Which serum change is commonly associated with syndrome of inappropriate antidiuretic hormone (SIADH)?

In the context of the syndrome of inappropriate antidiuretic hormone (SIADH), the common serum change observed is hyponatremia. SIADH is characterized by excessive secretion of antidiuretic hormone (ADH), which leads to increased water reabsorption by the kidneys. This excess water retention dilutes serum sodium levels, resulting in hyponatremia.

In this condition, despite normal or even elevated total body sodium levels, the effective serum sodium concentration decreases due to the increased volume of water. Patients may not show significant symptoms until the hyponatremia becomes severe, leading to complications such as confusion, seizures, or even brain edema in severe cases.

This understanding helps in recognizing SIADH in clinical scenarios, especially in patients with conditions like certain cancers, pulmonary diseases, or medications that might stimulate ADH release. Thus, the hallmark alteration in serum associated with SIADH is effectively the state of hyponatremia.