ABSTRACT Objective Our objective was to evaluate the trend of antithyroglobulin antibodies (TgAb) during follow-up of patients with differentiated thyroid cancer (DTC) treated without RAI, as well as their role in the risk of recurrence. Subjects and methods This was a prospective, descriptive study. A total of 152 consecutive patients with DTC treated in a single institution undergoing total thyroidectomy without RAI and followed for a median of 2.3 years (0.5-10.3) were divided in two groups: TgAb(-) (n = 111) and TgAb(+) (n = 41). Patients were classified according to AJCC 7th and 8th editions, as well as to their risk of recurrence and response to treatment categories. Results Both groups, TgAb(-) and TgAb(+), were similar regarding patient and tumor characteristics. At the end of follow-up, 90 (59.2%), 57 (37.5%), 3 (2%) and 2 (1.3%) patients achieved excellent, indeterminate, biochemically incomplete and structurally incomplete response, respectively. The risk of structural recurrence was similar in both groups (TgAb[-] 0.9% vs. TgAb[+] 2.4%, p = 0.46). In the TgAb(+) group, TgAb became negative in 10 (24.4%), decreased ≥ 50% without negativization in 25 (60.9%), decreased < 50% in 4 (9.8%) and remained stable or increased in 2 (4.9%) cases. The only incomplete structural response had increasing TgAb during follow-up. Conclusions In properly selected patients with DTC, TgAb concentration immediately after total thyroidectomy should not mandate RAI ablation, and their trend during follow-up may impact the risk of recurrence.

ABSTRACT Objective: Because serum calcitonin (CT) is a reliable marker of the presence, volume, and extent of disease in medullary thyroid cancer (MTC), both the ATA and NCCN guidelines use the 2-3 month post-operative CT value as the primary response to therapy variable that determines the type and intensity of follow up evaluations. We hypothesized that the calcitonin would nadir to undetectable levels within 1 month of a curative surgical procedure. Subjects and methods: This retrospective review identified 105 patients with hereditary and sporadic MTC who had at least two serial basal CT measurements done in the first three months after primary surgery. Results: When evaluated one year after initial surgery, 42 patients (42/105, 40%) achieved an undetectable basal calcitonin level without additional therapies and 56 patients (56/84, 67%) demonstrated a CEA within the normal reference range. In patients destined to have an undetectable CT as the best response to initial therapy, the calcitonin was undetectable by 1 month after surgery in 97% (41/42 patients). Similarly, in patients destined to have a normalize their CEA, the CEA was within the reference range by 1 month post-operatively in 63% and by 6 months in 98%. By 6 months after curative initial surgery, 100% of patients had achieved a nadir undetectable calcitonin, 98% had reached the CEA nadir, and 97% had achieved normalization of both the calcitonin and CEA. Conclusion: The 1 month CT value is a reliable marker of response to therapy that allows earlier risk stratification than the currently recommended 2-3 month CT measurement.