical manifestations of hyperthyroidism are unusual in SAT. When fever is of unknown origin, SAT should be considered even if classic features are absent.