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Diabetes insipidus (AVP deficiency or resistance)
Large volumes (more than 3 litres a day) of dilute urine (below 300 mOsm/kg) call for separating three possibilities — primary polydipsia, central AVP deficiency and renal AVP resistance — with a water deprivation test followed by desmopressin, or by measuring copeptin.
Why it matters in the exam
The question presents a patient with thirst and polyuria, sometimes after pituitary surgery or head injury, or a patient taking lithium. The first step is to confirm that the urine is dilute and to exclude high glucose. The sodium level gives a clue: a low-normal sodium points to primary polydipsia, and a high-normal or high sodium to diabetes insipidus. On water deprivation, a patient with primary polydipsia concentrates the urine unaided. If not — desmopressin is given: a rise of more than 50% in urine osmolality = central AVP deficiency; a rise of less than 10% = renal resistance. Copeptin measured after hypertonic saline stimulation is a more accurate alternative.
Causes: central — pituitary surgery or injury, tumours, infiltrative disease and idiopathic; renal — lithium, hypercalcaemia, hypokalaemia and congenital. After pituitary surgery a "triphasic" response may occur: polyuria, then a phase of SIADH, then polyuria again. Treatment: central — desmopressin, with sodium monitoring because overdosing causes hyponatraemia; renal — stop lithium if possible, a thiazide with a low-salt diet, and amiloride for lithium-induced disease. A patient who cannot drink (reduced consciousness) risks severe hypernatraemia.
Key points
- Urine above 3 litres a day and dilute, below 300 mOsm/kg.
- Low-normal sodium → primary polydipsia; high-normal → diabetes insipidus.
- Desmopressin: rise above 50% central; below 10% renal.
- Central: desmopressin; renal: thiazide, low-salt diet, amiloride for lithium.
- After pituitary surgery — a triphasic response.
Summary table
| State | Sodium | Deprivation / desmopressin | Treatment |
|---|---|---|---|
| Primary polydipsia | Low-normal | Concentrates urine with deprivation | Reduce drinking; look for a psychiatric cause |
| Central AVP deficiency | High-normal or high | No concentration on deprivation; rise above 50% after desmopressin | Desmopressin, monitor sodium |
| Renal AVP resistance | High-normal or high | Rise below 10% after desmopressin | Stop lithium, thiazide and low-salt diet, amiloride |
| Central causes | Pituitary surgery or injury, tumour | MRI | Triphasic response after surgery |
| Renal causes | Lithium, hypercalcaemia, hypokalaemia | Calcium, potassium | Correct electrolytes |
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Sources
- Endocrine Society / Pituitary Society position: Changing the name of diabetes insipidus, 2022 — AVP deficiency and AVP resistance
- Harrison's Principles of Internal Medicine, 21st edition (2022) — Disorders of the Neurohypophysis
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.