Question
Radioactive Iodine
Answer
Radioactive iodine (I-131) is used for definitive treatment of hyperthyroidism.
MOA: I-131 → taken up by the thyroid via the sodium-iodide symporter (the same physiological uptake mechanism as normal iodide) → concentrates specifically in thyroid tissue → beta-particle emission causes localized cellular damage/destruction over weeks → gradually reduces functional thyroid tissue.
Uses: definitive treatment of Graves’ disease and toxic nodular goitre, differentiated thyroid carcinoma (post-thyroidectomy ablation of residual tissue).
Adverse effects: hypothyroidism (the eventual, expected outcome of adequate treatment, requiring lifelong levothyroxine replacement), transient radiation thyroiditis. Absolutely contraindicated in pregnancy — crosses the placenta and concentrates in the fetal thyroid once it becomes iodine-avid in the second trimester, causing fetal hypothyroidism/cretinism.

