## Histopathology of Hashimoto Thyroiditis **Key Point:** Hashimoto thyroiditis is characterized by a chronic autoimmune lymphocytic infiltration with germinal centres (lymphoid follicles) and the pathognomonic presence of Hürthle cells (oxyphil cells). ### Microscopic Features | Feature | Hashimoto | Graves | Subacute | Riedel | | --- | --- | --- | --- | --- | | **Infiltrate** | Lymphocytes + plasma cells | Lymphocytes (mild) | Granulomas + giant cells | Fibrosis | | **Germinal centres** | Present (+++), prominent | Absent | Absent | Absent | | **Hürthle cells** | Present (pathognomonic) | Absent | Absent | Absent | | **Fibrosis** | Minimal | Minimal | Minimal | Extensive | | **Thyroid follicles** | Destroyed, atrophic | Hyperplastic | Disrupted | Replaced | **High-Yield:** Hürthle cells (also called Askanazy or oxyphil cells) are thyroid follicular cells with abundant mitochondria and eosinophilic cytoplasm — they appear in response to chronic autoimmune destruction and are virtually diagnostic of Hashimoto when present. ### Pathological Progression 1. Early: Lymphocytic infiltration around follicles 2. Active: Germinal centre formation, follicular destruction 3. Late: Fibrosis, atrophy, hypothyroidism **Clinical Pearl:** The presence of germinal centres with secondary lymphoid follicles indicates active, ongoing autoimmune destruction — this distinguishes Hashimoto from other forms of thyroiditis histologically. **Mnemonic:** **HASH** = **H**ürthle cells, **A**utoimmune, **S**econdary follicles (germinal centres), **H**ypofunction (hypothyroidism).
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