Cirrhosis of the liver
Congestive heart failure
Nephrotic syndrome in the kidneys
Excessive drinking of fluids

Normal volume
There is volume expansion in the body, no edema, but hyponatremia occurs
SIADH (and its many causes)
Hypothyroidism
Not enough ACTH
Beer potomania
Normal physiologic change of pregnancy
Reset osmostat

Low volume
Hypovolemia (extracellular volume loss) is due to total body sodium loss. Hyponatremia is caused by a relatively smaller loss in total body water. Any cause of hypovolemia such as prolonged vomiting, decreased oral intake, severe diarrhea
Diuretic use (due to the diuretic causing a volume depleted state and thence ADH release, and not a direct result of diuretic-induced urine sodium loss)
Addisons disease and congenital adrenal hyperplasia in which the adrenal glands do not produce enough steroid hormones (combined glucocorticoid and mineralocorticoid deficiency)
Pancreatitis
Prolonged exercise and sweating, combined with drinking water without electrolytes is the cause of exercise-associated hyponatremia (EAH). It is common in marathon runners and participants of other endurance events. The use of MDMA (ecstasy) can result in hyponatremia. Medication
Antipsychotics have been reported to cause hyponatremia in a review of medical articles from 1946 to 2016.Available evidence suggests that all classes of psychotropics, i.e., antidepressants, antipsychotics, mood stabilizers, and sedative/hypnotics can lead to hyponatremia. Age is a significant factor for drug induced hyponatremia. Other causes
Miscellaneous causes that are not included under the above classification scheme include the following:

False or pseudo hyponatremia is caused by a false lab measurement of sodium due to massive increases in blood triglyceride levels or extreme elevation of immunoglobulins as may occur in multiple myeloma.
This is because charged sodium ions attract around them up to 25 water molecules, thereby creating a large polar structure too large to pass through the cell membrane: "channels" or "pumps" are required. Cell swelling also produces activation of volume-regulated anion channels which is related to the release of taurine and glutamate from astrocytes. Diagnosis
The history, physical exam, and laboratory testing are required to determine the underlying cause of hyponatremia. A blood test demonstrating a serum sodium less than 135 mmol/L is diagnostic for hyponatremia. The history and physical exam are necessary to help determine if the person is hypovolemic, euvolemic, or hypervolemic, which has important implications in determining the underlying cause. An assessment is also made to determine if the person is experiencing symptoms from their hyponatremia. These include assessments of alertness, concentration, and orientation. False hyponatremia
False hyponatremia, also known as spurious, pseudo, hypertonic, or artifactual hyponatremia is when the lab tests read low sodium levels but there is no hypotonicity. In hypertonic hyponatremia, resorption of water by molecules such as glucose (hyperglycemia or diabetes) or mannitol (hypertonic infusion) occurs. In isotonic hyponatremia a measurement error due to high blood triglyceride level (most common) or paraproteinemia occurs. It occurs when using techniques that measure the amount of sodium in a specified volume of serum/plasma, or that dilute the sample before analysis. True hyponatremia
True hyponatremia, also known as hypotonic hyponatremia, is the most common type. It is often simply referred to as "hyponatremia."