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

Question

Accepted Answer

B. Fibrous capsular fibrosis extending beyond the thyroid. ## 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").

Answer

A. Hürthle cell change (oncocytic metaplasia)

Answer

C. Lymphocytic infiltration with germinal centers

Answer

D. Presence of anti-TPO and anti-thyroglobulin antibodies

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

Hashimoto Thyroiditis — Pathological Hallmarks

Clinical Pearl

Mnemonic: HASHIMOTO Features

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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