A 38-year-old woman presents with a 2-week history of severe neck pain, fever, and malaise. She reports a recent upper respiratory tract infection 3 weeks ago. On examination, the thyroid gland is tender, firm, and enlarged. Laboratory investigations show TSH 0.2 mIU/L (normal 0.5–5.0), free T4 8.5 ng/dL (normal 0.8–1.8), elevated ESR 68 mm/hr, and negative thyroid peroxidase (TPO) and thyroglobulin antibodies. Thyroid ultrasound shows heterogeneous echotexture with reduced vascularity. What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Subacute Granulomatous Thyroiditis ### Key Clinical Features **Key Point:** Subacute granulomatous thyroiditis (de Quervain's thyroiditis) is a viral-triggered, self-limited inflammatory disorder characterized by a triphasic course: thyrotoxicosis → euthyroidism → hypothyroidism. ### Diagnostic Criteria Met in This Case | Feature | Finding | Significance | |---------|---------|---------------| | **Onset** | 3 weeks post-URI | Viral prodrome typical | | **Neck pain** | Severe, tender thyroid | Hallmark feature | | **Fever & malaise** | Present | Systemic inflammation | | **TSH/T4 pattern** | Low TSH, high T4 | Phase 1: thyrotoxicosis | | **Inflammatory markers** | Elevated ESR | Non-specific but marked elevation | | **Antibodies** | Negative TPO, anti-Tg | Rules out autoimmune thyroiditis | | **Ultrasound** | Heterogeneous, reduced flow | Consistent with granulomatous inflammation | ### Pathophysiology 1. **Viral trigger** → lymphocytic infiltration of thyroid 2. **Granuloma formation** → disruption of follicles 3. **Release of preformed thyroid hormone** → thyrotoxicosis (Phase 1) 4. **Recovery of follicular function** → euthyroidism (Phase 2) 5. **Transient hypothyroidism** → antibody-negative (Phase 3) **High-Yield:** The **negative antibodies** are the key discriminator from Hashimoto's thyroiditis, which presents with positive TPO and anti-thyroglobulin antibodies. ### Clinical Pearl **Clinical Pearl:** Unlike Graves' disease (which has sustained thyrotoxicosis and positive TSI), subacute thyroiditis shows **triphasic evolution** with self-resolution in 3–6 months. Radioiodine uptake is **suppressed** (unlike Graves', where it is elevated). ### Management - **Mild cases:** NSAIDs (ibuprofen 400–600 mg TID) - **Moderate-to-severe:** Corticosteroids (prednisone 40 mg daily, tapered over 6–8 weeks) - **Beta-blockers:** For symptomatic thyrotoxicosis (propranolol 40–80 mg TID) - **No antithyroid drugs** (PTU/methimazole) — thyroid hormone is being released, not synthesized **Mnemonic:** **VIRAL** — **V**iral prodrome, **I**nflammatory markers elevated, **R**educed antibodies (negative), **A**cute neck pain, **L**imited course (self-resolving) [cite:Robbins 10e Ch 24]