Society of Diabetic Nephropathy Prevention Journal of Nephropathology 2251-8363 15 1 2026 01 01 Tubulointerstitial nephritis with hypocomplementemia in IgG4-related kidney disease; a case report and follow-up of a mysterious entity e25548 e25548 10.34172/jnp.2025.25548 EN Vitória Paes de Faria https://orcid.org/0000-0003-2746-4502 David João https://orcid.org/0000-0003-3195-1207 Susana Pereira https://orcid.org/0000-0001-9759-2177 Ana Marta Gomes https://orcid.org/0000-0002-8992-5792 Maria Clara Almeida Rute Carmo https://orcid.org/0000-0001-7541-7124 Journal Article 10.34172/jnp.2025.25548 2024 04 29 2024 11 23 Idiopathic tubulointerstitial nephritis (TIN) with hypocomplementemia is a rare cause of progressive renal failure, with IgG4-related disease (IgG4-RD) emerging as an important differential diagnosis. Within IgG4-RD spectrum, renal involvement frequently reported however, renal involvement alone is uncommon. We present a case of a 66-year-old male with progressive kidney dysfunction. Clinical evaluation revealed bilateral inguinal adenopathies and laboratory findings showed elevated IgG and IgG subclass 4 levels, along with low-complement. Renal biopsy confirmed extensive TIN with marked lymphoplasmacytic infiltrates and elevated IgG-positive cells, consistent with IgG4-RD. The patient was initiated on glucocorticoid therapy, resulting in partial recovery of renal function. Long-term follow-up demonstrated stable renal function and absence of systemic manifestations of IgG4-RD. This case highlights the importance of considering IgG4-RD in the differential diagnosis of TIN with hypocomplementemia. Glucocorticoids remain the first-line treatment, although alternative regimens may be considered to minimize relapse rate and long-term steroid related toxicity. Timely recognition and management of IgG4-related TIN are essential to prevent irreversible kidney damage and improve long-term outcomes.