During a routine thyroid screening program in a hospital, multiple patients with hypothyroidism are identified. Serological testing confirms elevated anti-TPO antibodies in most cases. What is the most common histopathological finding in the thyroid gland of patients with this autoimmune thyroid disease?

Explanation

## Histopathology of Hashimoto Thyroiditis **Key Point:** Lymphocytic infiltration with germinal centers and progressive fibrosis is the pathognomonic histopathological hallmark of Hashimoto thyroiditis. ### Microscopic Features The histological progression in Hashimoto thyroiditis follows a characteristic pattern: 1. **Early Stage:** Lymphocytic infiltration (T cells and B cells) around thyroid follicles 2. **Active Stage:** - Formation of germinal centers (secondary lymphoid follicles) - Plasma cell infiltration - Destruction of thyroid follicular epithelium (Hürthle cells / oncocytic metaplasia) 3. **Late Stage:** - Progressive fibrosis and atrophy - Replacement of thyroid parenchyma by fibrous tissue - Reduced follicular size and colloid content ### Comparative Histopathology | Condition | Key Histological Feature | Antibodies | |-----------|--------------------------|------------| | **Hashimoto Thyroiditis** | Lymphocytic infiltration with germinal centers + fibrosis | Anti-TPO, anti-thyroglobulin | | **Graves Disease** | Lymphocytic infiltration (less organized), hyperplastic follicles, increased colloid | TSH receptor antibodies | | **Subacute Thyroiditis** | Granulomatous inflammation (non-caseating), giant cells, minimal fibrosis | Absent (viral trigger) | | **Riedel Thyroiditis** | Dense fibrosis extending beyond thyroid capsule, minimal inflammation | Absent | **High-Yield:** Germinal centers are the distinguishing feature that separates Hashimoto from other thyroid conditions. They represent active B-cell proliferation and antibody production. ### Clinical Pearl The presence of Hürthle cells (oncocytic metaplasia of follicular epithelium) is a characteristic finding in Hashimoto thyroiditis. These cells appear due to increased mitochondrial content as a response to autoimmune attack. **Mnemonic:** **GIF** = **G**erminal centers, **I**nfiltration (lymphocytic), **F**ibrosis — the three cardinal features of Hashimoto histology. [cite:Robbins 10e Ch 24]