Radioactive Iodine Dosing for Toxic Nodule
For toxic adenomas (autonomously functioning toxic thyroid nodules), administer a fixed dose of 10-30 mCi of I-131, with most patients requiring 10 mCi for nodules ≤3 cm and up to 30 mCi for larger nodules. 1, 2
Standard Dosing Protocol
The most practical approach is a fixed-dose regimen rather than complex calculations:
- For nodules ≤3 cm: Administer 10 mCi (370 MBq) as initial therapy 1
- For nodules >3 cm: Administer 20-30 mCi (740-1110 MBq) as initial therapy 2, 3
- Alternative calculation method: 3.7 MBq per gram of thyroid tissue, corrected to 100% 24-hour I-131 uptake 3
The fixed-dose approach has proven highly effective, with complete cure rates of 90% for smaller nodules after a single 10 mCi dose 1. For larger nodules, a second 10 mCi dose typically achieves complete resolution if the first dose is insufficient 1.
Pre-Treatment Requirements
Before administering RAI, confirm the diagnosis with:
- Low/suppressed TSH with elevated FT4/T4 2
- Hot nodule on thyroid scintigraphy with suppression of contralateral lobe uptake 4
- Measure 24-hour radioiodine uptake (typical range: 7-54%) 5
- Ultrasound measurement of nodule diameter 3
Expected Outcomes by Time Point
At 3 months post-treatment:
At 6 months post-treatment:
At 12 months post-treatment:
- 77% achieve euthyroid status 3
- Total nodule volume reduction of 54% (from mean 26 ml to 12 ml) 3
- 10% develop hypothyroidism 3
- Complete nodule regression in 56% of cases 5
Retreatment Strategy
If hyperthyroidism persists at 6 months, administer a second dose of 10-30 mCi. 1, 2 The failure rate requiring retreatment is approximately 10-25% 1, 2. Most patients who require retreatment achieve cure after the second dose 1.
Critical Clinical Considerations
Key advantages of the fixed-dose approach:
- Eliminates need for complex dosimetry calculations 1, 2
- Highly reproducible outcomes across patient populations 1, 3
- Low hypothyroidism risk (10-35%) compared to Graves' disease treatment 1, 5, 3
Common pitfall to avoid: Do not use the higher doses (100-200 mCi) recommended for thyroid cancer, as these are inappropriate for benign toxic nodules and will result in excessive hypothyroidism 6, 7. The toxic nodule dosing is fundamentally different from differentiated thyroid cancer protocols.
Contraindications: Pregnancy must be excluded before treatment 7.