💧 Pituitary & Water Balance Disorders
SIADH, Diabetes Insipidus (central and nephrogenic), hyponatraemia and hypernatraemia, the water deprivation test — MRCS high-yield.
Overview — ADH and Water Balance
Antidiuretic hormone (ADH), also called vasopressin (AVP), is produced by the hypothalamus (supraoptic and paraventricular nuclei) and stored and released by the posterior pituitary. Its primary role is to regulate serum osmolality by controlling water reabsorption in the renal collecting duct via V2 receptors → aquaporin-2 channels → water follows.
| Feature | SIADH | Diabetes Insipidus |
|---|---|---|
| ADH level | ↑ (inappropriately elevated) or ↑ ectopic production | ↓ (central DI) or kidneys unresponsive (nephrogenic DI) |
| Effect on collecting duct | Excess water reabsorption → dilution of blood | No water reabsorption → massive water loss in urine |
| Serum sodium | ↓ (hyponatraemia — dilutional) | ↑ (hypernatraemia — dehydration) |
| Serum osmolality | ↓ (<275 mOsm/kg) | ↑ (>295 mOsm/kg) |
| Urine osmolality | ↑ (inappropriately concentrated, >100 mOsm/kg; often >300) | ↓ (dilute, <300 mOsm/kg; often <150) |
| Urine volume | Low (concentrated) | Massive (polyuria 3–20 L/day) |
| Fluid status | Euvolaemic (normal body fluid volume) | Hypovolaemic (dehydrated) |
| Symptom | Confusion, seizures (from hyponatraemia) | Polydipsia, polyuria, dehydration, confusion (from hypernatraemia) |
| ECG | Low Na → seizures (not a primary ECG abnormality) | High Na → no specific ECG; tachycardia from dehydration |
The Core Concept — “What Is ADH Doing to the Urine?”
The simplest framework: ADH keeps water in the body by concentrating urine. Too much ADH (or ADH effect) → too much water kept → blood diluted → hyponatraemia → concentrated urine. Too little ADH (or renal resistance) → water pours out → blood concentrated → hypernatraemia → dilute urine.
The defining diagnostic feature of SIADH: urine osmolality > plasma osmolality — the kidney is concentrating urine despite the blood being already dilute. This is physiologically inappropriate and defines the syndrome.