Hyponatremia – 3 Lab Approach to Diagnosis

Original talk by Brandon Fainstad, MD
Edits, updates and graphics by Yilin Zhang, MD


  • Learn the “3 lab approach” to diagnosis of hyponatremia (this does not cover treatment!)
  • Understand the role of ADH in hyponatremia

Hyponatremia is a common finding in hospital admissions. Most of the time it occurs in the setting of an acute illness and improves with appropriate resuscitation and treatment of the presenting illness. So we usually don’t spend too much time thinking about it. But sometimes it is severe, unexplained or does not improve when we expect it would.

Many of us are taught to think of hyponatremia in terms of hyper/hypo/euvolemic states. Unfortunately, physicians are classically bad at determining volume status and this method does not sufficiently address the physiology.

Chalk Talk 3 Lab Approach

Board Set-up:

Lab 1 (Serum Osm) – Na represents a large portion of our serum osm. If Na is low we should expect the total serum osm to be low too. If the serum osm are normal or high we know there is something in the blood that is either usually in lower quantities or not there at all.  I usually don’t go into much detail with this category because it is relatively complicated and low yield. It should suffice to say that if the serum osm are nml or high, think about hyperglycemia, hypertriglyceridemia, uremia and etc.

Lab 2 (Urine Osm) – The only causes of hyponatremia that would allow a patient to be urinating mostly water (UOsm <100) are if the patient is consuming large quantities of free water (i.e. psychogenic polydipsia) or if they are not consuming enough solute (i.e. tea and toast) for the kidney to filter.

Lab 3 (Urine Na) – At this point we can blame it all on ADH.  The question is whether the pituitary is appropriately releasing ADH (i.e. low arterial pressure) in order to expand intravascular volume or inappropriately releasing ADH (i.e. normal arterial pressure).  The best way to determine this is by the Urine Na.  If the UNa is low (<25) we know that there is low perfusing pressure in the glomerulus,  activating the RAA system and aldosterone is telling the kidney to reabsorb all the Na, thus low urine Na.  This is essentially verifying that ADH is being released appropriately.

Low UNa – Now our assessment of volume status matters.  Do they have true hypovolemia, heart failure (elevated JVD), cirrhotic or on a thiazide diuretic?

Normal UNa – Is this SIADH from a CNS source (CVA, infection, trauma…), malignancy, psych drugs or pulmonary infection?  Do they have a hormone derangement (i.e. hypothyroidism, pregnancy or adrenal insufficiency)?



40 yo F with nausea/vomiting and diarrhea

Serum Na: 125
Serum Osm: 265
Urine Osm: 400
Urine Na: <10


A 21 yo M with history of type 1 DM who presents with nausea/vomiting and abdominal pain, found to have a blood glucose of 500 and + ketones on urinalysis

Serum Na: 125
Serum Osm: 310
Urine Osm: 800
Urine Na: 25


A 68 yo homeless M with history of EtOH abuse presents to with EtOH withdrawal

Serum Na: 125
Serum Osm: 270
Urine Osm: 60
Urine Na: 65


A 52 yo M with a history of hypertension on hydrochlorothiazide presents to clinic with increased falls

Serum Na: 125
Serum Osm: 257
Urine Osm: 350
Urine Na: 65