A 42-year-old woman from Mumbai presents with a 3-week history of severe anterior neck pain, fever, and malaise. On examination, the thyroid gland is tender and firm. Laboratory investigations show TSH 0.2 mIU/L (normal 0.5–5), free T4 8.5 ng/dL (normal 0.8–1.8), elevated ESR 68 mm/hr, and CRP 12 mg/dL. Thyroid peroxidase (TPO) antibodies are negative. Ultrasound shows heterogeneous echotexture with hypoechoic areas. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Subacute Granulomatous Thyroiditis ### Key Clinical Features **Key Point:** Subacute granulomatous thyroiditis (also called de Quervain's thyroiditis) is characterized by the classic triad of severe neck pain, thyroiditis, and thyrotoxicosis in the absence of autoimmune markers. ### Diagnostic Hallmarks | Feature | Subacute Granulomatous | Hashimoto's | Postpartum | Riedel's | |---------|------------------------|-------------|-----------|----------| | **Neck pain** | Severe, prominent | Absent | Absent | Minimal | | **Fever** | Common | Absent | Absent | Absent | | **TPO antibodies** | Negative | Positive | May be positive | Negative | | **ESR/CRP** | Markedly elevated | Normal/mildly elevated | Normal | Mildly elevated | | **Thyroid texture** | Heterogeneous, hypoechoic | Hypoechoic, diffuse | Variable | Rock-hard, fibrotic | | **Thyrotoxic phase** | Early, prominent | Absent | Mild, transient | Absent | ### Pathophysiology 1. Viral infection (mumps, coxsackievirus, adenovirus) triggers immune response 2. Destruction of thyroid follicles → release of preformed thyroid hormones → **thyrotoxicosis** 3. Granulomatous inflammation with giant cells on histology 4. Self-limited course: 4–12 weeks typically ### Clinical Phases **High-Yield:** The disease progresses through three phases: - **Phase 1 (Thyrotoxic):** 2–6 weeks — neck pain, fever, elevated free T4, suppressed TSH - **Phase 2 (Hypothyroid):** 2–4 weeks — TSH rises, free T4 falls, symptoms resolve - **Phase 3 (Recovery):** Normal thyroid function restored (90% of cases) ### Why This Patient Has Subacute Thyroiditis 1. **Severe neck pain** — pathognomonic feature, absent in Hashimoto's and postpartum thyroiditis 2. **Negative TPO antibodies** — rules out autoimmune thyroiditis (Hashimoto's, postpartum) 3. **Marked elevation of inflammatory markers (ESR 68, CRP 12)** — characteristic of viral-induced granulomatous inflammation 4. **Thyrotoxicosis (TSH 0.2, free T4 8.5)** — early phase with follicular destruction 5. **Heterogeneous ultrasound echotexture** — reflects patchy granulomatous inflammation ### Management Approach ```mermaid flowchart TD A[Subacute Thyroiditis Diagnosed]:::outcome --> B{Severity of Symptoms?}:::decision B -->|Mild| C[NSAIDs + Supportive care]:::action B -->|Moderate-Severe| D[Corticosteroids: Prednisolone 40-60 mg daily]:::action D --> E[Taper over 4-6 weeks]:::action F[Beta-blockers for thyrotoxic symptoms]:::action G[Avoid antithyroid drugs - hormone release, not synthesis]:::urgent E --> H[Monitor TSH, free T4 monthly]:::action H --> I[Most recover normal function]:::outcome ``` **Clinical Pearl:** Do NOT use antithyroid drugs (PTU, methimazole) in subacute thyroiditis — the thyrotoxicosis is from hormone **release**, not increased synthesis. Beta-blockers provide symptom relief; corticosteroids reduce inflammation and accelerate recovery. **Mnemonic: "PAIN" in Subacute Thyroiditis:** - **P**ain (severe neck pain) - **A**cute onset (viral prodrome) - **I**nflammatory markers elevated (ESR, CRP) - **N**egative antibodies (TPO, thyroglobulin) [cite:Robbins 10e Ch 24]