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USMLE Step 1EndocrinologyEP 13 of 14

SIADH vs diabetes insipidus: central vs nephrogenic DI

SIADH and diabetes insipidus are both ADH problems that run in opposite directions. Serum sodium, urine osmolality and the response to desmopressin tell them apart.

  • 5:44 video
  • 3 min read
  • 7 sources
  • Published 15 Nov 2026
Video lesson · 5:44 · Watch on YouTubeSeries: Endocrinology · @medservatory
Chapters (12)
By the end of this lesson

Distinguish SIADH, central diabetes insipidus and nephrogenic diabetes insipidus by serum sodium and osmolality, urine osmolality, and the response to desmopressin.

The short version

  1. SIADH is water excess: low serum sodium, low serum osmolality and an inappropriately concentrated urine, in a euvolaemic patient.
  2. Diabetes insipidus is hypotonic polyuria, with a normal or high serum sodium.
  3. Central DI means ADH is not released; nephrogenic DI means ADH is present but the kidney ignores it.
  4. After water deprivation and desmopressin, central DI concentrates the urine (rise above 50 percent) and nephrogenic DI barely does (rise under 15 percent).

SIADH vs diabetes insipidus: two opposite problems, one hormone

A 60-year-old smoker is drowsy and confused. His serum sodium is 122 mEq/L, yet his urine is concentrated. Down the corridor, a man after pituitary surgery is passing litre after litre of pale urine.

These are two opposite problems with one hormone behind them. This lesson is about SIADH vs diabetes insipidus: how to tell SIADH, central diabetes insipidus and nephrogenic diabetes insipidus apart using sodium, urine osmolality and desmopressin.

The one question behind every case: is ADH present, and is the kidney listening?

ADH is made by hypothalamic neurons and released by the posterior pituitary when plasma osmolality rises.[1][4]

It binds the V2 receptor on collecting-duct principal cells, aquaporin-2 moves to the membrane, and water is reabsorbed.[2][4]

So every case in this lesson asks the same thing: is ADH present, and is the kidney listening?

SIADH: too much ADH

In SIADH, ADH keeps acting on dilute plasma, so the kidney keeps water it should excrete. The problem is a free-water excess, not salt loss.[4][5]

  • Serum sodium below 135 mEq/L and serum osmolality below 275 mOsm/kg.
  • Urine osmolality above 100 mOsm/kg: the urine is not dilute, which is inappropriate for such dilute plasma.
  • Urine sodium above 20 mEq/L, often above 40.
  • The patient is euvolaemic: no oedema, no dehydration.

Common causes include small cell lung cancer, brain disease, lung infection and SSRIs.[4][5][6]

In a stable euvolaemic patient, treatment starts with fluid restriction. Severe symptoms need hypertonic saline, which the hyponatraemia lessons cover.[4][5]

Diabetes insipidus: hypotonic polyuria

In diabetes insipidus there is too little ADH effect. The collecting duct stays waterproof, so the kidney cannot concentrate urine.[2][3]

  • The urine is hypotonic, under 300 mOsm/kg, and the volume exceeds 3 litres a day.
  • Patients drink to thirst, so serum sodium is usually normal or mildly high, and it rises if they cannot drink.

Central vs nephrogenic diabetes insipidus: causes

The three diagnoses side by side
DisorderWhat is wrong with ADHSerum sodium (mEq/L)Urine (osmolality, mOsm/kg)
SIADHToo much ADH effectLow (below 135)Osmolality above 100, not dilute
Central DIHypothalamus or posterior pituitary fails to release ADHNormal or mildly highHypotonic, under 300
Nephrogenic DIADH present, kidney ignores itNormal or mildly highHypotonic, under 300

In central DI, now also called arginine vasopressin deficiency, the hypothalamus or posterior pituitary fails to release ADH. Common causes are head injury, pituitary surgery and tumours.[2][3][7]

In nephrogenic DI, ADH is present but the kidney ignores it. Lithium is the classic cause, along with high calcium, low potassium, and V2 receptor or aquaporin-2 mutations.[2][3]

Water deprivation, then desmopressin

To separate the two kinds of DI, deprive the patient of water, then give desmopressin, a synthetic ADH.[2][3]

Healthy people concentrate urine above 800 mOsm/kg. Patients with DI stay under 300.[2][3]

  1. Deprive waterWithhold water and measure the urine osmolality. If the serum sodium is already above 147 mEq/L, skip this step and give desmopressin.
  2. Give desmopressinMeasure the urine osmolality again.
  3. Compare the riseA rise of more than 50 percent means central DI. A rise of under 15 percent means nephrogenic DI. A rise in between is indeterminate: partial DI or primary polydipsia.

Hormone missing, the urine concentrates. Receptor deaf, it does not. In nephrogenic DI the kidney cannot hear the hormone.[2][3]

Worked example: a rise from 180 to 560

A patient after pituitary surgery passes large volumes of dilute urine. His serum sodium is 151 mEq/L: above 147 with hypotonic polyuria already points to DI, so water deprivation is skipped. His urine osmolality is 180 mOsm/kg, and after desmopressin it is 560 mOsm/kg.

