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Objective: To investigate the preoperative diagnostic value of ultrasound-guided fine-needle aspiration biopsy wash-out fluid (FNA-CT) in medullary thyroid carcinoma (MTC) with mildly elevated serum calcitonin levels. Methods: 267 patients with MTC diagnosed in Tianjin Cancer Hospital between Jan 2015 and Jan 2024 were enrolled in the study. Based on serum calcitonin, patients were divided into two groups:＞100 ng/L and 10-100 ng/L. Sonographic features, clinicopathological characteristics and prognostic outcomes were evaluated between the two groups. The diagnostic efficacy of serum calcitonin and FNA-CT in MTC and non-medullary thyroid carcinoma with slightly high serum calcitonin were evaluated. The best cutoff values of serum calcitonin and FNA-CT were calculated by subject operating characteristic curve (ROC). The diagnostic efficacy was analyzed for MTC with mildly elevated serum calcitonin according the best cut off points. Results: Compared to the >100 ng/L group, MTC patients with serum calcitonin 10-100 ng/L exhibited distinct ultrasonographic features: taller-than-wide shape, non-adjacent to thyroid capsule, less vascularity, and CACA-TIRADS 4 (all P＜0.05). Clinicopathologically, the serum calcitonin 10-100 ng/L group demonstrated single focus rather than multifocality, smaller tumor size, higher prevalence of microcarcinoma, a lower degree of local invasion, earlier tumor stages (T stage, N stage, TNM stage), smaller extent of thyroidectomy and lymphadenectomy and lower diagnostic accuracy of FNA (all P＜0.05). The group also showed higher biochemical cure rate (90.0% and 54.2%)and lower rates of biochemical recurrence (5.5% and 23.1%) and tumor recurrence (3.6% and 22.6%, P＜0.001). Among MTC with serum calcitonin 10-100 ng/L, the median of serum calcitonin levels were 40.0 ng/L (19.5, 65.8 ng/L) for MTC vs. 16.6 ng/L (13.2, 20.8 ng/L, P＜0.001) for non-medullary thyroid carcinoma. The median of FNA-CT were 2 000.0 ng/L (1 334.0, 2 000.0 ng/L) vs. 0.8 ng/L (0.5, 2.0 ng/L, P＜0.001). The best cutoff points were 22.9 ng/L for serum calcitonin and 58.7 ng/L for FNA-CT. The sensitivity, specificity, negative predictive value, positive predictive value and the area under curve (AUC) were 67.3%, 82.9%, 65.4%, 61.2% and 0.829 (95% CI: 0.731-0.903) at the best cutoff point of 22.9 ng/L for serum calcitonin. The sensitivity, specificity, negative predictive value, positive predictive value and AUC were 98.2%, 97.6%, 97.61%, 98.1% and 0.998 (95% CI: 0.989-0.998) at the best cutoff point of 58.7 ng/L for FNA-CT. Conclusions: MTC with slightly elevated serum calcitonin commonly correlates with an early-stage disease. FNA-CT has demonstrated near-perfect diagnostic performance and improved the early detection in this population.