RESUMO
Extramedullary plasmacytomas most commonly occur in the nasal cavity, nasopharynx, paranasal sinuses, and larynx. Solitary extramedullary plasmacytoma (SEP) of the thyroid gland is rare. The diagnosis of SEP of the thyroid by cytology is typically difficult before surgery, and the entity is often confused with different cytology findings. We report a case of a 59-year-old man with primary plasmacytoma of the thyroid presented as a rapidly enlarging thyroid gland with Hashimoto's thyroiditis. He had been suffering from anterior neck swelling for 1 month. Several fine-needle aspiration biopsies yielded Hashimoto's thyroiditis. During a follow-up period of 3 years, the size of the thyroid gland increased and a mass lesion in right thyroid gland was detected. A total thyroidectomy was performed based on a diagnosis of a thyroid tumor with Hashimoto's thyroiditis. Permanent pathology identified the mass as an extramedullary plasmacytoma associated with Hashimoto's thyroiditis. Skeletal survey and serum electrophoresis tests were normal, and a bone marrow biopsy yielded no evidence of multiple myeloma. The patient underwent definitive radiotherapy and remained free from any recurrences during follow-up.