All of the following are characteristic features of Hashimoto thyroiditis EXCEPT:

Explanation

## Hashimoto Thyroiditis — Pathological Features ### Correct Answer: Fibrous capsular fibrosis extending beyond the thyroid **Key Point:** Fibrous capsular fibrosis that extends beyond the thyroid gland is a hallmark of **Riedel thyroiditis**, not Hashimoto thyroiditis. Hashimoto shows intrathyroidal fibrosis, but the characteristic "woody" hard fibrosis extending into surrounding tissues is pathognomonic for Riedel. ### Hashimoto Thyroiditis — Pathological Hallmarks | Feature | Presence | Notes | |---------|----------|-------| | Lymphocytic infiltration | ✓ Yes | Dense infiltrate with germinal centers | | Hürthle cell change | ✓ Yes | Oncocytic metaplasia of follicular epithelium | | Fibrosis | ✓ Yes | **Intrathyroidal only** — does not breach capsule | | Anti-TPO antibodies | ✓ Yes | Present in ~95% of cases | | Anti-thyroglobulin antibodies | ✓ Yes | Present in ~60% of cases | | Capsular extension fibrosis | ✗ No | **This is Riedel thyroiditis** | ### Clinical Pearl **High-Yield:** Riedel thyroiditis is the only form of thyroiditis that causes **extrathyroidal fibrosis** — the "woody" hard gland is fixed to surrounding structures (trachea, esophagus, recurrent laryngeal nerve). This can mimic thyroid cancer clinically. Hashimoto, by contrast, causes a firm but mobile gland with autoimmune inflammation confined within the capsule. ### Mnemonic: HASHIMOTO Features - **H**ürthle cells (oncocytic change) - **A**ntibodies (anti-TPO, anti-Tg) - **S**quamous metaplasia (occasional) - **H**yperplastic germinal centers - **I**ntrathyroidal fibrosis (NOT extrathyroidal) - **M**onolayer lymphocytes - **O**ncotic follicular cells - **T**hyroid atrophy (late stage) - **O**utcome: hypothyroidism **Warning:** Do not confuse Hashimoto fibrosis (intrathyroidal, mild) with Riedel fibrosis (extrathyroidal, severe, "woody").