Which of the following is the most common cause of hypothyroidism worldwide?

Explanation

## Epidemiology of Hypothyroidism **Key Point:** Iodine deficiency remains the most common cause of hypothyroidism globally, particularly in developing nations and areas without iodine supplementation programs. ### Geographic Distribution - **Worldwide:** Iodine deficiency accounts for >90% of hypothyroidism cases in iodine-deficient regions - **Developed nations (USA, Europe, India with salt iodization):** Hashimoto's thyroiditis is the leading cause - **India:** The prevalence varies by region; iodine deficiency is still significant in non-iodized areas, but Hashimoto's is increasingly recognized in urban populations ### Mechanism of Iodine Deficiency Hypothyroidism Iodine is an essential component of thyroid hormones (T~3~ and T~4~). Without adequate iodine: 1. Thyroid peroxidase cannot incorporate iodine into tyrosine residues 2. Thyroid hormone synthesis decreases 3. TSH rises (secondary to low free T~4~) 4. Thyroid enlarges (goiter formation) in an attempt to increase hormone production ### Other Causes by Frequency (in iodine-replete areas) | Cause | Frequency | Notes | |-------|-----------|-------| | Hashimoto's thyroiditis | Most common in developed nations | Autoimmune; anti-TPO and anti-thyroglobulin antibodies | | Iodine deficiency | Most common worldwide | Preventable with iodine supplementation | | Post-thyroidectomy/RAI | Variable | Iatrogenic | | Central hypothyroidism | <5% | Pituitary or hypothalamic disease | | Medications (amiodarone, lithium) | <5% | Drug-induced | **High-Yield:** When asked about the "most common cause of hypothyroidism worldwide," the answer is iodine deficiency. When asked about "most common in developed countries" or "most common autoimmune cause," the answer is Hashimoto's thyroiditis. **Clinical Pearl:** In India, the prevalence of iodine deficiency hypothyroidism depends on iodization coverage. Salt iodization programs have reduced but not eliminated iodine deficiency in certain regions.