Renal Physiology Pearls/Cases

What physiologic mechanisms normally prevent hyponatremia when plasma osmolality falls?

Drop in osmolality suppresses ADH → increased free water excretion → restoration of plasma osmolality.

What distinguishes true hyponatremia from pseudo-hyponatremia?

True hyponatremia is always hypotonic (hypo-osmolar)

What are the steps towards evaluating hyponatremia?

1. Serum osmolality to determine whether the hyponatremia is hypo-(<275), iso-(275-295), or hypertonic(>295) (is it true hyponatremia with hypoosmolarity)

2. Urine osmolarity (is ADH level suppressed, appropriately elevated, or inappropriately elevated)

3. Assess volume status

4. Assess urine Na content (is Na concentrated/is kidney working?)

What does urine osmolality reveal in the evaluation of hyponatremia?

Whether ADH is appropriately suppressed (<100 mOsm/kg)→water excreted or present/inappropriately elevated (>100 mOsm/kg)→less free water excretion

What urine osm finding suggests normal suppression of ADH? What level suggest ADH Present?

Urine osm <100 mOsm/kg. Urine osm >100 = ADH present

What conditions cause hyponatremia with suppressed ADH and very dilute urine?

Primary polydipsia and low-solute intake (tea-and-toast, beer potomania).

What is the physiologic reason SIADH causes hyponatremia?

Non-osmotic ADH release → water retention → dilutional hyponatremia despite normal volume status.

What urine sodium level helps differentiate renal vs extrarenal volume loss in hypovolemic hyponatremia?

Urine Na <20 suggests extrarenal loss. Urine Na>20-30 suggests Renal loss (because kidney damage leading to Na release when it shouldnt)

Why does hypovolemic hyponatremia produce high urine osm?

Volume depletion triggers ADH release → concentrated urine.

What is the major danger of rapidly correcting chronic hyponatremia?

Osmotic demyelination syndrome due to rapid water movement out of brain cells.

What is the safe correction goal for chronic hyponatremia?

6–8 mEq/L per 24 hours (never exceed 10–12 mEq/L).

What is the physiologic adaptation of the brain in chronic hyponatremia?

Loss of electrolytes (hours) and organic osmolytes (days) to prevent cerebral edema.

What finding differentiates SIADH from hypovolemic hyponatremia?

SIADH has urine Na >20–30 and euvolemia

What distinguishes water diuresis from osmotic diuresis in hypernatremia?

Water diuresis: Uosm <600

What physiologic defect causes central diabetes insipidus?

Absent ADH production → inability to concentrate urine.

What physiologic defect causes nephrogenic diabetes insipidus? Common causes?

Kidneys fail to respond to ADH → inability to concentrate urine. Causes: Lithium, Hypercalcemia,

Why does beer potomania cause hyponatremia?

Low solute intake limits renal free water excretion → dilutional hyponatremia.

Why does primary polydipsia(urge to drink fluids/excessive thirst) cause hyponatremia?

Excess water intake overwhelms maximal renal free water excretion → dilute urine and low serum Na.

Why does SIADH produce high urine osm and high urine Na?

ADH causes water retention

Common causes of SIADH

CANCER, pulmonary disorders, Drugs(SSRIs, HCTZ), CNS disorders,

Why does vomiting/diarrhea often cause hypovolemic hyponatremia?

Loss of Na-rich fluid + ADH activation → water retention → dilutional hyponatremia.

Why does adrenal insufficiency cause hyponatremia?

Low cortisol → increased ADH

Why does thiazide-induced hyponatremia occur?

Impaired urinary dilution + volume depletion → ADH activation.

Why does furosemide NOT worsen hyponatremia in SIADH?

Loop diuretics reduce medullary gradient → impair urine concentration → increase free water excretion.

Hypernatremia

>145 meq/l; results from water deficit; risk factors include: elderly with reduced thirst, poor access tof luids, diabetes insipidus

Renal forms of water loss

Osmotic diuresis: hyperglycemia, mannitol

Water diuresis: Central or Nephrogenic DI

Non-renal water loss

Diarrhea, burns, sweat, respiratory ventilation

Na correction for hyperglycemia

For every 100 inc in glucose over 100, decrease Na by factor of 1.6 (for blood glucose <400) or 2.4 (for blood glucose >400)

Acute vs Chronic Hyponatremia timeline

<48hrs is acute; >48hrs is chronic