In addition to discontinuing fluoxetine, fluid restriction is the appropriate treatment for this patient. She is euvolemic and has hyponatremia with a decreased plasma osmolality and an inappropriately increased urine osmolality. This clinical and laboratory presentation is highly suggestive of the syndrome of inappropriate antidiuretic hormone secretion (SIADH). SIADH has many causes, including various drugs such as selective serotonin reuptake inhibitors like fluoxetine. The initial treatment of asymptomatic patients with SIADH includes management of the underlying cause if possible and free water restriction (in practical terms, fluid restriction) without limiting sodium intake. Discontinuation of fluoxetine should result in resolution of the SIADH with normalization of the serum sodium. In the interim, fluid restriction with a decrease in intake less than urine output will result in a gradual increase in the serum sodium.

Although the serum sodium is significantly decreased, she has no specific neurologic symptoms, which suggests that the hyponatremia is chronic. Rapid normalization of this patient's serum sodium with hypertonic saline would place her at risk for osmotic demyelination syndrome, which may result in severe neurologic symptoms such as paraplegia, dysarthria, dysphagia, diplopia, and locked-in syndrome. Because of this risk, treatment with hypertonic saline is usually limited to patients with severely symptomatic hyponatremia (such as mental status changes or seizures) to rapidly increase the serum sodium.

Treatment with isotonic saline may correct hyponatremia if it is secondary to hypovolemia, but this patient is euvolemic and most likely has SIADH. In this patient, isotonic saline alone, without concomitant fluid restriction, results in volume expansion but may not correct and may possibly worsen the hyponatremia because of inappropriate retention of the water associated with the infusion.

Oral demeclocycline results in renal resistance to antidiuretic hormone and can be effective in treating patients with SIADH. However, it has been associated with acute kidney injury and is generally reserved for patients who have failed other therapies. It should be used with caution in patients with preexisting kidney or liver disease.

Tolvaptan, a vasopressin receptor antagonist, results in the excretion of electrolyte free water and is effective in raising the serum sodium in patients with SIADH. It should be used with caution in the treatment of severe, symptomatic hyponatremia, which is not seen in this patient, and as a last resort when other treatments have failed. Because severe liver injury has been reported with its use, the FDA recommends that it not be used in patients with liver disease and that it be used for no more than 30 days.