Monoclonal gammopathy in Hashimoto's thyroiditis and malignant lymphoma of the thyroid.

Serum protein electrophoresis was performed in 681 patients (43 men and 638 women) with an initial diagnosis of Hashimoto's thyroiditis between April and November 1983. All patients whose thyroid size was estimated to be greater than 50 g underwent biopsy; 1 man was found to have thyroid prelymphoma, and 13 patients (4 men and 9 women) were found to have malignant lymphoma of the thyroid. Monoclonal gammopathy (M-component) was demonstrated in 5 of 667 patients (0.7%) with Hashimoto's thyroiditis (1 man and 4 women), 1 man with thyroid prelymphoma, and 3 of the 13 patients (23.1%) with malignant lymphoma of the thyroid (2 men and 1 woman). Intracytoplasmic monoclonal immunoglobulin was found in the 1 thyroid prelymphoma and in all 3 malignant lymphoma of the thyroid in patients who had M-component in their serum, but not in thyroid tissue from any of the 5 patients with Hashimoto's thyroiditis who had M-component in their serum. Thus, the finding of monoclonal intracytoplasmic immunoglobulin in tissue sections permitted the diagnosis of malignant lymphoma of the thyroid or thyroid prelymphoma.