Solitary Extramedullary Plasmacytoma of the Thyroid Gland in a Patient With Subclinical Hyperthyroidism

Solitary extramedullary plasmacytoma (SEP) of the thyroid is rare, and its clinical and pathologic features are not entirely understood. Generally, patients with SEP of the thyroid also present with Hashimoto thyroiditis. We report, for the first time, a case of SEP of the thyroid in a patient with subclinical hyperthyroidism.

A 46-year-old woman presented to the emergency department with symptoms consistent with a panic attack. Following a physical examination, neck ultrasonography, and laboratory tests, we determined that the patient was hyperthyroid and possessed a toxic multinodular goiter. The patient elected to undergo a total thyroidectomy to treat the subclinical hyperthyroidism and to remove the nodules.

SEP diagnosis requires consideration of the systemic spread of multiple myeloma and a combination of immunohistochemical examination, imaging, and other laboratory tests. Upon further examination, we confirmed SEP through the following criteria: the absence of kidney failure or hypercalcemia, a biopsy demonstrating plasma cell histology, a lack of bone or tissue involvement, and low serum myeloma protein concentration. The immunophenotype of the tumor cells further confirmed SEP diagnosis, with positivity for CD138, CD79a, and lambda light chain antibodies and no reactivity toward cyclin D1.

This case fulfilled the criteria for SEP, and we were able to rule out multiple myeloma. Currently, no standard treatment exists for SEP. Yet, the prognosis remains encouraging. Going forward, SEP of the thyroid should be considered in the differential diagnosis of a patient with subclinical hyperthyroidism and a toxic multinodular goiter.