A 35-year-old woman from North India presents with fatigue, weight gain, and cold intolerance for 6 months. Thyroid function tests show elevated TSH and low free T4. What is the most common cause of hypothyroidism in this patient?

Explanation

## Most Common Cause of Hypothyroidism **Key Point:** In iodine-sufficient regions (including urban and semi-urban India), chronic autoimmune thyroiditis (Hashimoto's thyroiditis) is the most common cause of hypothyroidism, accounting for >90% of cases in developed countries and an increasing proportion in India. ### Pathophysiology of Hashimoto's Thyroiditis 1. Autoimmune destruction of thyroid follicles 2. Presence of anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin antibodies 3. Lymphocytic infiltration and fibrosis of thyroid tissue 4. Progressive loss of thyroid function ### Epidemiology & Risk Factors | Feature | Hashimoto's | Iodine Deficiency | |---------|-------------|-------------------| | **Prevalence in iodine-sufficient areas** | Most common | Rare | | **Geographic pattern** | Worldwide | Endemic regions | | **Female predominance** | Yes (5–10:1) | No | | **Antibody positivity** | Anti-TPO, anti-Tg | Absent | | **Thyroid ultrasound** | Heterogeneous, hypoechoic | Diffuse enlargement | **High-Yield:** The female predominance (5–10 times more common in women) and presence of circulating autoantibodies are diagnostic hallmarks. ### Clinical Presentation **Key Point:** Insidious onset with: - Fatigue and lethargy - Weight gain despite normal appetite - Cold intolerance - Dry skin and hair loss - Bradycardia and constipation - Elevated TSH with low/low-normal free T4 ### Diagnostic Confirmation **Clinical Pearl:** Anti-TPO antibodies are present in >90% of Hashimoto's cases; anti-thyroglobulin in ~60%. Their presence confirms autoimmune etiology. **Mnemonic:** **HASH** = **H**ashimoto's, **A**utoimmune, **S**ubacute/chronic, **H**ypothyroidism ### Why Hashimoto's is Most Common in India While iodine deficiency remains endemic in certain regions (Himalayan foothills, parts of Northeast India), most urban and semi-urban Indian patients with hypothyroidism have adequate iodine intake. In these populations, autoimmune thyroiditis dominates, reflecting global epidemiological trends and improved iodine fortification programs. **Tip:** Always check anti-TPO and anti-thyroglobulin antibodies in newly diagnosed hypothyroidism to confirm autoimmune etiology and assess risk of other autoimmune conditions (celiac disease, type 1 diabetes, Addison's disease).