A 38-year-old woman presents with a 2-week history of severe anterior neck pain, fever (38.5°C), and progressive difficulty swallowing. She reports a recent upper respiratory tract infection 1 week ago. On examination, the thyroid is tender, firm, and enlarged. Laboratory investigations show: TSH 0.2 mIU/L (normal 0.4–4.0), free T4 8.5 ng/dL (normal 0.8–1.8), ESR 68 mm/hr, CRP 12 mg/dL. Thyroid ultrasound shows heterogeneous echotexture with reduced vascularity. Fine-needle aspiration cytology reveals granulomatous inflammation with giant cells. 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 or viral thyroiditis) is characterized by the classic triad of: 1. Severe neck pain with tenderness 2. Systemic inflammation (fever, elevated ESR/CRP) 3. Thyrotoxicosis followed by hypothyroidism ### Pathological Hallmark **High-Yield:** The presence of **granulomatous inflammation with giant cells** on FNAC is pathognomonic. This distinguishes it from other forms of thyroiditis: - Hashimoto's: lymphocytic infiltration, no granulomas - Acute suppurative: acute inflammatory cells, bacteria, abscess formation - Riedel's: fibrosis with IgG4-positive cells, painless ### Laboratory Pattern **Clinical Pearl:** The thyroid hormone profile shows **thyrotoxicosis (elevated free T4, suppressed TSH)** in the acute phase due to release of preformed thyroid hormones from damaged follicles—NOT thyroid hormone synthesis. This distinguishes it from Graves' disease (where synthesis is increased). ### Timeline and Etiology **Key Point:** Typically follows a viral upper respiratory infection by 1–3 weeks. Proposed viral triggers include mumps, measles, coxsackievirus, and adenovirus. The disease is self-limited, lasting 4–12 weeks. ### Comparison Table: Forms of Thyroiditis | Feature | Subacute Granulomatous | Hashimoto's | Acute Suppurative | Riedel's | |---------|------------------------|-------------|-------------------|----------| | **Onset** | Acute (days–weeks) | Insidious | Acute | Insidious | | **Pain** | Severe, tender | Mild/absent | Severe, localized | Absent | | **Fever** | Yes | No | Yes | No | | **ESR/CRP** | Markedly elevated | Mildly elevated | Markedly elevated | Normal/mild | | **Pathology** | Granulomas, giant cells | Lymphocytes, plasma cells | Acute inflammation, PMNs | Fibrosis, IgG4 cells | | **TSH/T4 phase 1** | ↓ TSH, ↑ T4 (release) | ↑ TSH, ↓ T4 | ↓ TSH, ↑ T4 | Variable | | **Etiology** | Viral | Autoimmune | Bacterial/fungal | Unknown (IgG4-related?) | ### Management Approach **Tip:** NSAIDs (ibuprofen, indomethacin) are first-line for pain and inflammation. Corticosteroids (prednisolone 40–60 mg/day tapered over 6–8 weeks) are reserved for severe cases with significant systemic symptoms. Beta-blockers manage thyrotoxic symptoms. The disease is self-resolving; long-term hypothyroidism occurs in 5–10% of cases. ### Why the Diagnosis Fits This Case 1. **Prodrome:** Recent URTI (classic viral trigger) 2. **Clinical triad:** Neck pain + fever + thyroid tenderness 3. **Lab pattern:** Thyrotoxicosis (elevated free T4, suppressed TSH) with marked inflammation (ESR 68, CRP 12) 4. **Pathology gold standard:** Granulomatous inflammation with giant cells on FNAC 5. **Ultrasound:** Heterogeneous echotexture with reduced vascularity (typical in subacute phase) [cite:Robbins 10e Ch 24]