Water and Sodium Balance in Endocrine Disease

Expansion

Hyponatraemia

• SIADH: euvolaemic, concentrated urine, high urine sodium
• Adrenal insufficiency: hypovolaemic, with hyperkalaemia and acidosis; cortisol deficiency also removes the normal inhibition of antidiuretic hormone
• Hypothyroidism: reduced free water clearance, usually mild

Hypernatraemia

• Diabetes insipidus: large volumes of dilute urine; sodium rises only if thirst is impaired or water is unavailable
• Primary hyperaldosteronism: sodium is usually normal or only slightly raised, because aldosterone escape limits retention; the striking abnormality is hypokalaemia with hypertension and alkalosis

The general framework is that sodium concentration is a water problem while sodium content is a volume problem. Endocrine disorders can affect either: antidiuretic hormone disorders act on water, while aldosterone and cortisol disorders act on sodium content and on potassium and acid-base balance.

A practical rule: always check cortisol and thyroid function in unexplained hyponatraemia before diagnosing SIADH, since both are formally required exclusions.