Abnormally low blood sodium concentration is known specifically as hyponatremia.
The word hypo- means 'under,' so basically 'not enough' of something. Natrium is the Latin word for sodium, so the word hyponatremia means 'not enough sodium.' Hypertension is abnormally high blood pressure, hyperkalemia refers to elevated concentrations of potassium, and water intoxication doesn't affect your sodium levels.
