Use of Oral Urea in Hyponatremia Caused by the Syndrome of Inappropriate Antidiuretic Hormone Secretion: A Case Report

Abstract

Introduction Syndrome of inappropriate antidiuretic hormone (ADH) secretion (SIADH) is characterized by excessive ADH activity, impaired water excretion, euvolemic hyponatremia, concentrated urine [1]. Most common causes: malignancy, CNS disorders, medications, postoperative states, head trauma [2,3]. SIADH is frequently diagnosed, yet effective correction of hyponatremia is often difficult. Case Presentation A 62-year-old man was admitted after a seizure during cranial CT in April 2025. Anamnesis: left nephrectomy due to renal cell carcinoma (RCC) was performed in December 2024. Initial laboratory tests – hyponatremia (127 mmol/L). Brain MRI showed a left occipital lobe metastasis, which was resected and histologically confirmed as RCC metastasis. After surgery, severe chronic hyponatremia persisted (115–122 mmol/L). Further endocrinological evaluation: hyponatremia (116 mmol/L), low serum osmolality (254 mOsm/kg), high urine osmolality (614 mOsm/kg), and elevated urine sodium (86 mmol/L), consistent with SIADH, most likely related to malignancy or recent neurosurgery. Hyponatremia was managed by a multidisciplinary team. Potentially causative medications were discontinued, fluid restriction, hypertonic saline and furosemide 20 mg/day were initiated without adequate response. SGLT2 inhibitors were discontinued due to a generalized pruritic rash. An oral urea formulation prepared by the LSMU pharmacy was administered (30 g in the morning and 15 g in the evening), representing the first use of urea for SIADH treatment at Kauno Klinikos. Serum sodium increased to 126 mmol/L. Target sodium range was defined as 120–129 mmol/L and the patient was discharged for outpatient follow-up on the same treatment. Discussion SIADH treatment includes fluid restriction, hypertonic saline or sodium supplementation in symptomatic patients, and loop diuretics [1]. Oral urea has re-emerged as an effective, safe, and cost-efficient option for chronic or refractory SIADH. Yet it is limited by uncertain long-term outcomes and side effects such as allergic reactions [4, 5], while SGLT2 inhibitors represent an adjunctive option with limited evidence [6]. Conclusions In this case SIADH management was challenging and required a multidisciplinary approach. Oral urea was effective and could be considered as an alternative treatment for hyponatremia caused by SIADH.