Supplementary Material for: NELL1-Associated Membranous Nephropathy in a Patient with Papillary Thyroid Carcinoma: A Case Report and Literature Review

Introduction: Membranous nephropathy (MN) is a leading cause of nephrotic syndrome in adults. NELL1 accounts for approximately 10% of MN cases, and is emerging as a distinct subtype often linked to secondary conditions, such as malignancies and drugs. Here, we describe a case of MN in a patient with metastatic papillary thyroid carcinoma. Case Presentation: A 78-year-old man with metastatic papillary thyroid carcinoma and a distant history of treated syphilis presented with nephrotic syndrome. A renal biopsy revealed segmental MN with positive NELL1 staining, while the thyroid carcinoma tissue also demonstrated NELL1 staining at weak to moderate intensity. The patient was managed conservatively without immunosuppression due to the lack of oncologic treatment options and syphilis was treated with doxycycline due to positive serologies, though active infection was not suspected. Follow-up showed stabilization of renal function and marked reduction in proteinuria. Conclusion: NELL1-positive MN is increasingly recognized for its association with malignancy. The patient achieved partial remission with supportive, non-immunosuppressive therapy. Clinicians should maintain a high index of suspicion for secondary conditions when NELL1-positive MN is encountered. This case report reviews secondary NELL1 associations and underscores the diagnostic challenges in MN when multiple potential secondary causes exist.