Background: Hyalinizing trabecular adenoma (HTA) is a rare benign epithelial tumor of the thyroid which shows a prominent trabecular growth pattern and stromal hyalinization. On fine-needle aspiration cytology, HTA is frequently misdiagnosed as either papillary thyroid carcinoma (PTC) or medullary carcinoma. We present both the cytologic and the histopathologic features of HTA in a patient with Hashimoto’s thyroiditis. Case: Cytologically, the tumor cells showed a low nucleocytoplasmic ratio and eccentrically located nuclei, nuclear grooves, and eosinophilic pseudoinclusions. Lymphocyte-dominant inflammatory cells were present in the background, raising the possibility of thyroiditis. Histologically, the tumor was a 0.5 × 0.4 cm-sized mass and showed a trabecular and nested pattern of tumor cells separated by scant hyaline material in the background of Hashimoto’s thyroiditis. Tumor cells showed abundant eosinophilic granular cytoplasms, nuclear grooves, and pseudoinclusions, as well as immunoreactivity for MIB-1 on the cell membrane. We diagnosed this lesion as HTA in a patient with Hashimoto’s thyroiditis. Conclusion: Although distinction of HTA from PTC in the cytologic specimen is difficult, especially in cases associated with Hashimoto’s thyroiditis, cohesive cell aggregates with a low nucleocytoplasmic ratio and eccentrically located nuclei may be helpful to consider the possibility of HTA.

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