Unmeasurable Stimulated Thyroglobulin Before Radioactive Iodine Ablation Predicts Excellent Long-Term Outcomes in...

Background: There is ongoing interest in limiting the extent of treatment for patients with low-risk differentiated thyroid cancer (DTC) and in redesigning individualised follow-up strategies. This study assessed long-term outcomes in patients with excellent response to surgical treatment demonstrated by reaching unmeasurable stimulated thyroglobulin (sTG) levels before proceeding with radioactive iodine ablation. Methods: This is a retrospective cohort study of consecutive patients treated for DTC in a tertiary referral centre. Radioactive iodine ablation (RIA) was done after hormone withdrawal (before 2015) or after Thyrogen stimulation (in recent years). The biochemical assay for TG changed from a lower limit of detectability of 5 ng/mL to 0.2 ng/mL in 2012. Results: Of 331 patients operated on between 2001 and 2019, unmeasurable sTG was measured in 70 of 138 patients (51%) when using an assay with threshold of 5 ng/mL and in 38 of 193 patients (20%) based on the threshold of 0.2 ng/mL. Compared with patients whose sTG was >5 ng/mL, those with sTG <5 ng/mL (187 of 331 patients) were less likely to have T3-T4 tumours or positive lymph node disease (N1a-N1b) and had a lower MACIS score (5.73 ± 1.26 vs. 6.45 ± 1.69, p < 0.001) and much lower mortality with metastatic disease during follow-up for 100 ± 48 months (3/187 vs. 23/144 patients, p = 0.001). Conclusions: Patients with unmeasurable sTG have excellent prognosis, with very low incidence of adverse events. With the wide use of TG assay with a threshold of 0.2 ng/mL (or lower), this subgroup could have patient-initiated follow-up rather than embark on regular assessments.