MEMBRANOUS NEPHROPATHY IN A PATIENT WITH ELEVATED CARCINOEMBRYONIC ANTIGEN

AN UNUSUAL PRESENTATION OF MEDULLARY THYROID CARCINOMA

Abstract

INTRODUCTION/BACKGROUND
Membranous nephropathy is an important cause of nephrotic syndrome where immune complexes are deposited at the subepithelial space of the glomerular basement membrane. Membranous nephropathy is classified into primary and secondary causes, including infections, autoimmune, neoplasms, drugs or idiopathic. Medullary thyroid carcinoma (MTC) is a relatively rare malignant tumour subtype originating from the parafollicular cells of the thyroid gland, producing tumour markers such as calcitonin, carcinoembryonic antigen (CEA) and chromogranin A. The co-occurrence of membranous nephropathy with MTC is extremely uncommon, and such an association may reflect a paraneoplastic manifestation or an underlying monoclonal gammopathy.

CASE
We report the case of a 68-year-old woman who presented with progressive shortness of breath, bilateral lower limb edema, frothy urine, and periorbital puffiness. She denied orthopnea, paroxysmal nocturnal dyspnea, constitutional symptoms, or features suggestive of autoimmune disease. Initial workup revealed nephrotic-range proteinuria, and a renal biopsy demonstrated early membranous nephropathy. Notably, her CEA level was persistently elevated; however, upper and lower gastrointestinal endoscopies showed only benign findings, including a hiatal hernia and sigmoid colon diverticulum. Contrast-enhanced CT imaging revealed a retrosternal goitre, while FDG-PET scanning identified an FDG-avid lesion in the left thyroid lobe with ipsilateral cervical lymphadenopathy. Thyroid ultrasound showed a TIRADS 4 nodule, and subsequent core needle biopsy confirmed MTC. She underwent total thyroidectomy and modified neck dissection without complications. At one-month postoperative follow-up, her proteinuria had slightly improved.

CONCLUSION
This case underscores the importance of a thorough malignancy workup in atypical presentations of nephrotic syndrome and highlights a rare paraneoplastic link between MTC and glomerular disease. In the context of raised CEA with negative findings despite extensive investigations for gastrointestinal tract causes, one might need to consider other non-gastrointestinal related causes for raised CEA, such as medullary thyroid carcinoma.