Abstract Background Patients with sporadic or hereditary Medullary Thyroid Carcinoma (MTC) often undergo total thyroidectomy and lymph node dissection, with postoperative surveillance relying on serum calcitonin levels and ultrasound examinations. According to American Thyroid Association (ATA) guidelines, detectable levels of serum calcitonin and Carcinoembryonic Antigen (CEA) post-thyroidectomy suggest potential disease persistence or recurrence, necessitating frequent monitoring for early detection. This study sought to reassess the role of postoperative calcitonin levels in identifying possible tumor recurrence. Methods A retrospective analysis was conducted using Siemens Atellica to test calcitonin levels in 562 samples collected between 09/27/2022 and 08/11/2023, at least 3 months post-total thyroidectomy. Imaging studies performed within 6 months of the calcitonin assessment were also reviewed. Sample pools with calcitonin concentrations near 3.00 and 5.00 pg/mL were utilized to determine the limit of quantification (LoQ). CEA results were included for comparison. Additionally, 16 serum samples were analyzed using the Roche Cobas system at a reference laboratory. Results Precision analysis around 3.00 and 5.00 pg/mL revealed coefficients of variation (CVs) of 16.49% and 8.87%, respectively. Comparison of calcitonin levels between Siemens Atellica and Roche Cobas systems demonstrated consistent levels < 5.00 pg/mL in 15 out of 16 samples, indicating alignment and reliability between the platforms. A calcitonin cutoff of 1.89 pg/mL yielded 43% sensitivity and 67% specificity, while a cutoff of 5.00 pg/mL resulted in 0% sensitivity and 100% specificity. Conclusions Our findings suggest that a 5 pg/mL calcitonin cutoff on the Siemens Atellica platform may be suitable for identifying tumor persistence or recurrence in post-thyroidectomy patients within our institution. However, individual laboratories should establish their LoQ criteria when interpreting calcitonin levels for postoperative monitoring of tumor recurrence.