Background: Medullary thyroid carcinoma (MTC) is a rare malignancy derived from parafollicular C-cells, with calcitonin (Ct) as its key biomarker. While basal Ct (bCt) levels above 100 pg/mL strongly suggest MTC, intermediate elevations (10–100 pg/mL) may reflect C-cell hyperplasia (CCH) or other benign conditions, making diagnostics challenging. Although calcium stimulation testing enhances sensitivity, the optimal cut-off values and comparative efficacy of calcium gluconate (CG) versus calcium chloride (CC) remain insufficiently researched. Methods: Data on 176 patients who underwent total thyroidectomy between 2009 and 2025 were retrospectively analyzed. BCt values ranged from 10 to 100 pg/mL, and stimulated Ct (sCt) values were above 100 pg/mL. CG was used from 2009 to 2019, and CC was used from 2020 to 2025. Definitive pathohistological findings divided patients into those with MTC, CCH, or no C-cell pathology. Receiver operating characteristic (ROC) analysis identified optimal Ct thresholds for predicting MTC for each stimulatory agent. Results: Of the 176 patients, 36 (20.5%) had confirmed MTC. A bCt threshold of 31.1 pg/mL yielded 69.4% sensitivity and 87.1% specificity. For sCt, optimal cut-offs were 810.8 pg/mL for CG and 1076 pg/mL for CC. Lower thresholds (388.4 pg/mL for CG and 431.5 pg/mL for CC) improved sensitivity (≥76.9%) and negative predictive value (>91%). Conclusions: Calcium stimulation testing improves MTC detection in patients with moderate bCt elevation. Although CG showed marginally better diagnostic performance, CC remains a practical and reliable alternative, especially when higher cut-off values are considered. Early surgical intervention should be considered when sensitivity-driven thresholds are met.

Background/Objectives: Papillary thyroid carcinoma (PTC) is the most common endocrine malignancy. The classic variant (cPTC) is characterized by indolent behavior and excellent prognosis. However, rare subtypes of PTC most often exhibit adverse clinical behavior. The aim of the study was to assess the aggressiveness of rare variants of PTC by analyzing clinicopathological characteristics (CPCs) and survival outcomes. Methods: We analyzed 80 patients with rare PTC variants treated between 2009 and 2019 who were compared with cPTC and matched with a control group for age and tumor size. The variants were categorized into high-risk (HRV: tall cell, diffuse sclerosing, columnar cell, and hobnail variants), intermediate-risk (IRV: solid variant (SV)), and low-risk (LRV: oncocytic (OV) and Warthin-like (WLV)) variants. Different CPCs (capsule and blood vessel invasion, lymphonodal metastases, microscopic and macroscopic extrathyroid extension, multifocal and bilateral presentation) and survival outcomes—overall (OS), disease-specific (DSS), and disease-free survival (DFS) were compared. Results: HRVs exhibited significantly more aggressive CPCs and worse OS, DSS, and DFS compared to cPTC (p < 0.001). IRVs showed no significant difference in CPCs or survival outcomes compared to cPTC. LRVs showed excellent survival but were associated with several unfavorable CPCs. Multivariate analysis identified classification in HRVs as the only independent predictor of recurrence (p = 0.014). Conclusions: Tumors in the HRV group should retain their status as aggressive PTC variants due to unfavorable behavior and poorer prognosis. SVs, despite earlier assumptions, do not exhibit aggressive characteristics. Although the OV and WLV have similar survival to cPTC, their potential for adverse CPCs requires caution.