Hypernatremia By James L Lewis III, MD, Brookwood Baptist Health and Saint Vincent's Ascension Health, Birmingham Reviewed / Revised Sep 2021 | Modified Sep 2022 View Patient Education Etiology Symptoms and Signs Diagnosis Treatment Key Points Topic Resources Water Deficit in Hypernatremia Osmolality Estimator (serum) Principal Causes of Hypernatremia Hypernatremia is a serum sodium concentration > 145 mEq / L (> 145 mmol/L).