Calcium-stimulated calcitonin and procalcitonin in borderline hypercalcitoninemia: histopathologic and nodal correlates in a thyroidectomy cohort.

Mild basal calcitonin elevation represents a diagnostic gray zone in patients undergoing thyroid surgery for non-medullary thyroid carcinoma (MTC) indications. We evaluated calcium-stimulated calcitonin and procalcitonin kinetics according to final histopathology and nodal status in this setting. This retrospective study included 37 adults with reproducible borderline basal calcitonin elevation (< 50 pg/mL) selected from 577 patients who underwent total thyroidectomy for non-MTC indications between 2021 and 2024. All patients underwent calcium stimulation testing with measurement of calcitonin and procalcitonin at baseline and after stimulation. Biochemical responses were compared with final histopathology. Median basal calcitonin was 13.1 [11.0-16.8] pg/mL and median peak stimulated calcitonin (psCT) was 480 [320-790] pg/mL. Final histopathology showed MTC in seven patients (18.9%), all microcarcinomas, C-cell hyperplasia (CCH) without MTC in nine, and no C-cell pathology in 21. Median psCT was highest in MTC, intermediate in CCH without MTC, and lower in patients without C-cell pathology (820 [640-1040], 460 [310-670], and 360 [270-540] pg/mL, respectively; nominal p = 0.047 for MTC vs CCH). psCT values ≥ 500 pg/mL were observed in 6 of 7 MTC cases compared with 1 of 9 CCH cases without MTC. Central lymph-node metastasis was documented in 3 of 7 MTC patients and was accompanied by numerically higher psCT and fold-rise values. wf45g6 Procalcitonin increment and fold-rise were greater in MTC than in CCH without MTC, but no stable procalcitonin-based threshold or combined rule was identified. Occult micro-MTC may show marked calcium-stimulated calcitonin responses despite mild basal hypercalcitoninemia. These findings are exploratory and do not validate diagnostic or surgical thresholds.