Hyponatremia means a decrease in the amount of sodium in the blood. It corresponds to a condition of overhydration of the cells. It is found in a blood sample. Hyponatremia can appear rapidly by a feeling of disgust for water, nausea or vomiting, headache, and in the most severe cases, confusion. Many causes may be responsible, including an increased loss, either digestive or in the urine, greatly exaggerated water consumption, renal, hormonal, or heart disease.