A Low Postoperative Nonstimulated Serum Thyroglobulin Level Does Not Exclude the Presence of Radioactive Iodine Avid Metastatic Foci in Intermediate-Risk Differentiated Thyroid Cancer Patients

Background: Postsurgical thyrotropin (TSH)-stimulated serum thyroglobulin (Tg) level can be used to predict the likelihood of finding radioactive iodine (RAI) avid metastatic foci on postablation scanning. However, there is little data regarding the predictive value of a nonstimulated postoperative Tg obtained on levothyroxine therapy in patients being considered for recombinant human TSH (rhTSH)-assisted remnant ablation. Methods: The study included 290 intermediate-risk differentiated thyroid cancer (DTC) patients with a postsurgical nonstimulated Tg<10 ng/mL prior to rhTSH-assisted remnant ablation. Patients were stratified into four groups based on the postsurgical nonstimulated Tg value: Tg<0.6 ng/mL (n=146), Tg 0.6–0.9 ng/mL (n=76), Tg 1–5 ng/mL (n=51), and Tg>5–10 ng/mL (n=17). RAI avid metastatic foci were identified using post-therapy scanning with SPECT/CT (single photon emission computed tomography). Results: RAI avid metastases were identified in 16% (46/290) of patients, including 12% (17/146) with Tg<0.6 ng/mL, 14% (11/76) with Tg 0.6–0.9 ng/mL, 25% (13/51) with Tg 1–5 ng/mL, and 29% (5/17) with Tg>5–10 ng/mL (p=0.02). While 99% of the RAI avid foci were located in the neck, lung uptake was seen in one patient with Tg<0.6 ng/mL (0.7%, 1/146), one patient with Tg 0.6–0.9 ng/mL (1.3%, 1/76), and 2 patients with Tg>5–10 ng/mL (11%, 2/17 patients). Conclusions: A postoperative nonstimulated Tg<0.6 ng/mL does not exclude identification of RAI avid metastatic foci on postablation SPECT/CT scanning in intermediate-risk DTC patients. Therefore, patient selection for RAI ablation in the intermediate-risk group must be based on an integration of multiple risk factors rather than any single clinicopathologic risk factor.