Medullary thyroid cancer is a rare neuroendocrine tumor with aggressive 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. Computed 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, adenopathy, 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 helpful in elucidating a diagnosis. Due to its severity, medullary thyroid cancer should always be approached and managed aggressively, even microcarcinomas.