Sodium; Aldosterone–ADH–Cortisol axis

Serum Na+ controls the water balance of the body, not as a total body sodium. Hyponatremia = excess water relative to sodium. First assess tonicity, then volume status 

• Hypotonic hyponatremia = too much water, not sodium deficiency

• Isotonic hyponatremia = lab artefact due to high lipids/proteins

• Hypertonic hyponatremia = osmotic shift of water out of cells due to hyperglycemia/mannitol

• Hypovolemic hyponatremia = dry mucosa. If urine sodium <20 means extra renal loss, >20 means renal loss. Treatment - isotonic saline

• Euvolemic hyponatremia = no oedema. Urine sodium would be >20, consider SIADH, polydipsia, and cortisol deficiency. Treatment - fluid restriction

• Hypervolaemic hyponatremia = oedema. Urine sodium <20, consider organ failure. Treatment - fluid + salt restriction

• Treatment for severe hyponatremia (<120) or symptomatic = 3% sodium. maximum correction 10 Meq/day. Overcorrection (speed of correcion) - osmotic demyelination syndrome

Hypernatremia (thirst) =  water loss > Na loss. Treatment; calculate water deficit = 0.6 x wt (1-140/Na), and correct it with 5D

Volume-Pressure Regulation

• RAAS + ADH + SNS integrate renal perfusion, Na⁺, and water balance.

• Endocrine causes of hypertension:

  • • Primary aldosteronism

    • Cushing’s

    • Phaeochromocytoma

    • Liddle’s syndrome (Na⁺ channel mutation mimicking aldosterone excess)

The kidney is the final effector organ for almost the entire endocrine system

• ADH → Water

• Aldosterone → Sodium, Potassium, Hydrogen

• PTH → Calcium, Phosphate

• Cortisol → Volume & pH

Pearls;

• “Hypertension + hypokalaemia + alkalosis” → Aldosterone or mimic.

• “Hyponatraemia + high urine osmolality” → SIADH or Addison’s.

• “Hypernatraemia + dilute urine” → Diabetes insipidus.

• “High K⁺ + acidosis + low BP” → Aldosterone failure (Addison’s / RTA 4).

• “Aldosterone keeps sodium, kills potassium.”

• “ADH keeps water, not sodium.”

• “Cortisol mimics aldosterone when in excess.”

• “SIADH → concentrated urine in diluted plasma.”

• “Addison’s → salt loss, potassium gain, acid retention.”

• “Conn’s → salt retention, potassium loss, hydrogen loss.”

• “Liddle’s → aldosterone effects without aldosterone.”