This patient's history and clinical presentation are most consistent with the syndrome of inappropriate antidiuretic hormone secretion (SIADH). On physical examination, she is clinically euvolemic. The appropriate physiologic response to hypo-osmolality in a euvolemic patient is suppression of antidiuretic hormone (ADH) with resultant increase in free water clearance, with urine osmolality less than plasma osmolality. In contrast, this patient demonstrates evidence for increased ADH with hyponatremia and urine osmolality significantly and inappropriately greater than plasma osmolality in spite of her euvolemic status and no apparent stimulus for ADH release.

Because water excretion is, in part, solute dependent, severe limitations in solute intake decrease free water excretion, and hyponatremia may develop in this setting with only modest increases in fluid intake. This syndrome is termed beer potomania when observed in patients with chronic alcohol abuse and low solute intake. This patient's relatively high urine osmolality makes beer potomania an unlikely diagnosis.

The most common renal consequence of chronic lithium ingestion is nephrogenic diabetes insipidus; this disorder presents with polyuria and hypernatremia, which are not seen in this patient.

Primary polydipsia should always be considered in the differential diagnosis of patients with mental illness and hyponatremia, particularly those with schizophrenia who are taking psychotropic drugs. Primary polydipsia presents with hyponatremia, decreased plasma osmolality, and decreased urine osmolality, reflecting suppressed ADH levels in response to water overload. Patients with primary polydipsia may also present with abnormalities of ADH regulation such as transient stimulation of ADH release during psychotic episodes and increased renal response to ADH so that at the same levels of ADH, patients who are psychotic may have higher urine osmolalities and a downward resetting of the osmostat that regulates ADH release. Thus, the urine of patients who are psychotic and have primary polydipsia may not be as dilute as would be expected. The significant elevation in urine osmolality makes SIADH the more likely diagnosis in this patient.