Radioactive Iodine Uptake Scan Isotope
Iodine-123 (I-123) is the preferred isotope for radioactive iodine uptake (RIU) scans, though Iodine-131 (I-131) can also be used but is less desirable.
Primary Isotope: Iodine-123 (I-123)
I-123 is strongly preferred over I-131 for diagnostic radioiodine uptake scanning because it provides superior image quality, delivers significantly lower radiation exposure to patients, and avoids the risk of "thyroid stunning" that can interfere with subsequent therapeutic interventions 1.
Key Advantages of I-123:
Superior imaging characteristics: The 159 keV photon emission is optimally detected by standard gamma cameras, producing higher quality images than I-131 1
Lower radiation burden: I-123 allows administration of higher activities (200-400 MBq in adults) while still delivering substantially less radiation dose compared to I-131 1
Faster imaging protocol: Scanning can be performed at 24 hours post-injection with I-123, compared to longer delays required with I-131 1
Better SPECT capability: I-123 is more feasible for SPECT imaging, which provides improved anatomic localization 1
No thyroid stunning: I-123 emits minimal particulate radiation and does not cause cellular injury that would reduce subsequent therapeutic I-131 uptake 2, 3
Alternative Isotope: Iodine-131 (I-131)
I-131 can be used for radioiodine uptake scans but is less preferred due to several limitations 1:
Higher radiation exposure: Delivers approximately 10-fold higher effective dose (0.14 mSv/MBq for I-131 versus 0.013 mSv/MBq for I-123) 1
Inferior image quality: The higher energy emissions and beta particles degrade image resolution 1
Risk of stunning: Diagnostic doses of I-131 larger than 3 mCi can cause cellular injury and reduce subsequent therapeutic uptake 3
Lower administered activity: Radiation safety concerns limit the activity that can be given, resulting in lower count rates 1
Clinical Context for RIU Scanning
When RIU Scanning is Indicated:
RIU scans should only be performed when TSH is suppressed (low) to differentiate causes of thyrotoxicosis 4, 5. The scan helps distinguish between:
- Graves' disease (diffuse increased uptake)
- Toxic adenoma (single hot nodule)
- Toxic multinodular goiter (multiple hot areas)
- Thyroiditis (low or absent uptake) 4, 5
Critical Pitfall to Avoid:
Never perform RIU scanning in euthyroid patients with thyroid nodules—it has low positive predictive value for malignancy and does not guide biopsy decisions 1, 4, 5. The majority of nodules are "cold" on scanning, and the majority of cold nodules are benign, making this test unhelpful for cancer risk stratification 1.
Practical Protocol Considerations
Typical I-123 Protocol:
- Administered activity: 200-400 MBq (approximately 5-10 mCi) orally 1, 6
- Imaging timing: 24 hours post-administration (some protocols include 4-6 hour early images) 1, 7
- Thyroid blocking: Requires thyroid blockade with potassium iodide or perchlorate to prevent free iodine uptake 1