This study aims to evaluate whether SIADH in elderly patients with urinary retention is associated with increased urinary aquaporin 2 (U-AQP2) levels and whether the insertion of an indwelling catheter with fluid restriction, without the administration of 3% saline, can lower the U-AQP2 level, leading to the resolution of SIADH. The gene discussed is AQP2; the disease is inappropriate ADH syndrome.