Medullary thyroid carcinoma (MTC) is a rare neuroendocrine tumor. However, non-elevated calcitonin levels have been reported in the literature. We present the case of an 81-year-old woman with chronic elevations in carcinoembryonic antigen (CEA) levels for the past 15 years, despite normal calcitonin levels, who was ultimately diagnosed with MTC. The patient had a remote history of breast cancer and presented with symptoms of unintentional weight loss, fatigue, and joint pain. A positron emission tomography (PET) scan revealed low fluorodeoxyglucose (FDG) uptake in partially calcified thyroid nodules, and fine needle aspiration cytology was consistent with medullary carcinoma. The patient underwent total thyroidectomy, with pathology revealing a pT1aN0M0 medullary thyroid microcarcinoma with negative margins. After thyroidectomy, CEA levels decreased to within the normal range, and calcitonin levels remained normal. This case highlights the importance of considering MTC in patients with unexplained chronic elevations in CEA levels, even with normal calcitonin levels.