Hyponatremia may happen because of an excess of water, which is more common or loss of Na. If sodium level was low, water level in the body will rise and this will lead to swelling of the cells. In addition, Dilution of urine occurs in the thick ascending limb of loop of Henle by re-absorption of solutes by Na-K-2Cl transporter, in the distal convoluted tubule by NaCl transporter and in the collecting tubule by absence of ADH. Dysfunction in these steps maximum amount of urine that can be excreted and this will causes hyponatremia.

Hyporvolemic:
Here, there is decrease in total body water and sodium.

Euvolemic:
Total body water increase but Na stays the same.
Sodium concentration is controlled by secretion of ADH, mechanisms of the renin-angiotensin-aldosterone