Euthyroid atypical subacute thyroiditis simulating systemic or malignant disease.

An unusual form of atypical subacute thyroiditis (SAT) occurred in 13 patients. The clinical presentation in all patients simulated systemic or malignant disease, involving fever of long duration and loss of weight without localized thyroid tenderness and without signs or symptoms of thyrotoxicosis. Eleven of the 13 patients had normal serum free thyroxine values. In the ten patients in whom a needle biopsy was done, the histologic findings were the same as in typical SAT. In the other three patients, the diagnosis was made based on the following clinical findings: high erythrocyte sedimentation rate, low radioactive iodine uptake, and good response to salicylate (aspirin, 2 g/day) or steroid (prednisone, 30 mg/day) treatment. Early recognition of this variation of atypical SAT may save the patients unnecessary investigations in search of systemic or malignant disease.