SIADH vs Diabetes Insipidus: How to Tell Them Apart
SIADH and diabetes insipidus are mirror-image disorders of antidiuretic hormone (ADH). In SIADH there is too much ADH effect, causing water retention and dilutional hyponatremia; in DI there is too little ADH effect, causing free water loss and a tendency toward hypernatremia. The core axis is simple: SIADH concentrates urine and drops serum sodium, while DI produces dilute urine and raises serum osmolality.
How to tell them apart
| Feature | SIADH (Syndrome of Inappropriate ADH Secretion) | Diabetes Insipidus (DI) |
|---|---|---|
| ADH problem | Too much ADH (inappropriate secretion), including ectopic production | Not enough ADH effect—either deficient ADH (central) or renal unresponsiveness (nephrogenic) |
| Serum sodium | Hyponatremia | Normal sodium if water access is maintained; hypernatremia if water access is limited |
| Serum osmolality | Low serum osmolality (<275 mOsm/kg) | High-normal to high serum osmolality (often >290 mOsm/kg when water access is limited) |
| Urine osmolality | Inappropriately concentrated urine (>100 mOsm/kg) | Inappropriately dilute urine (<300 mOsm/kg) despite high serum osmolality |
| Urine output / thirst | Normal urine output; not characterized by massive diuresis | Polyuria (often 5–20 L/day) with intense polydipsia |
| Volume status | Euvolemic—no edema, no dehydration | Euvolemic if water access maintained; risk of dehydration/hypovolemia if water access is limited |
| Classic causes | Small cell lung cancer (ectopic ADH), CNS disease (meningitis, encephalitis, stroke, trauma), post-pituitary surgery, certain drugs (carbamazepine, SSRIs, cyclophosphamide) | Central: post-surgical, trauma, tumors (craniopharyngioma, metastases), infiltrative (sarcoidosis, histiocytosis), idiopathic. Nephrogenic: lithium, hypercalcemia, hypokalemia, hereditary, chronic kidney disease |
| Response to desmopressin (DDAVP) | No meaningful further urine concentration (endogenous ADH already maximal) | Urine concentrates in central DI but remains dilute in nephrogenic DI |
The reasoning
Start with serum sodium and osmolality. Hyponatremia with low serum osmolality points to SIADH; a high serum osmolality (with normal or high sodium) points to DI. Next, check whether the urine is appropriate for the serum state. In SIADH the urine is inappropriately concentrated (>100 mOsm/kg) while the blood is dilute, and urine sodium exceeds 40 mEq/L in a euvolemic patient—confirming inappropriate ADH. In DI the urine is inappropriately dilute (<300 mOsm/kg) despite a concentrated serum, confirming inadequate ADH effect. Once DI is established, give desmopressin: if the urine concentrates, the kidney was simply lacking ADH (central DI); if it stays dilute, the kidney cannot respond (nephrogenic DI). Anchor on the classic vignette—euvolemic hyponatremia with concentrated urine in a small cell lung cancer patient is SIADH; polyuria with dilute urine and rising sodium is DI.
Key tests
- Serum osmolality: low (<275 mOsm/kg) in SIADH versus high (>290 mOsm/kg) in DI—this single axis separates them.
- Urine osmolality relative to serum: inappropriately concentrated (>100 mOsm/kg, urine osmolality > serum) in SIADH; inappropriately dilute (<300 mOsm/kg despite high serum osmolality) in DI.
- Urine sodium: typically >40 mEq/L in euvolemic SIADH (kidneys not conserving sodium); in DI the defect is free water handling and urine sodium is not diagnostic.
- Desmopressin (DDAVP) challenge in DI: urine concentrates in central DI (kidney was waiting for ADH) but stays dilute in nephrogenic DI. A water deprivation test can also confirm DI—urine concentrates in primary polydipsia but stays dilute in DI.
What they share
- Both are disorders centered on ADH (vasopressin) and its action on the kidney
- Both can be caused by CNS/pituitary disease—trauma, tumors, and pituitary surgery can precipitate either condition
- Both can occur after transsphenoidal pituitary surgery, and the post-surgical pattern may oscillate in a triphasic response (DI, then SIADH, then DI)
Pitfalls
- Confusing the direction of the sodium: SIADH causes hyponatremia, whereas DI trends toward hypernatremia—memorize which is which.
- In DI, if water access is maintained, sodium may look relatively normal, masking the disorder; the giveaway is high-volume dilute urine, so don't wait for hypernatremia.
- After pituitary surgery the pattern can follow a triphasic course (transient DI, then SIADH, then permanent DI), so a single lab snapshot can mislead—reassess over time.
- Nephrogenic DI can be mistaken for a pituitary problem: ADH levels are actually high, but the kidney doesn't respond—use the desmopressin challenge to separate nephrogenic from central DI.
- Don't forget reversible metabolic causes of nephrogenic DI such as lithium, hypercalcemia, and hypokalemia (e.g., hypokalemia from primary aldosteronism can produce polyuria).
- In DI workup, primary polydipsia mimics DI with dilute urine—use the water deprivation test, in which the polydipsia patient's urine concentrates but the DI patient's does not.
Practice this the way the exam tests it — on branching cases where your decisions shape the patient.