Radioactive Iodine Uptake Scan with I-123
A radioactive iodine uptake scan with I-123 is a nuclear medicine diagnostic procedure that measures how much iodine-123 the thyroid gland absorbs from the bloodstream and creates images showing the distribution of iodine within the thyroid tissue. 1
What the Test Involves
The patient receives an oral dose of I-123 sodium iodide, typically 3.7 to 14.8 MBq (100-400 µCi), with the lower dose (100 µCi) used for uptake measurements alone and the higher dose (400 µCi) when imaging is also needed. 1 Measurements are initiated at 6 hours after administration and should be performed according to standardized procedures. 1
Clinical Applications
The I-123 uptake and scan serves multiple diagnostic purposes:
Primary Indications
- Evaluating thyroid function and morphology 1
- Differentiating causes of thyrotoxicosis (Graves disease, toxic adenoma, toxic multinodular goiter, versus destructive thyroiditis) 2
- Confirming goiter tissue composition, particularly when associated with thyrotoxicosis 2
- Planning radioactive iodine therapy by providing baseline thyroid activity measurements 2
Specific Clinical Scenarios
In thyrotoxicosis, I-123 radionuclide uptake and scan is usually appropriate as initial imaging and is complementary with ultrasound. 2 The scan directly measures thyroid activity rather than inferring it based on blood flow, making it superior to Doppler ultrasound for this purpose. 2
For multinodular goiter, the scan should be compared with ultrasound to identify hypofunctioning or isofunctioning nodules that require biopsy. 2
In suspected recurrence of differentiated thyroid cancer, I-123 whole body scan is usually appropriate and complementary with ultrasound. 2
Advantages of I-123 Over I-131
I-123 is preferred over I-131 because of its superior imaging quality. 2 As a pure gamma emitter with excellent characteristics for modern scintillation cameras, I-123 provides better image resolution. 3
I-123 substantially reduces thyroid radiation exposure compared to I-131 while providing equally reliable clinical results. 4, 3 Studies demonstrate that 50 MBq (1.5 mCi) I-123 produces superior imaging compared to 111 MBq (3 mCi) I-131, detecting all foci of thyroid tissue that I-131 identifies and occasionally revealing additional lesions. 3
Normal Values and Interpretation
Normal thyroidal I-123 uptake ranges from 1% to 8.8% at 2 hours and 4% to 27% at 24 hours in euthyroid subjects. 4 These values help distinguish between overactive thyroid conditions (elevated uptake) and destructive processes (low uptake). 5
Radiation Dosimetry
The estimated absorbed radiation dose to the thyroid from the maximum 14.8 MBq (400 µCi) dose varies with thyroid uptake: 25 mGy at 5% uptake, 77 mGy at 15% uptake, and 130 mGy at 25% uptake. 1 These doses are substantially lower than comparable I-131 studies. 1
Important Limitations
Cold nodules on uptake scans have low positive predictive value for malignancy—while cold nodules are more likely to be malignant, the majority of nodules are cold and the majority of cold nodules are benign. 2 Therefore, uptake scans are not helpful in determining malignancy or deciding whether to biopsy a nodule. 2
There is no role for radioiodine uptake studies in primary hypothyroidism, as all causes will show decreased uptake without providing diagnostically useful information. 2
Medullary thyroid cancer cells do not take up iodine, making I-123 scans inappropriate for this cancer type. 2