Adult CCRN Practice Exam

Excess ADH in SIADH makes the kidneys hold onto water, which dilutes the blood sodium. That dilutional effect produces hyponatremia, the hallmark finding. In SIADH, patients are typically euvolemic, with low serum osmolality and inappropriately concentrated urine (urine osmolality that remains high despite low sodium). The low sodium is the primary clue clinicians look for because it reflects the excess water relative to sodium. Hypernatremia would suggest the opposite problem (ADH deficiency or water loss), polyuria points to diabetes insipidus, and polyphagia is unrelated to SIADH.