Intrathyroidal oncocytic parathyroid adenoma: a diagnostic pitfall on fine-needle aspiration

Oncocytic parathyroid adenoma is a rarely seen benign neoplasm of the parathyroid and intrathyroidal location of this lesion is also uncommon. It can be easily misdiagnosed as Hürthle cell thyroid neoplasm on fine-needle aspiration (FNA). Here, an intrathyroidal oncocytic parathyroid adenoma in a 32-year-old male is reported. Ultrasonography showed a nodule 2 cm in largest diameter in the left thyroid lobe. FNA revealed cellular smears containing monotonous oncocytic cells arranged in monolayered sheets, pseudopapillary structures and clusters within a rich vascular network. The case was reported as "suspicious for Hürthle cell thyroid neoplasm" on FNA. Frozen section examination showed intrathyroidal oncocytic parathyroid adenoma with a rim of normal parathyroid tissue. The diagnosis was also confirmed with immunohistochemical stains (TTF-1, PTH, thyroglobulin) performed on permanent sections. Cytopathologic features of oncocytic parathyroid adenoma and Hürthle cell thyroid neoplasm are similar. It is important to keep oncocytic parathyroid adenoma in mind in the differential diagnosis of Hürthle cell thyroid neoplasm. Prior knowledge of clinical, radiological and laboratory data will avoid wrong cytopathologic diagnosis.