A patient presents to a clinic with complaints of headache, fatigue. Lab data show serum sodium, 122 mEq/L; serum osmolality, 240 mOsm/L; urine osmolality, 455 mOsm/L. which condition best correlates with these data?: (A) Neurogenic diabetes insipidus, (B) Nephrogenic diabetes insipidus, (C) Diabetes mellitus, (D) SIADH
Answer is D. Hyponatremia, hypo-osmolality, and inappropriately concentrated urine are characteristic of SIADH. The syndrome results from excessive secretion of vasopres­sin. This could be due to ectopic tumor secretion of vasopressin, various central nervous system complica­tions, or it could be drug induced. The increased vasopressin causes excessive water retention, resulting in dilutional hyponatremia and hypo-osmolality. Vol­ume expansion inhibits the renin-angiotensin system. Plasma aldosterone therefore tends to be low, which partly explains the inappropriately concentrated urine.