Percentage rise after desmopressin
  1. 560 − 180 = 380
  2. 380 / 180 ≈ 2.1, a rise of about 211 percent
  3. 560 / 180 ≈ 3.1: the urine osmolality roughly triples
  4. More than 50 percent → central DI

Practice question

Original · USMLE style
Your turnQB-MS-S1-ENDO-008-01

A 41-year-old woman comes to the clinic because of constant thirst and frequent urination for several months. She has bipolar disorder and has taken lithium for 12 years. She drinks large amounts of water and passes about 5 L of urine daily. She has no headache or visual change and has had no head injury. Serum studies show:

Results
Na⁺149 mEq/L
Osmolality306 mOsm/kg H₂O
Glucose92 mg/dL
Calcium9.4 mg/dL
Urine osmolality is 150 mOsm/kg H₂O. After subcutaneous administration of desmopressin, urine osmolality is 160 mOsm/kg H₂O. Which of the following best explains these findings?
Pick one option. Your answers stay on this device.

Common questions

What is the difference between SIADH and diabetes insipidus?

SIADH is water excess: low serum sodium and an inappropriately concentrated urine. Diabetes insipidus is hypotonic polyuria with a normal or high serum sodium.

How does the desmopressin test separate central from nephrogenic DI?

After water deprivation, desmopressin is given. In central DI the urine osmolality rises by more than 50 percent; in nephrogenic DI it rises by under 15 percent.

What are the lab findings in SIADH?

Serum sodium below 135 mEq/L, serum osmolality below 275 mOsm/kg, urine osmolality above 100 mOsm/kg and urine sodium above 20 mEq/L, in a euvolaemic patient.

What causes central and nephrogenic diabetes insipidus?

Central DI commonly follows head injury, pituitary surgery or a tumour. Nephrogenic DI has lithium as its classic cause, along with high calcium, low potassium, and V2 receptor or aquaporin-2 mutations.

Why is serum sodium normal or high in diabetes insipidus?

Patients drink to thirst, so sodium is usually normal or mildly high. It rises if they cannot drink.

Transcript

Read the full narration, by chapter645 words

0:00Two patients, one hormone: low sodium with concentrated urine, and pale polyuria

A 60-year-old smoker is drowsy and confused. His serum sodium is 122, yet his urine is concentrated. Down the corridor, a man after pituitary surgery is passing litre after litre of pale urine. Two opposite problems, one hormone.

0:18Today: three diagnoses, three clues

Today: tell SIADH, central diabetes insipidus and nephrogenic diabetes insipidus apart, using sodium, urine osmolality and desmopressin.

0:30Last time: ADH, V2 receptor and aquaporin-2

Last time, we followed ADH into the collecting duct. Hypothalamic neurons make it, and the posterior pituitary releases it when plasma osmolality rises. It binds the V2 receptor, aquaporin two moves to the membrane, and water is reabsorbed. So every case today asks one question: is ADH present, and is the kidney listening?

0:54SIADH: too much ADH (sodium, osmolality, urine osmolality, euvolaemia)

First, too much ADH: SIADH. ADH keeps acting on dilute plasma, so the kidney keeps water it should excrete. The extra water dilutes the blood: serum sodium falls below 135, and serum osmolality below 275. The key finding: the urine is not dilute. Urine osmolality stays above 100, inappropriate for such dilute plasma. Urine sodium is above 20, often above 40, and the patient is euvolaemic: no oedema, no dehydration.

1:33SIADH causes and first treatment

Causes include small cell lung cancer, brain disease, lung infection and SSRIs. In a stable euvolaemic patient, treatment starts with fluid restriction; severe symptoms need hypertonic saline, which the hyponatraemia lessons cover.

1:50Diabetes insipidus: hypotonic polyuria

Now too little ADH effect: diabetes insipidus. The collecting duct stays waterproof, so the kidney cannot concentrate urine. Urine is hypotonic, under 300 milliosmoles per kilogram, and the volume exceeds three litres a day. Patients drink to thirst, so sodium is usually normal or mildly high, rising if they cannot drink.

2:15Central DI vs nephrogenic DI: causes

In central DI, now also called arginine vasopressin deficiency, the hypothalamus or posterior pituitary fails to release ADH, after head injury, pituitary surgery or a tumour. In nephrogenic DI, ADH is present but the kidney ignores it: lithium is the classic cause, along with high calcium, low potassium, and V2 or aquaporin two mutations.

2:42Water deprivation, then desmopressin

To separate them, deprive the patient of water, then give desmopressin, a synthetic ADH. Healthy people concentrate urine above 800. Patients with DI stay under 300. After desmopressin, central DI responds: urine osmolality rises by more than 50%. In nephrogenic DI it barely moves, under 15%, because the kidney cannot hear the hormone. Hormone missing, the urine concentrates. Receptor deaf, it does not.

