Hashimoto Thyroiditis
Hashimoto thyroiditis (chronic lymphocytic thyroiditis) is the most common cause of hypothyroidism in iodine-sufficient areas. It is an autoimmune disorder characterized by the destruction of thyroid follicles.
1. Pathophysiology & Genetics
- Mechanism: T-cell-mediated autoimmunity (Type IV hypersensitivity) and autoantibodies against thyroid peroxidase (TPO) and thyroglobulin.
- Associations: HLA-DR3, HLA-DR5; increased risk of other autoimmune diseases (e.g., Type 1 DM, Addison disease).
2. Histological Hallmarks
| Finding | Description |
|---|---|
| Lymphocytic Infiltrate | Diffuse infiltration with formation of lymphoid follicles (germinal centers). |
| Hürthle Cells | Metaplastic, eosinophilic, granular follicular epithelial cells. |
3. Exam Must-Knows
- Clinical Course: Often presents as a painless, diffuse goiter. Patients may experience a transient hyperthyroid phase (“Hashitoxicosis”) due to initial follicular rupture before moving into a hypothyroid state.
- Malignancy Risk: Significantly increased risk of thyroid lymphoma (typically Marginal Zone/MALT lymphoma).
Pathology Board Hint: For board exams, if you see Hürthle cells and lymphoid aggregates with germinal centers, it is Hashimoto. Always keep in mind that a patient with Hashimoto’s who develops a rapidly enlarging, firm thyroid mass should be evaluated for thyroid lymphoma.