A 52-year-old woman from rural Maharashtra presents with a 6-month history of progressive neck swelling, dysphagia, and voice hoarseness. On examination, the thyroid is hard, fixed, and non-tender. Laboratory tests show TSH 8.2 mIU/L, free T4 0.6 ng/dL, negative TPO antibodies, and normal thyroglobulin. Thyroid ultrasound reveals diffuse heterogeneous echotexture with fibrosis extending into the anterior mediastinum. A fine-needle aspiration cytology (FNAC) shows fibrous tissue with sparse lymphocytes and no malignancy. What is the most likely diagnosis?

Question

Accepted Answer

D. Riedel's thyroiditis. ## Clinical Diagnosis: Riedel's Thyroiditis

### Key Diagnostic Features

**Key Point:** Riedel's thyroiditis (Riedel's struma) is a rare, progressive fibroinflammatory disorder of the thyroid characterized by extensive fibrosis that extends beyond the thyroid capsule into surrounding structures (mediastinum, trachea, esophagus).

### Diagnostic Criteria Met

| Feature | Finding | Riedel's Signature |
|---------|---------|-------------------|
| **Gland consistency** | Hard, woody, fixed | Pathognomonic |
| **Extent of fibrosis** | Extends to mediastinum | Extrathyroidal extension |
| **Duration** | Insidious, progressive (6 months) | Chronic inflammatory process |
| **Compressive symptoms** | Dysphagia, voice hoarseness | Mass effect on adjacent structures |
| **Antibody status** | Negative TPO, normal Tg | Non-autoimmune |
| **Hypothyroidism** | Present (TSH 8.2, low T4) | Due to fibrosis, not inflammation |
| **FNAC findings** | Fibrous tissue, sparse lymphocytes | Fibrosis predominates over cellular infiltrate |
| **Ultrasound** | Heterogeneous with mediastinal extension | Confirms extrathyroidal involvement |

### Pathophysiology

1. **Chronic lymphocytic infiltration** → progressive fibrosis
2. **Replacement of thyroid parenchyma** by fibrous tissue
3. **Extrathyroidal extension** → compression of trachea, esophagus, recurrent laryngeal nerve
4. **Secondary hypothyroidism** → from loss of functional thyroid tissue
5. **Association with systemic fibrosis** → retroperitoneal fibrosis, mediastinal fibrosis, sclerosing cholangitis

**High-Yield:** Riedel's thyroiditis is part of a **systemic fibrosing syndrome** — always screen for retroperitoneal fibrosis (flank pain, renal insufficiency) and other fibrosing conditions.

### Clinical Pearl

**Clinical Pearl:** The **hard, woody, fixed thyroid** that extends beyond the capsule is the distinguishing feature. Unlike Hashimoto's (soft, diffusely enlarged, painless), Riedel's causes **compressive symptoms** and **extrathyroidal fibrosis**. FNAC shows predominantly fibrous tissue, not lymphocytes.

### Differential Diagnosis Table

| Feature | Riedel's | Hashimoto's | Anaplastic CA |
|---------|---------|-------------|---------------|
| **Consistency** | Hard, woody, fixed | Firm, diffuse, mobile | Hard, fixed |
| **Extrathyroidal extension** | Yes (mediastinum) | No | Yes (aggressive) |
| **Antibodies** | Negative | Positive TPO | N/A |
| **FNAC** | Fibrosis, sparse cells | Lymphocytes, germinal centers | Malignant cells |
| **Malignancy risk** | No | No | 100% |
| **Systemic fibrosis** | Yes (common) | No | No |

### Management

1. **Corticosteroids** — prednisone 40–60 mg daily, tapered over weeks to months
2. **Tamoxifen** — 10–20 mg BID (anti-fibrotic effect in some cases)
3. **Levothyroxine** — for hypothyroidism replacement
4. **Surgical intervention** — if severe airway/esophageal obstruction (isthmusectomy, tracheal reconstruction)
5. **Screen for systemic fibrosis** — CT abdomen for retroperitoneal fibrosis, liver function tests for sclerosing cholangitis

**Mnemonic:** **RIEDEL** — **R**are fibroinflammatory, **I**nfiltration beyond capsule, **E**xtragoitral extension, **D**ifficult swallowing/hoarseness, **E**xtensive fibrosis, **L**arge systemic fibrosing syndrome association

[cite:Robbins 10e Ch 24]

Answer

A. Anaplastic thyroid carcinoma

Answer

B. Hashimoto's thyroiditis with fibrous variant

Answer

C. Iodine-deficiency goiter with secondary fibrosis

Explanation

Clinical Diagnosis: Riedel's Thyroiditis

Key Diagnostic Features

Diagnostic Criteria Met

Pathophysiology

Clinical Pearl

Differential Diagnosis Table

Management

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Feature	Finding	Riedel's Signature
Gland consistency	Hard, woody, fixed	Pathognomonic
Extent of fibrosis	Extends to mediastinum	Extrathyroidal extension
Duration	Insidious, progressive (6 months)	Chronic inflammatory process
Compressive symptoms	Dysphagia, voice hoarseness	Mass effect on adjacent structures
Antibody status	Negative TPO, normal Tg	Non-autoimmune
Hypothyroidism	Present (TSH 8.2, low T4)	Due to fibrosis, not inflammation
FNAC findings	Fibrous tissue, sparse lymphocytes	Fibrosis predominates over cellular infiltrate
Ultrasound	Heterogeneous with mediastinal extension	Confirms extrathyroidal involvement

Feature	Riedel's	Hashimoto's	Anaplastic CA
Consistency	Hard, woody, fixed	Firm, diffuse, mobile	Hard, fixed
Extrathyroidal extension	Yes (mediastinum)	No	Yes (aggressive)
Antibodies	Negative	Positive TPO	N/A
FNAC	Fibrosis, sparse cells	Lymphocytes, germinal centers	Malignant cells
Malignancy risk	No	No	100%
Systemic fibrosis	Yes (common)	No	No