The syndrome of inappropriate secretion of ADH (SIADH) is defined by the secretion of ADH in the absence of an appropriate physiologic stimulus. Its hallmark is an inappropriately concentrated urine despite the presence of a low serum osmolality and a normal circulating blood volume. Causes of SIADH include central nervous system disorders, pulmonary disease, drugs, stress, pain, and surgery. Therefore, the above patient with a known history of lung cancer and hyponatremia, most likely has SIADH and exhibits the following lab findings: serum osmolarity low, urine osmolarity high, urine sodium high.
Psychogenic polydipsia (D) is a rare cause of euvolemic hyponatremia and is seen in psychiatric patients who consume large amounts of free water (in excess of 1L/hr). This large consumption overwhelms the kidney’s ability to excrete free water. Patients will exhibit low serum osmolarity, low urine osmolarity, and low urine sodium. Diabetes insipidus (B) results in the loss of large amounts of dilute urine from the loss of concentrating ability in the distal nephron. This may be due to a central cause such as the lack of ADH secretion from the pituitary, or a nephrogenic cause, such as the lack of responsiveness to circulating ADH. Laboratory work-up invariably shows high serum osmolarity, low urine osmolarity, and low urine sodium. High serum osmolarity, high urine osmolarity, and low urine sodium (A) rarely occurs.
Gibbs MA, Tayal VS: Electrolyte Disturbances, in Marx JA, Hockberger RS, Walls RM, et al (eds): Rosen’s Emergency Medicine: Concepts and Clinical Practice, ed 7. St. Louis, Mosby, Inc., 2010, (Ch) 123:p 1615-1632