Electrolyte Patterns in Endocrine Disease

Hyponatremia means a serum sodium concentration below 135 mmol/L. In endocrine disease, the problem is often not that the body has simply lost sodium. More often, it has retained too much water relative to sodium. Antidiuretic hormone (ADH) tells the kidney to conserve water. When ADH activity is excessive, or when the kidney cannot excrete free water effectively, water dilutes serum sodium and plasma osmolality falls. Water then moves into cells. Brain cells are especially vulnerable because the skull cannot expand; cerebral oedema can produce headache, confusion, seizures, coma, respiratory arrest, and brain herniation. Three endocrine-related patterns deserve close attention: - Adrenal insufficiency: Cortisol normally suppresses ADH release and supports vascular tone. Cortisol deficiency promotes ADH-mediated water retention. In primary adrenal insufficiency, mineralocorticoid deficiency may also cause renal sodium loss and volume depletion. - Hypothyroidism: Reduced cardiac output and impaired renal water excretion can contribute to hyponatremia, particularly in severe disease. - SIADH: Persistent ADH effect causes water retention despite a low serum sodium concentration. SIADH is classically euvolemic, but adrenal insufficiency and hypothyroidism must be considered before attributing hyponatremia...