Background Occult thyroid cancer is a particularly rare type of thyroid carcinomas. The absolute majority is presented by papillary thyroid carcinoma. We report a case of a medullary thyroid carcinoma (MTC) that was detected as a primary tumor neither on preoperative ultrasound examination nor on postoperative morphological examination. Case A 57-year-old patient with no personal or family history of thyroid disease presented to the surgeon with the complaint of swelling in the cervical lymph nodes. Ultrasonography of the thyroid showed a hypoechoic 4 mm zone with peripheral calcification in the right lobe and two suspicious malignant lymph nodes 14×12 mm and 10×7 mm in the right paratracheal region. A blood test revealed no abnormalities in thyroid function (TSH 0.9 mUI/L, fT3 4.25 pmol/l, fT4 10.2 pmol/l). However, the calcitonin (CT) and the carcinoembryonic antigen (CEA) levels were significantly elevated – 144 pg/ml (N for male <12) and 28 pg/ml (N <5). We performed an ultrasound-guided fine-needle aspiration (FNA) cytology from the thyroid nodule with subsequent FNA-CT measurement which showed no signs of MTC (cells, adequate CT level). On the other hand, FNA of the lymph node revealed cytomorphology characteristics of MTC and according to FNA-CT measurement, the CT concentration exceeded the upper reporting range of 2000 pg/ml. We carried out the measurement of stimulated CT. Taking into account the significant CT level increase (0 min – 168 pg/ml, 2 min – 1050 pg/ml, 5 min – 857 pg/ml), we excluded the extrathyroidal CT production. Genetic analysis demonstrated the absence of usual RET mutation. Computed tomography was carried out showing multiple calcifications up to 6 mm in diameter in the lung. On the cervical scan: the right upper paratracheal lymph node (11 mm) and a 6 mm mass with calcification in the structure along the front line of the right main bronchus (at the level of the bifurcation), the rest of the lymph nodes were not enlarged. The patient underwent total thyroidectomy with central lymph node dissection. On the section in the middle third of the lobe, there was a dense, yellowish-grey knot of 5 mm in size and 8 lymph nodes 5-15 mm. Surprisingly the right lobe node was represented by colloid goiter with foci of fibrosis and calcifications. In two of the eight cervical nodes, we identified nodal metastases that contained a component of MTC. The primary tumor was not detected even after thorough and complete pathologic examination and immunohistochemical study of the thyroid. Postoperative CT level (third day after surgery) has fallen to 5.45 pg/ml. Conclusion Metastatic medullary thyroid carcinoma without detectable primary tumor is a rare but real occurrence that might be encountered in practice. The lack of malignant findings in the thyroid gland during preoperative examination complicates the diagnostic. Presentation: No date and time listed