Hypoosmotic hyponatremia

Hyponatremia has occurred in the settings of hypo-osmotic, hyperosmotic, and isosmotic plasma thus the measurement (or occasionally a calculation) of plasma osmolality is an important initial step in.the assessment of hyponatremia. Of these, the most common is hypoosmotic hyponatremia, since Na is the primary determinant of plasma osmolality. Figure 46-2 describes an algorithm for laboratory measurements and physical examination findings in the differential diagnosis of a plasma Na" <135 mmol/L. 

In hypoosmotic hyponatremia with a normal volume status, the most common etiologies are the syndrome of inappropriate ADH (SIADH), primary polydipsia, hypothyroidism, and adrenal insufficiency (see Figure 46-2). SIADH is usually a result of ectopic or otherwise inappropriate ADH production arising from a variety of conditions (see Chapters 45 and 50) and results in excessive H2O retention. SIADH is often diagnosed when a urine osmolality that is greater than plasma osmolality (usually by more than >i00 mOsmol/kg) is observed in the setting of hyponatremia, but only when renal, adrenal, and thyroid functions are normal. Hypothyroidism impairs free H2O excretion, whereas in adrenal insufficiency, Na" is lost in preference to IC reabsorption. Finally, euvolemic hyponatremia can be... 

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