subacute thyroiditis

**Etiology/Pathophysiology:** Thought to be triggered by a viral infection (e.g., coxsackievirus, mumps, adenovirus, influenza) or post-viral inflammatory process, often preceded by an upper respiratory infection. Associated with HLA-B35. Most common in women aged 30-50. Viral-induced destruction of thyroid follicles causes release of preformed thyroid hormone (T4/T3) into the circulation, leading to thyrotoxicosis (NOT increased synthesis). TSH suppression then reduces iodine uptake.

**Histology:** Granulomatous inflammation with multinucleated giant cells, disruption of follicles, and lymphocyte/neutrophil infiltrate, followed by fibrosis. Minimal fibrosis compared with Riedel thyroiditis.

**Clinical course (4 phases):** 1. Thyrotoxic phase (weeks 2-6): painful, tender, firm, enlarged thyroid; pain may radiate to the jaw/ear; fever, malaise, palpitations, tremor, heat intolerance. 2. Hypothyroid phase (weeks to a few months): transient, as follicular stores are depleted. 3. Recovery/euthyroid phase. 4. Most patients (~90%) return to euthyroid; a minority develop permanent hypothyroidism.

**Labs/Diagnosis:** - Elevated ESR and CRP (markedly elevated ESR is classic). - Early: increased T4/T3, suppressed TSH. - LOW radioactive iodine uptake (RAIU) on scan (key discriminator from Graves disease, which shows high, diffuse uptake). - Thyroglobulin elevated; anti-TPO/TRAb typically negative or low titer (no TSH receptor antibodies). - Mild leukocytosis and mild anemia may occur.

**Management:** - Supportive: NSAIDs/aspirin for pain and inflammation. - Glucocorticoids (prednisone) for severe pain or if NSAIDs fail; rapid response supports diagnosis. - Beta-blockers (propranolol) for adrenergic symptoms of thyrotoxicosis. - Antithyroid drugs (methimazole, PTU) are NOT indicated since hormone synthesis is not increased. - Levothyroxine only if symptomatic hypothyroidism develops (often temporary).

**Differential/High-yield comparisons:** - Graves disease: painless, high RAIU, TRAb positive, exophthalmos, pretibial myxedema. - Hashimoto thyroiditis: painless, anti-TPO/antithyroglobulin antibodies, Hürthle cells, lymphocytic infiltrate with germinal centers; HLA-DR5; risk of B-cell (MALT) lymphoma. - Painless (silent/lymphocytic) thyroiditis and postpartum thyroiditis: painless, low RAIU, normal ESR, anti-TPO often positive. - Riedel thyroiditis: rock-hard, fixed "woody" thyroid due to fibrosis extending into neck structures; IgG4-related; may cause compressive symptoms. - Acute suppurative thyroiditis: bacterial infection with fever, erythema, abscess; usually normal thyroid function and normal RAIU; associated with pyriform sinus fistula (4th branchial pouch anomaly), most commonly left-sided. - Thyrotoxicosis factitia: low RAIU, low thyroglobulin. - Amiodarone-induced thyroiditis, and exogenous hormone use also cause low RAIU thyrotoxicosis.

**Pearls:** Painful thyroid + high ESR + low RAIU + thyrotoxicosis = de Quervain. Self-limited, usually resolves in 2-6 months; recurrence is uncommon.