Medullary thyroid cancer (MTC) accounts   for only 4–6% of all thyroid malignancies and has a more aggressive course compared to differentiated forms of carcinomas. MTC responsible for 8–15% of the causes of death in the structure of total mortality from thyroid cancer.  Significant progress has been achieved in the diagnosis of the disease due to the integrated use of biochemical, instrumental and cytological research methods, which in some cases makes it possible to diagnose the pathology at the earliest stages and avoid its progression.  The main methods for identifying of MTC are the determination of the serum concentration of basal and stimulated calcitonin, as well as cytological examination. However, each of these methods has a number of limitations that border their diagnostic accuracy. Thus, the informative value of cytological examination in the diagnosis of medullary carcinoma is much lower compared to differentiated forms of carcinomas (60–70% versus 90–95%). In some cases, high or low serum calcitonin (S-Ct) levels can be observed in various diseases not related to MTC. The diagnostic accuracy of FNA analysis markedly increased by analysis of the FNA specimen and additionally by measuring calcitonin levels in the FNA washout fluid (Ctn). The aim of our study was to assess the diagnostic accuracy of calcitonin in In Fine-Needle Washout Fluids comparison with cytological examination in the diagnosis of thyroid cancer and determination the optimal diagnostic level of Ctn or clinical practice. The data of patients who were examined and treated at the National Medical Research Center of Endocrinology in the period from 2019 to 2025 were studied (n 1039) with high S-Ct, MTC was verified in 332 patients.  The main advantages of our study are the stratification of groups based on histological verification of the diagnosis, as well as comparison with other standard diagnostic procedures (FNA and determination of basal calcitonin levels). As an optimal threshold diagnostic level for determining malignancy in thyroid nodules, values of more than 2000 pg/mL were proposed. The results of our study showed that in all cases of histologically verified MTC, there are increased levels of calcitonin in the FNA washout fluid (FNA-Ctn) both in primary forms (sensitivity and specificity 100%) and in lymph node metastases (sensitivity 88.5% and specificity 100%).