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?

Explanation

## 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]