SIADH is caused by:

• A. An increase in ADH
• B. A decrease in ADH
• C. An increase in aldosterone
• D. A decrease in cortisol

Answer: (A)

SIADH occurs when there is excess ADH, which causes water retention and dilution of serum sodium. ADH acts on the kidney’s collecting ducts to insert aquaporin-2 channels, increasing water reabsorption. When ADH is inappropriately high, more water is reabsorbed than solutes, leading to hyponatremia with a low serum osmolality, while the person often remains euvolemic. The urine is inappropriately concentrated (high urine osmolality) and contains higher sodium, reflecting continued water reabsorption despite low serum osmolality. Because the driving problem is too much ADH, the correct explanation is an increase in ADH.

Decreased ADH would cause diabetes insipidus with dilute urine and hypernatremia. An increase in aldosterone would tend to retain sodium and water differently and is not the primary driver of the hyponatremia seen in SIADH. Decreased cortisol can contribute to hyponatremia via ADH pathways, but SIADH is defined by an excess of ADH, making the rise in ADH the defining feature.