A child with symptoms of SIADH is most likely to present with which electrolyte abnormality?

• A. Hyponatremia
• B. Hypoglycemia
• C. Hypophosphatemia
• D. Hypomagnesemia

Answer: (A)

The main idea is that SIADH causes dilutional hyponatremia. When excess antidiuretic hormone is secreted, the kidneys reabsorb more free water without a corresponding increase in sodium, so the plasma becomes more watery and the sodium concentration drops even though total body sodium may be normal or even slightly increased. This leads to a low serum sodium level (hyponatremia) with typically inappropriately concentrated urine and a euvolemic or mildly fluid-overloaded state.

Hyponatremia fits SIADH best because it directly reflects the water retention driven by excess ADH. The other options involve different electrolytes that aren’t specifically tied to the pathophysiology of SIADH: hypoglycemia is a glucose issue, not an electrolyte abnormality; hypophosphatemia and hypomagnesemia are separate disturbances that aren’t characteristic features of SIADH.