Diagnosis, Hyponatremia
<p>The plasma osmolality is reduced in most hyponatremic patients because it is primarily determined by the plasma sodium concentration and accompanying anions. The normal response to hyponatremia is to suppress AVP secretion, resulting in a maximally dilute urine (osmolality less than 100 mmol/kg). Higher urine osmolality values indicate an inability to excrete free water that is generally caused by continued AVP secretion. In patients with hyponatremia and a low plasma osmolality, the urine osmolality can distinguish between impaired water excretion and primary polydipsia, in which water intake is so high that it exceeds excretory capacity. In patients with impaired water excretion caused by hypovolemia, the urine osmolality often exceeds 450 mmol/kg. Osmolality measurements are also useful in beer/diet potomania, i.e. hyponatremia with a low solute intake, when overt polyuria may be absent despite the production of dilute urine.
 
SIADH:                                Uosm <b>></b> Sosm and U<sub>Na</sub> > 20
Water Intoxication:    Uosm <b><</b> Sosm and U<sub>Na</sub> > 20 
 
Hypervolemic hyponatremia
    •cirrhosis<br />
    •congestive heart failure
    •nephrotic syndrome
    •massive edema of any cause
Euvolemic hyponatremia
    •states of severe pain or nausea
    •in the setting of trauma or other damage to the brain
    •SIADH (and its many causes)
Hypovolemic hyponatremia
    •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)
Miscellaneous causes of hyponatremia that are not included under the above classification scheme include:
    •factitious hyponatremia (due to massive increases in blood triglyceride levels, extreme elevation of immunoglobulins as may occur in multiple myeloma, and extreme hyperglycemia)
   •hypothyroidism and adrenal insufficiency (both thyroid hormone and cortisol are required to excrete free water)
    •beer potomania and other malnourished states where poor dietary protein intake leads to inadequate urine solute formation thereby impeding the kidney's ability to excrete free water.
    •primary polydipsia (where the amount of urine solute required to excrete huge quantities of ingested water exceeds the body's ability to produce it; this typically occurs when 12 or more litres of water are ingested per day)</p>