Severe hypothyroidism (unknown mechanism, possibly secondary to low cardiac output and glomerular filtration rate) and adrenal insufficiency are also associated with nonosmotic vasopressin release and impaired sodium reabsorption, leading to hypotonic hyponatremia. Hyponatremia associated with cortisol deficiency, such as primary or secondary hypoadrenalism, commonly presents subtly and may go undiagnosed. A random cortisol level check, especially in acute illness, can be misleading if the level is normal (when it should be high). Testing for adrenal insufficiency and hypothyroidism should be part of the hyponatremic workup, because these disorders respond promptly to hormone replacement. Depending on the etiology, mineralocorticoid will also need replacement.
Hypovolemic hypotonic hyponatremia usually indicates concomitant solute depletion, with patients presenting with orthostatic symptoms. The pathophysiology underlying hypovolemic hypotonic hyponatremia is complex and involves the interplay of carotid baroreceptors, the sympathetic nervous system, the renin-angiotensin system, antidiuretic hormone (ADH [vasopressin]) secretion, and renal tubular function. In the setting of decreased intravascular volume (eg, severe hemorrhage or severe volume depletion secondary to gastrointestinal or renal loss, or diuretic use) owing to decreased stretch on the baroreceptors in the great veins, aortic arch, and carotid bodies, an increased sympathetic tone to maintain systemic blood pressure generally occurs.
Normovolemic (euvolemic) hypotonic hyponatremia is common among patients who are hospitalized. It is associated with nonosmotic and non–volume-related ADH secretion (ie, syndrome of inappropriate ADH secretion [SIADH]) secondary to various clinical conditions, including the following:
CNS disturbances (eg, hypopituitarism )
Diuretics may induce hypovolemic hyponatremia. Note that thiazide diuretics, in contrast to loop diuretics, impair the diluting mechanism without limiting the concentrating mechanism, thereby impairing the ability to excrete a free water load. Thus, thiazides are more prone to causing hyponatremia than are loop diuretics. This is particularly so in elderly persons, who already have impaired diluting ability.
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Cite this: Romesh Khardori. Fast Five Quiz: Hyponatremia Essentials - Medscape - Oct 14, 2019.