Which laboratory findings would you expect in a patient with SIADH?

In the clinical context of Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH), the expected laboratory findings include decreased serum osmolality and hyponatremia.

SIADH is characterized by the excessive release of antidiuretic hormone (ADH), which leads to increased water reabsorption in the kidneys. This consequently results in dilutional hyponatremia, as the body's sodium levels are reduced relative to the excess fluid volume. The retained water dilutes serum sodium concentrations, causing hyponatremia (a sodium level of less than 135 mEq/L).

Additionally, the serum osmolality would be decreased due to the dilution effect from the excess fluid, which is a hallmark finding in SIADH. The urine osmolarity, conversely, would be increased because the kidneys are concentrating urine due to the excess ADH, which promotes water retention.

This combination of findings—decreased serum osmolality and hyponatremia—is critical for diagnosing SIADH and distinguishing it from other conditions that may cause similar symptoms but have different underlying pathophysiologies.