SIADH

 

 

Syndrome of Inappropriate Antidiuretic Hormone (SIADH)

• Mechanism: Excess ADH → water retention → dilutional hyponatremia.

• Volume status: Euvolemic (mild ↑ECF, but no edema due to natriuresis).

• Symptoms:

  • Mild (Na⁺ >120 mEq/L) → lethargy, forgetfulness.

  • Severe (Na⁺ <120 mEq/L) → confusion, seizures, coma → ↑ risk of cerebral edema & herniation.

• Treatment:

  • Mild → fluid restriction (<800 mL/day), salt tablets.

  • Severe (<120 with neuro symptoms) → urgent correction with hypertonic (3%) saline.

• Labs:

  • Serum osmolality: low (<275 mOsm/kg)

  • Urine osmolality: inappropriately high (>100 mOsm/kg)

  • Urine sodium: elevated (>40 mEq/L)

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Primary Polydipsia

• Mechanism: Excess water intake overwhelms kidneys’ ability to excrete water.

• Common in: Psychiatric patients (e.g., schizophrenia) due to disordered thirst regulation.

• Symptoms: Confusion, lethargy, psychosis, seizures (if Na⁺ falls significantly).

• Treatment: Water restriction, behavioral modification.

• Labs:

  • Serum osmolality: low

  • Urine osmolality: very dilute (<100 mOsm/kg)

  • Urine sodium: variable, often low

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Key Differentiator

• SIADH: urine concentrated despite hyponatremia.

• Primary polydipsia: urine dilute (<100 mOsm/kg).

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✅ Memory hook:

• SIADH → “S = Sticky urine” (concentrated).

• Polydipsia → “P = Peeing pure water” (dilute).