A 42-year-old woman presents with a 2-year history of progressive fatigue, weight gain, and cold intolerance. Examination reveals a firm, rubbery, non-tender thyroid enlargement. Serum TSH is elevated at 8.2 mIU/L with low-normal free T4. Anti-TPO antibodies are positive. A thyroid biopsy is performed showing diffuse lymphocytic infiltrate with germinal center formation and atrophic follicles. The cells marked **C** in the diagram demonstrate abundant granular eosinophilic cytoplasm with numerous mitochondria and pyknotic nuclei. What is the pathological significance of these cells marked **C** in Hashimoto thyroiditis?

Explanation

## Why Hurthle (Askanazy) cells representing oncocytic metaplasia of follicular epithelium due to autoimmune destruction is right The cells marked **C** are Hurthle (Askanazy) cells, which are a hallmark histopathological finding in Hashimoto thyroiditis. These cells arise from oncocytic metaplasia of the remaining thyroid follicular epithelium in response to chronic autoimmune destruction. Their characteristic abundant granular eosinophilic cytoplasm is due to accumulation of numerous mitochondria, reflecting increased metabolic activity. This finding is pathognomonic for Hashimoto thyroiditis and indicates the chronic autoimmune process targeting the thyroid follicles (Robbins Basic Pathology 11e; Williams Textbook of Endocrinology 14e). ## Why each distractor is wrong - **Langerhans cells indicating granulomatous inflammation characteristic of de Quervain thyroiditis**: Langerhans cells and granulomatous inflammation are features of de Quervain (subacute) thyroiditis, not Hashimoto thyroiditis. Hashimoto is characterized by lymphocytic infiltration, not granulomas. The morphology described (eosinophilic cytoplasm, mitochondria) does not match Langerhans cells. - **Plasma cells producing anti-thyroid peroxidase antibodies in response to thyroid antigens**: While plasma cells are present in the lymphocytic infiltrate and do produce anti-TPO antibodies, they are not the cells marked **C**. Plasma cells have a different morphology (eccentric nucleus, basophilic cytoplasm) and are not characterized by abundant mitochondria or eosinophilic cytoplasm. - **Lymphoid follicle center cells representing the site of B-cell activation and antibody production**: Although germinal centers (marked **B**) are prominent in Hashimoto thyroiditis and contain follicle center cells, the cells marked **C** are follicular epithelial cells, not lymphoid cells. The eosinophilic cytoplasm and mitochondrial content are features of Hurthle cells, not lymphoid follicle center cells. **High-Yield:** Hurthle (Askanazy) cells = oncocytic metaplasia of thyroid follicles in Hashimoto thyroiditis; abundant mitochondria → granular eosinophilic cytoplasm; pathognomonic finding distinguishing Hashimoto from other thyroiditis types. [cite: Robbins Basic Pathology 11e; Williams Textbook of Endocrinology 14e]