In SIADH, which description best fits the patient's volume status and serum osmolality?

• A. Hypovolemic
• B. Hypervolemic
• C. Euvolemic and hypoosmolar with inappropriately elevated ADH
• D. Normovolemic and hyperosmolar

Answer: (C)

SIADH causes hyponatremia with low serum osmolality because ADH is secreted inappropriately, promoting water reabsorption even when the body should be excreting free water. This dilutes the serum sodium, giving a hypoosmolar state, while the extracellular fluid volume stays normal—euvolemic—since the excess water is balanced by natriuresis and there is typically no edema. The key feature is ADH that remains elevated despite hyponatremia, leading to inappropriately concentrated urine. So the description that best fits is euvolemic and hypoosmolar with inappropriately elevated ADH. The other scenarios describe true volume depletion, true volume overload, or hyperosmolar states, which do not align with SIADH.