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Radioiodine therapy (I-131)

aka radioiodine, radioactive iodine, radioiodine ablation, radioiodine therapy, I-131, 131I, iodine-131, RAI-refractory, radioiodine-refractory, RAI-avid, radioiodine-avid, iodine-avid, iodine uptake, redifferentiation, recombinant TSH, rhTSH

Swallowing a capsule of radioactive iodine after thyroid surgery: thyroid cells (including cancer cells) are the only ones that soak up iodine, so the radiation destroys leftover thyroid tissue and metastases while sparing everything else. It was the first targeted radiotherapy, in 1946.

Differentiated (papillary and follicular) thyroid cancer retains iodine uptake, so I-131 ablates remnant tissue after total thyroidectomy in intermediate- and high-risk patients and treats iodine-avid metastases; low-risk patients no longer need it (ESTIMABL2, IoN). Thyroglobulin and whole-body scans monitor for recurrence. 'Radioiodine-refractory' disease (about 5-15%, more in poorly differentiated and BRAF-mutant tumours) is treated with lenvatinib, sorafenib or targeted drugs for RET, NTRK and BRAF alterations, and MEK/BRAF inhibitors can restore iodine uptake ('redifferentiation'). Side effects are salivary gland damage, dry mouth and a small excess of secondary cancers at high cumulative doses.

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