A 35-year-old woman presents with fatigue, weight gain, and cold intolerance. Investigations reveal elevated TSH (8.2 mIU/L) and low free T4 (0.6 ng/dL). What is the most common cause of primary hypothyroidism in India?

Explanation

## Most Common Cause of Primary Hypothyroidism in India **Key Point:** Iodine deficiency remains the single most common cause of hypothyroidism worldwide and in India, despite salt iodization programs. It accounts for >90% of hypothyroidism cases in iodine-deficient regions. ### Epidemiology in India **High-Yield:** India is classified as an iodine-deficient country by WHO standards. While salt iodization (introduced in 1983) has reduced the burden, endemic goiter and iodine deficiency hypothyroidism persist in many regions, particularly in hilly and coastal areas with poor iodization compliance. ### Comparison of Causes | Cause | Prevalence in India | Mechanism | TSH | Free T4 | |-------|-------------------|-----------|-----|----------| | **Iodine deficiency** | >70% of hypothyroidism | Insufficient substrate for thyroid hormone synthesis | ↑ | ↓ | | Hashimoto thyroiditis | ~15–20% | Autoimmune destruction; TPO/thyroglobulin antibodies | ↑ | ↓ | | Central hypothyroidism | <5% | Pituitary/hypothalamic dysfunction | ↓ or normal | ↓ | | Thyroid hormone resistance | <1% | Defective thyroid hormone receptor β | ↑ | ↑ (paradoxically) | ### Clinical Pearl **Clinical Pearl:** In iodine deficiency, the thyroid gland is often enlarged (goiter) as a compensatory response to low iodine availability. Hashimoto thyroiditis, by contrast, may present with a firm, non-tender gland and positive TPO antibodies. ### Pathophysiology Iodine is an essential component of thyroid hormones (T3 and T4). Without adequate dietary iodine: 1. Thyroid hormone synthesis decreases 2. Free T4 and T3 levels fall 3. TSH rises (negative feedback loss) 4. Thyroid gland enlarges (goiter) in attempt to trap more iodine **Mnemonic:** **IODINE** = **I**nsufficient **O**utput **D**ecreases **I**ncreases **N**egative feedback → **E**nlargement (goiter) ### Prevention in India Universal salt iodization (USI) is the WHO-recommended strategy. However, compliance remains variable across states, making iodine deficiency the leading preventable cause of hypothyroidism in the Indian population.