Silent thyroiditis(ST) is a self-limiting condition lasting weeks to several months.Typically patients present with variable features of thyrotoxicosis including weight loss, fatigue, malaise, nervousness, increase sweating, goiter, tachycardia and hyperreflexia. The precise etiology of ST is unknown, although thyroid biopsies have revealed lymphocytic infiltration. A majority of the cases reported occur in the postpartum period.

We report a 12 year old girl who was being evaluated for FUO after two weeks of fever, malaise and lower extremity nodular rash. Her thyroid gland was nontender and minimally enlarged. Clinically she was not thyrotoxic. Her mother had a history of Graves disease. Laboratory studies were significant for a T4=13.6μg/dL, fT4=2.1ng/dL, T3=100ng/dL, TSH<0.03μIU/mL and a ESR=78mm. Antithyroglobulin, antithyroid peroxidase antibodies and thyroid stimulating immunoglobulin were negative. During the 9th week of illness her thyroid I-123 uptake was 3.7% at 6 hours and 4.6% at 24 hours.Biopsies of the skin lesions showed panniculitis with primarily lymphocytic infiltration of the fat cells. Studies for other causes of FUO were negative including a bone scan, abdominal CT, complete upper endoscopy, bone marrow examination, bacterial, fungal and parasitic cultures of blood, stool, urine and bone marrow, viral serologic studies for EBV, CMV, parvovirus and HIV, ANA, ANCA, HLA-B27 and serum complement levels. The fever and panniculitis waxed and waned over several months with a steadily declining ESR without treatment.

ST should be included in the differential diagnosis of FUO. Patients with ST can lack-overt symptoms of thyrotoxicosis and therefore the diagnosis can be missed unless thyroid function tests are performed. Early consideration of ST as a cause of FUO could avoid an extensive and costly diagnostic evaluation.