Value of 18F-Fluorodeoxyglucose (FDG) Positron Emission Tomography (PET)/Computed Tomography (CT) in the Detection of Recurrence in Medullary Thyroid Carcinoma With Negative Conventional Imaging: A Case Report.

Medullary thyroid carcinoma (MTC) is a rare neuroendocrine tumor with variable clinical presentation, posing both diagnostic and therapeutic challenges. Disease recurrence is common and may manifest solely as elevated tumor markers in the absence of clinical findings or positive morphological imaging. We present the case of a 56-year-old woman diagnosed with MTC in 2011, treated with total thyroidectomy and adjuvant therapy. In 2021, a locoregional recurrence was detected, requiring left-sided neck dissection. During follow-up in 2025, elevated tumor markers were identified despite negative findings on computed tomography (CT). Consequently, an F-fluorodeoxyglucose (FDG) positron emission tomography (PET)/CT was performed, revealing disease recurrence with multiple lesions suggestive of bone metastases. This case underscores the value of F-FDG PET/CT in the follow-up and detection of MTC recurrence.