Procalcitonin as a Tumour Marker in Medullary Thyroid Carcinoma: A Comparative Study with Calcitonin and Carcinoembryonic Antigen

Medullary thyroid carcinoma is a rare neuroendocrine tumor of parafollicular C-cells. Calcitonin is the primary tumor marker but presents several limitations, including assay variability and false positives in renal dysfunction, proton pump inhibitor use and smoking. Procalcitonin may offer advantages in stability and specificity. To evaluate the diagnostic performance of procalcitonin compared to calcitonin and carcinoembryonic antigen in patients with medullary thyroid carcinoma, we conducted a retrospective study of 60 patients with histologically confirmed medullary thyroid carcinoma at a single endocrine center. Calcitonin, procalcitonin, and carcinoembryonic antigen levels were analyzed pre- and postoperatively over a 4-year period (2015-2019). Statistical analyses included Spearman's correlation and receiver operating characteristic curve analysis. Subgroup analyses examined the effects of renal dysfunction, proton pump inhibitors, and smoking. Calcitonin and procalcitonin showed a strong correlation (<i>r</i>=0.874 and <i>p</i><0.001). Procalcitonin maintained high specificity and sensitivity (area under the curve>0.95 across all years) and remained unaffected by the proton pump inhibitor use or renal impairment. Carcinoembryonic antigen correlated with tumor progression but lacked sufficient specificity alone. The combined use of calcitonin and procalcitonin improved diagnostic accuracy. In all patients with detectable tumor burden, procalcitonin was positive. False-positive calcitonin results were observed in patients without evidence of active diseases but with renal dysfunction or proton pump inhibitor use; procalcitonin remained negative in these cases. Procalcitonin is a reliable tumor marker for medullary thyroid carcinoma, especially in postoperative surveillance. Its stability and independence from common confounders make it a valuable complement to calcitonin. The combined assessment of calcitonin and procalcitonin enhances diagnostic performance and should be considered in routine clinical practice.