Which laboratory result would most likely indicate SIADH?

The most indicative laboratory result for Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) is low serum sodium. In SIADH, there is an excessive secretion of antidiuretic hormone (ADH), which leads to water retention in the body. This dilutional effect causes hyponatremia, or low serum sodium levels, due to the excess water diluting the sodium concentration in the blood.

In patients with SIADH, despite having enough or even increased total body water, the serum sodium levels drop because the kidneys retain water, resulting in a higher urine concentration and lower serum sodium. Monitoring serum sodium is crucial in managing patients with SIADH, as hyponatremia can lead to serious neurologic complications if left untreated.

Understanding the interrelationship between sodium levels, ADH secretion, and fluid balance is essential in recognizing and managing SIADH effectively.