3:16Worked example: sodium 151, urine osmolality 180 to 560

Our surgical patient has a sodium of 151. After deprivation, his urine osmolality is 180. After desmopressin, 560. A threefold rise: central DI.

3:32Your turn: a practice question

Your turn. A 41-year-old woman has taken lithium for bipolar disorder for 12 years. She is constantly thirsty and passes about five litres of urine a day. Serum sodium is 149, serum osmolality 306, and glucose and calcium are normal. Urine osmolality is 150, and 160 after desmopressin. Which of the following best explains these findings? A, autonomous secretion of antidiuretic hormone. B, destruction of antidiuretic-hormone-producing neurons. C, resistance of the collecting duct to antidiuretic hormone. D, excessive water intake suppressing antidiuretic hormone release. E, osmotic diuresis from glucose in the filtrate. Pause here and decide.

4:29The answer, one option at a time

A is out: excess ADH would dilute her blood and concentrate her urine, but her sodium is high and her urine dilute. B is out: central DI would concentrate the urine by more than 50% after desmopressin, and hers rose by only about 7%. D is out: drinking too much lowers the sodium, and hers is high. E is out: her glucose is normal, so there is no osmotic load. So the answer is C. Hypotonic urine, a high sodium and no response to desmopressin mean the kidney is not listening, and lithium is the cause.

5:10Recap pocket card and next lesson

Three things to keep. One: SIADH is water excess, with low sodium and an inappropriately concentrated urine. Two: diabetes insipidus is hypotonic polyuria, with a normal or high sodium. Three: after desmopressin, central DI concentrates the urine and nephrogenic DI does not. Next, the hyponatraemia diagnostic approach, where SIADH meets volume status.

Sources

Numbers in the text, e.g. [1], point here
  1. OpenStax Anatomy and Physiology 2e, 17.3 The Pituitary Gland and Hypothalamus. openstax.org/books/anatomy-and-physiology-2e/pages/17-3-the-pituitary-gland-and-hypothalamus
  2. Flynn K et al. Central and nephrogenic diabetes insipidus: updates on diagnosis and management. Front Endocrinol 2024. doi.org/10.3389/fendo.2024.1479764
  3. Priya G et al. Diabetes Insipidus: A Pragmatic Approach to Management. Cureus 2021. doi.org/10.7759/cureus.12498
  4. Sumi H et al. Pathophysiology, symptoms, outcomes, and evaluation of hyponatremia: comprehension and best clinical practice. Clin Exp Nephrol 2025. doi.org/10.1007/s10157-025-02624-9
  5. Al Zaman K et al. SIADH in Chronic Respiratory Diseases: A Comprehensive Review. Cureus 2025. doi.org/10.7759/cureus.77407
  6. Capinha M et al. Drug-Induced Hyponatremia. J Clin Med 2025. doi.org/10.3390/jcm14186584
  7. Approach to the Patient With Suspected Hypotonic Polyuria. J Clin Endocrinol Metab 2025 (arginine vasopressin deficiency, formerly central diabetes insipidus). europepmc.org/article/PMC/PMC11747752

Teaching values on screen: the hook, the plate bars and the worked example are illustrative. Thresholds follow the sources above; ranges vary by laboratory and guideline.

The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.

Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (cropped, recoloured, outlined): pituitary gland, nephron, lung cancer, brain, glass of water, bladder, kidney, syringe, woman (adult), urine specimen jar. Pill icon by Marcel Tisch (bioicons.com), CC0. The lab slip, specimen jars, beakers, number lines, principal-cell inset, bar charts, cards and all handwriting are drawn in code (original). No AI-generated teaching images.

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-ENDO-008.

For exam study only, not medical advice. Read the medical disclaimer.

In this series

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  1. EP 01Hypothalamic-pituitary axes and feedback: primary, secondary, tertiary5:05
  2. EP 02Primary vs secondary endocrine disorders: paired hormone levels5:13
  3. EP 03Insulin secretion in the beta cell: from glucose to granule5:06
  4. EP 04Insulin vs glucagon signalling: receptors and effects5:19
  5. EP 05DKA vs HHS pathophysiology: why only one makes acid5:58
  6. EP 06Diabetes drugs by mechanism: where each class acts5:58
  7. EP 07Thyroid hormone synthesis, step by step5:14
  8. EP 08Calcium homeostasis: PTH, vitamin D and calcitonin5:28
  9. EP 09Adrenal steroidogenesis: zones, enzymes and the 17α-hydroxylase fork5:29
  10. EP 10Congenital adrenal hyperplasia: three enzymes, three patterns5:44
  11. EP 11Cushing syndrome: the dexamethasone suppression test and ACTH5:29
  12. EP 12Primary hyperaldosteronism: high aldosterone, low renin5:27
  13. EP 13SIADH vs diabetes insipidus: central vs nephrogenic DI5:44
  14. EP 14MEN1, MEN2A and MEN2B: match the gene to its tumours5:13