Medullary thyroid cancer is a rare neuroendocrine tumor with ag-gressive behavior and an uncertain prognosis. Calcitonin is the associated tumor marker; however, neuroendocrine tumors of the lung or intestine can also present high values. We report the case of a 53-year-old patient presenting with dry cough for 3 years. Comput-ed tomography (CT) showed a 58-mm expansive lesion in the right anterior mediastinum and cervical ultrasound informed a 9-mm hypoechoic nodule with a lobulated edge and thick calcifications. Cytology reported Hashimoto’s thyroiditis. He underwent emergent surgery for respiratory failure. The pathological study informed G2 neuroendocrine carcinoma, immunohistochemistry showed: cytokeratin (+), vimentin (+), alpha-actin (-), synaptophysin (+), chromogranin (+), Ki-67: 10%. He progressed with bone metastasis visualized in scintigraphy and mediastinal tumor remnants, adenop-athy, and pulmonary nodules on CT scan, prompting chemotherapy. Due to elevated and increasing calcitonin, an ultrasound study with needle washout for calcitonin resulted in 1,724 pg/mL, and cytology of the nodule reported medullary carcinoma. He was finally diagnosed with medullary thyroid microcarcinoma with metastasis to the mediastinum. Coexistence with Hashimoto’s thyroiditis could alter the initial fine-needle aspiration results. We conclude that in the case of calcitonin-secreting neuroendocrine tumors or medullary thyroid cancer, calcitonin fine needle aspirate washout may be help-ful in elucidating a diagnosis. Due to its severity, medullary thyroid cancer should always be approached and managed aggressively, even microcarcinomas.