The standard therapeutic iodine isotope. The beta emissions (path length ~0.8 mm in tissue) deliver local cytotoxic dose to thyroid tissue and iodine-avid metastases; the 364 keV gamma allows post-therapy whole body scintigraphy to assess distribution.
High gamma energy (364 keV) is suboptimal for standard gamma cameras (designed for ~140 keV) — leads to septal penetration and degraded image quality. For this reason I-123 is preferred for diagnostic thyroid imaging when possible.
Thyroid stunning: a high diagnostic dose of I-131 can transiently reduce iodine uptake of remnant/metastatic tissue before therapeutic dosing. To avoid this, I-123 is used for pre-therapy diagnostic scans when available, or low-dose I-131 (≤3 mCi) is used if I-123 unavailable.
Radiation precautions required post-therapy due to long half-life and beta emission — patients typically isolated or discharged with strict instructions for the first few days.