## Most Common Cause of Hypothyroidism in Iodine-Sufficient Regions **Key Point:** Hashimoto thyroiditis (chronic autoimmune thyroiditis) is the most common cause of hypothyroidism in iodine-sufficient countries, including India. ### Epidemiology & Pathophysiology Hashimoto thyroiditis accounts for approximately 90% of hypothyroidism cases in regions with adequate iodine intake. It is an autoimmune condition characterized by: - Infiltration of thyroid follicles by lymphocytes and plasma cells - Production of anti-TPO (thyroid peroxidase) and anti-thyroglobulin antibodies - Progressive destruction of thyroid parenchyma - Female predominance (5–10:1 ratio) ### Diagnostic Features | Feature | Hashimoto Thyroiditis | Iodine Deficiency | |---------|----------------------|-------------------| | **Prevalence in India** | Most common in iodine-sufficient areas | Historically endemic; now rare due to salt iodization | | **Antibodies** | Anti-TPO, anti-thyroglobulin present | Absent | | **Thyroid size** | Often firm, diffuse enlargement | Diffuse goiter | | **TSH level** | Elevated | Elevated | | **Free T4** | Low | Low | | **Ultrasound** | Heterogeneous echotexture, hypoechogenicity | Diffuse parenchymal changes | **High-Yield:** Anti-TPO antibodies are present in >90% of Hashimoto cases and are the gold-standard diagnostic marker. ### Clinical Pearl Hashimoto thyroiditis often presents insidiously with gradual onset of hypothyroid symptoms. The presence of elevated TSH with low free T4 in a patient with no history of iodine deficiency or recent viral illness strongly suggests autoimmune etiology. **Mnemonic:** HASH = **H**ashimoto **A**utoimmune **S**ubacute **H**ypothyroidism — remember it as the most common cause in developed and iodine-sufficient developing nations. [cite:Harrison 21e Ch 405]
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