An electrolyte imbalance in which sodium concentration in the blood is too low. This can lead to nausea, confusion, muscle weakness, seizures, and even coma. Potential causes include conditions causing fluid retention (e.g. heart failure, cirrhosis, etc.), conditions causing fluid loss (e.g. diarrhea, vomiting), and various hormonal conditions (e.g. SIADH, Addison's disease).