A rare case of follicular thyroid carcinoma with unknown primary tumor location.

The most common follicular cell-derived malignant neoplasm of the thyroid is papillary thyroid carcinoma (PTC), which typically demonstrates early lymphatic dissemination. The second most common - follicular thyroid carcinoma (FTC), primarily spreads via hematogenous routes, often resulting in distant metastases, most commonly in the lungs and bones. The standard diagnostic and therapeutic process of patients with differentiated thyroid carcinoma (DTC) includes ultrasound examination followed by fine needle biopsy (FNB), subsequent thyroidectomy, and postoperative radioiodine (RAI) treatment. In cases of suspicion of non-radioiodine-avid metastases fluorine-18 fluorodeoxyglucose positron emission tomography/computed tomography ([18F]FDG PET/CT) is also relevant for patient assessment. This report presents a case of a patient in which the diagnosis was completely overturned, due to an unspecified diagnostic process.