Which is the most common cause of thyroiditis worldwide?

Explanation

## Most Common Thyroiditis: Hashimoto Thyroiditis ### Epidemiology and Prevalence **Key Point:** Hashimoto thyroiditis (chronic autoimmune thyroiditis) is the most common cause of thyroiditis and the leading cause of hypothyroidism in iodine-sufficient regions worldwide. **High-Yield:** In India and other iodine-sufficient countries, Hashimoto thyroiditis accounts for approximately 60–70% of all thyroiditis cases and is the most frequent autoimmune endocrine disorder. ### Pathophysiology Hashimoto thyroiditis is a cell-mediated and antibody-mediated autoimmune disorder characterized by: - Infiltration of thyroid by lymphocytes and plasma cells - Formation of germinal centers - Destruction of thyroid follicles - Fibrosis in advanced stages - Positive anti-TPO (thyroid peroxidase) and anti-thyroglobulin antibodies ### Clinical Presentation - Insidious onset of hypothyroidism - Diffuse firm goiter (may be absent in atrophic variant) - Female predominance (5–10:1) - Peak incidence: 30–50 years - Associated with other autoimmune conditions (celiac disease, type 1 diabetes, pernicious anemia) ### Comparison with Other Thyroiditis Types | Feature | Hashimoto | De Quervain | Graves | Riedel | |---------|-----------|-------------|--------|--------| | **Frequency** | Most common | Uncommon | Autoimmune but not thyroiditis | Rare | | **Etiology** | Autoimmune | Viral (post-viral) | Autoimmune (TSH-R antibodies) | Idiopathic fibrosis | | **Presentation** | Insidious hypothyroidism | Acute thyroiditis + pain | Hyperthyroidism | Fibrosis + hypothyroidism | | **Antibodies** | Anti-TPO, anti-Tg | Absent | TSI (TSH receptor) | Absent | | **Prognosis** | Permanent hypothyroidism | Self-limited | Relapsing-remitting | Progressive fibrosis | **Clinical Pearl:** Graves disease is an autoimmune disorder causing hyperthyroidism, but it is NOT classified as thyroiditis because it does not involve inflammation of the thyroid tissue itself — it is mediated by stimulating antibodies (TSI) rather than destructive inflammation. **Warning:** De Quervain (subacute granulomatous) thyroiditis is the second most common cause of symptomatic thyroiditis, but it is self-limited and accounts for only 5–15% of thyroiditis cases. It is often confused with Hashimoto in exams, but its viral etiology and acute presentation distinguish it. **Mnemonic:** HASH = **H**ashimoto is **A**utoimmune, **S**low (insidious), **H**ypothyroidism (outcome). ### Why Hashimoto Is Most Common 1. **Genetic predisposition:** HLA-DR3, HLA-DR4, HLA-DR5 associations 2. **Environmental triggers:** Iodine intake, infections, stress 3. **Female predominance:** Estrogen-mediated immune response 4. **Chronic nature:** Persistent autoimmune attack leads to long-term prevalence [cite:Robbins 10e Ch 24]