Carcinomatous pseudothyroiditis: a problem in differential diagnosis.

A 51-year-old woman presented with a painful, rapidly enlarging thyroid gland. The serum concentrations of the thyroid hormones were initially at the upper limit of normal, and the uptake of radioactive iodine by the thyroid was completely depressed. Although subacute thyroiditis was suspected, a biopsy specimen from the thyroid showed malignant disease and no evidence of inflammation. At the time of right subtotal lobectomy metastatic breast carcinoma was diagnosed. Thus, primary or secondary malignant disease of the thyroid can simulate thyroiditis and present a serious problem in differential diagnosis.