Polyuria and hypokalemia in an adolescent: deciphering the dual tubulopathy of primary Sjögren’s syndrome - a case report

Primary Sjögren’s syndrome (pSS) is a systemic autoimmune disorder most commonly affects older women. It can involve the kidneys, most commonly causing distal renal tubular acidosis (dRTA). Overt nephrogenic diabetes insipidus (NDI) is rare, and the simultaneous occurrence of both tubular defects as the presenting feature of pSS, particularly in adolescents, is exceptional and presents a challenging diagnostic puzzle. We present a case of a 17-year-old female with a 5-year history of polydipsia and polyuria (daily intake/output 6–7 L), who was found to have severe hypokalemia, hyperchloremic normal anion-gap metabolic acidosis, and inappropriately alkaline urine, confirming distal RTA. Water deprivation test with desmopressin demonstrated urine osmolality that remained below 150 mOsm/kg despite rising serum osmolality and lacked significant increase after desmopressin, strongly supporting the diagnosis of NDI. Positive anti-SSA/SSB antibodies, objective sicca findings, and renal biopsy revealing active tubulointerstitial nephritis led to the diagnosis of pSS with dual tubular manifestations. Treatment with prednisone, mycophenolate mofetil, hydrochlorothiazide, and high-dose potassium supplements led to significant clinical and biochemical improvement. This case illustrates a systematic approach to polyuria and hypokalemia in an adolescent, unraveling a rare dual tubulopathy as the initial manifestation of pSS. We hypothesize that NDI in this setting may result from a “two-hit” mechanism: direct autoimmune injury to the collecting duct and profound hypokalemia-induced downregulation of aquaporin-2. Recognizing this association and initiating early immunosuppression can partially reverse tubular dysfunction and improve long-term outcomes.