What is the normal Standardized Uptake Value (SUV) range for the thyroid in a full body I-131 (Iodine-131) Single Photon Emission Computed Tomography (SPECT) scan?

Normal SUV Range for Thyroid in Full Body I-131 SPECT Scan

There is no established normal standardized uptake value (SUV) range for the thyroid in full body I-131 SPECT scans, as SUV quantification is not typically used in I-131 imaging for thyroid evaluation. Instead, qualitative visual assessment and percentage uptake measurements are the standard methods for interpreting I-131 thyroid scans.

I-131 Thyroid Imaging: Standard Approach

• I-131 whole-body scintigraphy (WBS) is primarily used for post-treatment evaluation of differentiated thyroid cancer (DTC) patients after thyroidectomy and radioiodine ablation 1
• The scan is interpreted qualitatively by visual assessment of uptake patterns rather than through SUV quantification 1
• For patients who have undergone thyroidectomy and radioiodine ablation, any visible uptake in the thyroid bed on follow-up scans may indicate residual thyroid tissue 1

Thyroid Uptake Measurements in I-131 Imaging

• In diagnostic thyroid scanning, uptake is typically measured as a percentage of the administered dose rather than as an SUV 1
• Normal thyroid uptake is assessed through radioiodine uptake tests, which measure the percentage of administered dose in the thyroid at specific time points (typically 3-hour and 24-hour) 2
• The mean 3-hour radioiodine uptake in hyperthyroid patients is approximately 55.5 ± 23.69% (though this is elevated compared to normal thyroid function) 2

Why SUV Is Not Typically Used for I-131 Thyroid Imaging

• SUV quantification is more commonly used in FDG-PET/CT imaging rather than in I-131 SPECT scanning 1
• I-131 scans are typically interpreted based on the presence or absence of uptake in the thyroid bed or metastatic sites 3
• After complete thyroid ablation, no uptake should be detected in the thyroid bed on follow-up scans 3

Clinical Applications of I-131 Scanning

• I-131 whole-body scans are performed after radioiodine therapy to evaluate for residual disease 1
• In intermediate and high-risk DTC patients, whole-body scintigraphy has a role in evaluating initial response to radioiodine ablation 1
• If there has been excellent response to therapy, WBS is usually not performed for ongoing surveillance 1

Important Considerations for I-131 Scanning

• Diagnostic doses of I-131 typically range from 74 MBq (2 mCi) to 185 MBq (5 mCi) 3
• Higher doses (3.7 GBq or 100 mCi) may be used for therapy or for scanning patients with elevated thyroglobulin levels above 10 ng/mL 3
• Caution should be exercised with diagnostic I-131 doses as they may potentially cause "thyroid stunning," which could decrease further radioiodine uptake in subsequent therapeutic administrations 4

Alternative Imaging Approaches

• For routine surveillance of low-risk patients without suspicion for recurrence and normal thyroid ultrasound, I-131 whole-body scanning is not utilized 1
• Ultrasound is the preferred first-line imaging for early follow-up after treatment of differentiated thyroid cancer 1
• FDG-PET/CT scanning is not recommended for routine surveillance if there is no residual disease 1