Objective: The importance of identifying papillary carcinoma or lymphoma amidst background Hashimoto’s/lymphocytic thyroiditis (H/LT) is well documented. However, avoidance of overdiagnosing neoplasms on thyroid fine-needle aspiration (FNA) with only H/LT has not been adequately addressed. Study Design: This study aimed to identify cytomorphologic features leading to overdiagnosing neoplasms within background H/LT. Nine thyroid FNAs classified as suspicious or positive for neoplasm with subsequent thyroidectomy specimens having only H/LT were identified. Cytologic features of these cases were compared to 8 control cases from the same time period and FNAs from both groups were reevaluated for features from the cytology literature. Results: Features leading to overdiagnosing papillary carcinoma were: powdery chromatin, occasional nuclear grooves or holes, and paucity of background lymphocytes. One feature differentiating H/LT from neoplasm noted in most cases was lymphocytes infiltrating follicular groups. In contrast, true papillary carcinomas displayed characteristic features in multiple cell clusters. These clusters were devoid of infiltrating lymphocytes or displayed only rare lymphocytes at their periphery. A microfollicular pattern with paucity of background lymphocytes was the major pitfall in overdiagnosing follicular neoplasm. Conclusions: Features suspicious for neoplasm are often seen in FNA of H/LT, leading to unnecessary surgery. Recognizing this pitfall and its differentiating features should avert overdiagnosis.

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