A 35-year-old woman from a coastal region of India presents with a firm, diffuse enlargement of the thyroid gland without nodules. Laboratory investigations show elevated TSH with low-normal free T4 levels. Which is the most common cause of hypothyroidism in iodine-sufficient regions of India?

## Most Common Cause of Hypothyroidism in Iodine-Sufficient Regions **Key Point:** In iodine-sufficient areas like coastal India, chronic autoimmune thyroiditis (Hashimoto's thyroiditis) is the most frequent cause of hypothyroidism, accounting for approximately 80% of cases in developed and iodine-replete populations. ### Pathophysiology Hashimoto's thyroiditis is a cell-mediated and antibody-mediated autoimmune destruction of thyroid follicles. The condition involves: 1. Infiltration of thyroid tissue by CD8+ T lymphocytes and B cells 2. Production of anti-TPO (thyroid peroxidase) and anti-thyroglobulin antibodies 3. Progressive loss of thyroid parenchyma and fibrosis 4. Gradual decline in thyroid hormone synthesis and secretion ### Clinical Features - Diffuse, firm thyroid enlargement (goiter may be present initially) - Insidious onset of hypothyroid symptoms - Female predominance (10:1 ratio) - Often associated with other autoimmune conditions (celiac disease, type 1 diabetes, pernicious anemia) - Elevated TSH with low-normal or low free T4 ### Laboratory Findings | Parameter | Finding | | --- | --- | | TSH | Elevated (>4.0 mIU/L) | | Free T4 | Low-normal or low | | Anti-TPO antibodies | Positive (>35 IU/mL) | | Anti-thyroglobulin antibodies | Often positive | | Thyroid ultrasound | Heterogeneous echotexture, hypoechoic pattern | **High-Yield:** Anti-TPO antibodies are present in >90% of Hashimoto's thyroiditis cases and are the most sensitive diagnostic marker. Their presence confirms autoimmune etiology. ### Geographic Variation The epidemiology of hypothyroidism varies by iodine status: - **Iodine-sufficient regions (coastal India, developed nations):** Autoimmune thyroiditis > 80% - **Iodine-deficient regions (inland India, developing nations):** Iodine deficiency > 90% - **Post-iodine supplementation:** Autoimmune thyroiditis becomes the leading cause **Clinical Pearl:** The question specifies a coastal region (iodine-sufficient), making autoimmune thyroiditis the clear answer. The firm, diffuse goiter and elevated TSH with low-normal T4 are classic for Hashimoto's. ### Treatment L-thyroxine replacement therapy is the standard treatment, with dose titration based on TSH levels (target TSH 0.5–2.5 mIU/L in most patients). [cite:Harrison 21e Ch 405]