A Rare Case of Preeclampsia-Associated Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) in Twin Pregnancy Treated With Tolvaptan.

Severe hyponatremia is uncommon in pregnancy but has been described in association with preeclampsia, particularly in severe disease and multifetal gestation. We report a 38-year-old gravida 1 para 0 with dichorionic-diamniotic twins who presented at 29 weeks and four days of gestation and developed rapidly progressive hypotonic hyponatremia, with serum sodium decreasing from 126 to 110 mmol/L. Low serum osmolality with inappropriately concentrated urine, elevated urine sodium, normal thyroid testing, preserved renal function, and the initial clinical volume assessment supported a pattern consistent with syndrome of inappropriate antidiuretic hormone secretion (SIADH); however, cortisol was not obtained during the hospitalization, so adrenal insufficiency was not completely excluded biochemically. Fluid restriction, hypertonic saline, and furosemide were limited by pulmonary edema, and persistent severe hyponatremia prompted multidisciplinary use of tolvaptan despite limited pregnancy safety data. Cesarean delivery was subsequently performed in the setting of worsening maternal disease. Maternal serum sodium normalized postpartum without documented neurologic complications, supporting a pregnancy-mediated disturbance of water balance. This case highlights the diagnostic overlap between preeclampsia-associated hyponatremia and SIADH, the narrow therapeutic margin created by pulmonary edema, and the importance of close sodium monitoring and timely delivery when maternal status deteriorates.