Hypomagnesemia 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 Causes of Hypomagnesemia Hypomagnesemia is serum magnesium concentration < 1.8 mg / dL (< 0.70 mmol/L).