RESUMEN
INTRODUCTION: Medullary thyroid cancer (MTC) is a primary neuroendocrine tumor derived from parafollicular cells or C-cells of the thyroid gland. It accounts for 1% to 10% of all thyroid cancers and is the second most aggressive thyroid cancer after undifferentiated thyroid carcinoma. Serum calcitonin and carcinoembryonic antigen (CEA) concentrations are widely used as biomarkers to facilitate diagnosis and follow-up. However, in rare cases, serum levels of calcitonin or CEA can be normal. CASE PRESENTATION: We report the case of a 64-year-old male patient with MTC who presented brain metastasis and normal preoperative serum levels of calcitonin and CEA. The patient underwent total thyroidectomy with central compartment lymph-node dissection, resection of the single brain metastasis, and adjuvant holo-cranial radiotherapy. At 30 months' follow-up, he maintained normal serum calcitonin and CEA levels with increased procalcitonin levels. CONCLUSION: We describe a rare case of "calcitonin-negative" MTC with brain metastasis. The pathophysiology underlying normal serum levels of calcitonin in MTC is still not clearly understood. The lack of effective serum biomarkers for these patients makes diagnosis and treatment challenging.