Beyond IgAN: A Case of Thin Basement Membrane Disease with Overlapping Clinical Features

Introduction: Thin basement membrane disease (TBMD) is an autosomal dominant disorder that is more common than often recognized, though it is diagnosed in fewer than 1% of the population. It typically presents with hematuria and occasionally mild proteinuria. TBMD can closely resemble IgA nephropathy, which should be excluded due to its broader therapeutic implications. We present a case suggestive of IgA nephropathy but ultimately diagnosed as TBMD with coexisting focal segmental glomerulosclerosis (FSGS) Case Description: A 37-year-old male immigrant from the Middle East was seen in the renal clinic for sub-nephrotic range proteinuria and intermittent microscopic hematuria. Past medical history was significant for hypertension and family history for CKD with hematuria in his sister. He had not undergone prior renal biopsy. A recent renal ultrasound performed in his home country showed no significant abnormalities. Initial labs showed serum creatinine of 1.2 mg/dL, urine protein-to-creatinine ratio (UPC) of 356 mg/g, and urinalysis with 3+ blood and 3–10 RBCs/HPF intermittently. Home blood pressure readings were well controlled (120–130/70–80 mmHg) on a thiazide diuretic, angiotensin receptor blocker (ARB), and calcium channel blocker (CCB). Over the next two years, the patient experienced a gradual increase in proteinuria (UPC 1042–1081 mg/g) despite stable renal function and addition of a sodium-glucose cotransporter 2 inhibitor (SGLT2i). He declined cystoscopy after resolution of gross hematuria. Due to worsening proteinuria and persistent microscopic hematuria, with possibly implications for further treatment, a renal biopsy was performed. Of 21 glomeruli examined, two were globally sclerotic. Findings included minimal interstitial fibrosis/tubular atrophy, negative immunofluorescence, and diffuse foot process effacement, consistent with TBMD and secondary FSGS. The patient declined genetic testing. He remained on ARB and SGLT2i, and was counseled on weight loss. Discussion: This case underscores the importance of considering TBMD in the differential diagnosis of hematuria and proteinuria, particularly when contemplating immunosuppressive therapy. Patient counseling is essential, given the typically benign course and supportive nature of management.