Residency · Residency · Nuclear Medicine
Thyroid Cancer Surveillance: Thyroglobulin and Diagnostic I-131/I-123 Scans
Overview
Long-term surveillance of differentiated thyroid cancer relies on thyroglobulin measurement, neck ultrasound, and selective use of radioiodine diagnostic scans. Dynamic risk stratification reclassifies patients over time based on their response to initial therapy, allowing surveillance intensity to be tailored. FDG PET/CT is reserved for radioiodine-refractory disease with an elevated thyroglobulin level.
Thyroglobulin as a Tumor Marker
Principles
Thyroglobulin is produced exclusively by thyroid follicular cells, both normal and malignant. After total thyroidectomy and RAI ablation, thyroglobulin should be undetectable, below 0.2 ng/mL with sensitive assays. A rising or detectable thyroglobulin level indicates persistent or recurrent disease. When used with appropriate assays, thyroglobulin is a highly sensitive and specific tumor marker.
Stimulated vs. Suppressed Thyroglobulin
Suppressed thyroglobulin is measured while the patient is taking levothyroxine with TSH in the suppressed range. It is most useful for ongoing surveillance, and an undetectable suppressed thyroglobulin carries an excellent prognosis with a recurrence rate below 2%. Stimulated thyroglobulin is measured after TSH stimulation through either thyroid hormone withdrawal or recombinant TSH, with TSH above 30 mIU/L required. It is more sensitive for detecting small-volume disease. A stimulated thyroglobulin below 1 ng/mL after recombinant TSH indicates an excellent response. Values between 1 and 10 ng/mL represent an indeterminate response warranting follow-up. Values above 10 ng/mL indicate a biochemically incomplete response.
Thyroglobulin Antibodies (TgAb)
Thyroglobulin antibodies are present in 20 to 25% of DTC patients. They interfere with thyroglobulin immunometric assays and can cause falsely low thyroglobulin values. Trending TgAb levels serves as a surrogate marker: rising TgAb may indicate recurrence, while declining TgAb suggests disease clearance. Mass spectrometry-based thyroglobulin assays, an emerging technology, are not affected by TgAb interference.
<image>Dynamic risk stratification schema showing reclassification of DTC patients based on stimulated thyroglobulin levels, structural imaging findings, and response categories over time</image>
Dynamic Risk Stratification (Response to Therapy)
Excellent Response
An excellent response is defined by a stimulated thyroglobulin below 1 ng/mL (or suppressed thyroglobulin below 0.2 ng/mL) with negative imaging on ultrasound and diagnostic RAI scan. The recurrence rate is 1 to 4%. Management involves reduced surveillance intensity, and relaxing TSH suppression may be considered.
Indeterminate Response
An indeterminate response is characterized by a suppressed thyroglobulin of 0.2 to 1 ng/mL or stimulated thyroglobulin of 1 to 10 ng/mL, with nonspecific findings on imaging such as small, stable lymph nodes. Most of these patients will resolve spontaneously or remain stable, though 15 to 20% will be reclassified as biochemically or structurally incomplete.
Biochemically Incomplete Response
A biochemically incomplete response is defined by a suppressed thyroglobulin above 1 ng/mL or stimulated thyroglobulin above 10 ng/mL without structural disease identified on imaging. Rising thyroglobulin levels are most concerning and may warrant additional RAI therapy or further imaging.
Structurally Incomplete Response
A structurally incomplete response involves persistent or newly identified structural disease on imaging, including neck ultrasound, CT, or RAI scan. This is the most concerning category and may require additional surgery, RAI, or external beam radiation.
| Response Category | Suppressed Tg | Stimulated Tg | Imaging | Recurrence Risk | TSH Target |
|---|---|---|---|---|---|
| Excellent | <0.2 ng/mL | <1 ng/mL | Negative | 1–4% | 0.5–2.0 mIU/L |
| Indeterminate | 0.2–1 ng/mL | 1–10 ng/mL | Nonspecific findings | 15–20% reclassified | 0.1–0.5 mIU/L |
| Biochemically incomplete | >1 ng/mL | >10 ng/mL | No structural disease | Variable | <0.1 mIU/L |
| Structurally incomplete | Variable | Variable | Persistent/new disease | Highest | <0.1 mIU/L |
Diagnostic Radioiodine Scans
I-123 Diagnostic Whole-Body Scan
I-123 is preferred over I-131 for diagnostic scans to avoid the stunning effect. The dose is 1 to 2 mCi of I-123 administered orally, with imaging at 24 hours. Advantages include the absence of stunning, better image quality due to the lower energy 159 keV gamma emission with thin-crystal cameras, and the ability to identify residual thyroid bed tissue, cervical and mediastinal node uptake, and distant metastases.
I-131 Diagnostic Whole-Body Scan
The I-131 diagnostic scan uses a dose of 1 to 5 mCi with imaging at 48 to 72 hours. It is lower in cost and more widely available than I-123. However, there is a potential for stunning at higher diagnostic doses above 3 mCi, and its sensitivity is limited compared with post-therapy scans because of the lower administered activity.
