Intrathyroidal Relapse of Multiple Myeloma Mimicking Primary Thyroid Neoplasia: A Case Report and Literature Review

Background Thyroid nodules are highly prevalent in the general population, yet their diagnostic assessment may require accurate differential diagnosis in patients with hematologic malignancies. In this context, immune dysregulation, prior cytotoxic therapies, and the potential for atypical extramedullary involvement may confound conventional risk stratification algorithms. Although ultrasonography remains the first-line imaging modality, fine-needle aspiration (FNA) with ancillary studies is often required for definitive characterization. Case report A 47-year-old woman with relapsed multiple myeloma (MM), previously treated with multiagent chemotherapy, radiotherapy, and autologous stem cell transplantation, was evaluated for biochemical and clinical disease progression. Bone marrow biopsy demonstrated no evidence of plasma cell infiltration. However, 18 F-fluorodeoxyglucose positron emission tomography/computed tomography ( 18 F-FDG PET/CT) identified focal hypermetabolic uptake in the right thyroid lobe. Dedicated neck ultrasonography revealed a 26-mm solid, markedly hypoechoic nodule with peri-nodular vascularity and without definitive sonographic features of papillary thyroid cancer. Given the discordance between imaging findings and marrow status, ultrasound-guided FNA was performed. Cytologic evaluation demonstrated numerous atypical plasma cells, and immunocytochemistry confirmed lambda light chain restriction, consistent with clonal plasma cell proliferation. These findings established intrathyroidal involvement by MM, representing an isolated extramedullary relapse in the absence of bone marrow disease. Discussion Intrathyroidal manifestation of MM is rare but clinically significant. In patients with systemic malignancies, FDG-avid thyroid nodules should prompt consideration of secondary involvement, even when bone marrow evaluation is negative. Conventional ultrasound-based risk stratification systems are primarily designed to detect epithelial thyroid malignancies and may fail to identify non-epithelial neoplasms. Cytologic assessment supplemented by immunophenotypic analysis remains essential for accurate diagnosis. Conclusion This case highlights the importance of integrating clinical history, functional imaging, cytopathology, and immunocytochemistry to avoid diagnostic pitfalls and to guide appropriate oncologic management.