To determine the cytodiagnostic accuracy rate and pitfalls of Hashimoto's thyroiditis (HT), the files and smears prepared from the thyroid needle aspirates of 146 patients with suspected HT and/or clinically and serologically confirmed HT were reviewed. Of those patients, 105 presented with a diffuse and rubbery thyroid enlargement, and 41 with one or two prominent nodules. For the first group (105 patients), the needle aspiration biopsy (NAB) was performed on one or two thyroid lobes during their initial endocrinologic consultation, and for the second group (41 patients), the NAB was performed on and around the predominant nodules that were found either at initial physical examination or during the patients' routine follow‐ups. In 134 cases, a cytodiagnosis of HT was made on the first NAB. Among the 41 patients with a prominent thyroid nodule, a thyroid neoplasm was suspected clinically in four because their thyroid nodules increased in size. In the other 12 patients, a cytodiagnosis of follicular neoplasm (FN) was made in five cases, and a Hürthle cell tumor (HCT) was diagnosed or suspected in seven patients. All 16 patients had thyroid surgery, and a HT was histologically confirmed in all cases. In the first four patients, no tumor was found. Among five patients with a cytodiagnosis of FN, one had a hyperplastic follicular cell nodule (HFCN), two had follicular adenomas, and two had papillary carcinomas of follicular variant. For the seven patients with a cytodiagnosis of HCT, HCT was confirmed in three, three were found to have hyperplastic Hürthle cell nodules (HHCN), and one showed a benign colloid nodule with Hürthle cell changes and remote hemorrhagic necrosis. It is concluded that NAB is highly sensitive in diagnosing HT, with a diagnostic accuracy rate of 92% by the first biopsy attempt. The cytologic differential diagnosis between an HFCN and a follicular neoplasm and between an HHCN and an HCT is impossible in some cases. Diagn. Cytopathol. 16:531–536, 1997. © 1997 Wiley‐Liss, Inc.

We report a case of a solitary plasmacytoma arising from a thyroid with longstanding Hashimoto's disease, and diagnosed by fine-needle aspiration cytology. Serum protein electrophoresis revealed an M-spike in the gamma-globulin region due to monoclonal IgG-lambda immunoglobulin. The thyroid tumor was treated with near-total thyroidectomy and irradiation, and the patient was well 6 years after surgery without evidence of multiple myeloma. The serum M-spike disappeared after the tumor resection and radiation therapy. Plasma cell lesions of the thyroid reported in the world literature are extensively reviewed. Solitary plasmacytomas occur most commonly in patients with Hashimoto's disease, and must be distinguished from plasma cell granulomas and involvement of the thyroid in multiple myeloma. Plasmacytomas should be considered in the differential diagnosis of a rapidly enlarging thyroid mass in a patient with known Hashimoto's disease.

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Thyroid nodules (TN) are a common clinical problem. Fine needle aspiration (FNA) of the thyroid now is practiced worldwide and proves to be the most economical and reliable diagnostic procedure to identify TNs that need surgical excision and TNs that can be managed conservatively. The key for the success of thyroid FNA consists of an adequate or representative cell sample and the expertise in thyroid cytology. The FNA cytologic manifestations of TNs may be classified into seven working cytodiagnostic groups consisting of a few heterogenous lesions each to facilitate the differential diagnosis. Recent application of diagnostic molecular techniques to aspirated thyroid cells proved to be useful in separating benign from malignant TNs in several cases of indeterminate lesions.

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