Indications for Diagnostic RAI Scans
Diagnostic RAI scans are indicated for follow-up of patients with prior RAI-avid disease, evaluation of rising thyroglobulin with a negative ultrasound, and confirmation of RAI-avid disease before empiric RAI therapy. Routine diagnostic scans are not indicated for low-risk patients with an excellent response, meaning thyroglobulin below 1 ng/mL and a negative ultrasound.
When Diagnostic Scan Adds Value
Diagnostic scans are most valuable in intermediate- and high-risk patients during the initial 2 to 5 years of surveillance, in patients with TgAb where thyroglobulin is unreliable as a tumor marker, and as a pre-therapy scan when additional RAI treatment is planned.
<image>I-123 diagnostic whole-body scan at 24 hours showing focal uptake in the thyroid bed (remnant) and a small focus in the right lateral neck consistent with a lymph node metastasis</image>
Neck Ultrasound
Role in Surveillance
Neck ultrasound is the primary imaging modality for locoregional surveillance. It evaluates the thyroid bed, central compartment, and lateral neck lymph nodes. Initial imaging is performed at 6 to 12 months postoperatively, then periodically based on risk. Suspicious lymph node features include microcalcifications, cystic change, hypervascularity, loss of the fatty hilum, and a rounded shape.
Ultrasound-Guided FNA
Fine-needle aspiration is indicated for suspicious lymph nodes with a smallest dimension of 8 to 10 mm or greater. FNA with thyroglobulin measurement in the needle washout (Tg-FNA) confirms metastatic thyroid cancer. A Tg-FNA level above 10 to 100 ng/mL, depending on institutional cutoff, is considered positive for metastatic disease.
FDG PET/CT in Radioiodine-Refractory Disease
Thyroglobulin-Positive, RAI-Negative (TENIS Syndrome)
The TENIS syndrome refers to an elevated or rising thyroglobulin with a negative RAI diagnostic scan. This indicates dedifferentiated tumor that has lost NIS expression and RAI avidity. FDG PET/CT detects disease in 70 to 90% of patients with a stimulated thyroglobulin above 10 ng/mL, and the detection rate correlates with the thyroglobulin level.
Clinical Significance
FDG-avid, RAI-refractory disease carries a worse prognosis than RAI-avid disease. FDG PET/CT findings guide management decisions, including surgery for resectable disease, external beam radiation, or systemic therapy with tyrosine kinase inhibitors such as lenvatinib or sorafenib. Performing PET/CT under TSH stimulation through thyroid hormone withdrawal or recombinant TSH may increase sensitivity.
"Flip-Flop" Phenomenon
Well-differentiated DTC is typically RAI-avid and FDG-negative, while poorly differentiated or dedifferentiated DTC is FDG-avid and RAI-negative. Heterogeneous tumors may demonstrate both RAI-avid and FDG-avid components simultaneously. The presence of FDG-avid disease portends a less favorable prognosis.
<image>Flip-flop phenomenon: comparison of I-131 whole-body scan (showing RAI-avid pulmonary metastases) with FDG PET/CT (showing FDG-avid bone metastases not seen on RAI scan) in a patient with both differentiated and dedifferentiated disease components</image>
Surveillance Schedule (ATA Recommendations)
Low-Risk, Excellent Response
Suppressed thyroglobulin is measured every 6 to 12 months. Neck ultrasound is performed at 12 months, then every 1 to 2 years, then with decreasing frequency. Stimulated thyroglobulin testing and diagnostic RAI scans are generally not needed. The TSH target is 0.5 to 2.0 mIU/L, near the normal range.
Intermediate-Risk or Indeterminate Response
Suppressed thyroglobulin is checked every 6 to 12 months, with stimulated thyroglobulin every 1 to 2 years. Neck ultrasound is performed every 6 to 12 months. A diagnostic RAI scan every 1 to 2 years may be considered. The TSH target is 0.1 to 0.5 mIU/L.
High-Risk or Incomplete Response
Thyroglobulin is monitored frequently, every 3 to 6 months. Neck ultrasound is performed every 6 months. Cross-sectional imaging with CT, MRI, or FDG PET/CT is obtained as clinically indicated. The TSH target is below 0.1 mIU/L.
Clinical Pearls
Undetectable thyroglobulin with a negative neck ultrasound and declining or negative TgAb represents the best possible outcome, and further RAI scans are not warranted.
TgAb trends serve as the surrogate tumor marker when TgAb are present. Declining TgAb after surgery and RAI is a favorable sign.
FDG PET/CT should be performed under TSH stimulation for maximum sensitivity in the TENIS syndrome.
The flip-flop phenomenon has prognostic implications, and mixed RAI/FDG-avid disease requires multimodal treatment.
Mass spectrometry thyroglobulin assays may eventually eliminate the TgAb interference problem.
Highly sensitive thyroglobulin assays with functional sensitivity below 0.1 ng/mL may reduce the need for stimulated thyroglobulin testing.
References
- Haugen, B. R., et al. "2015 ATA Management Guidelines for DTC." Thyroid, 2016.
- Tuttle, R. M., et al. "Dynamic Risk Stratification in DTC." Endocrine Reviews, 2010.
- Giovanella, L., et al. "Thyroglobulin Measurement in DTC." European Thyroid Journal, 2019.
- Defined Role of FDG PET/CT in Thyroglobulin-Positive, RAI-Negative DTC. Journal of Nuclear Medicine, 2012.
