The Posterior Pituitary: ADH, Desmopressin & Diabetes Insipidus
One hormone controls how much water your body keeps: antidiuretic hormone. When it's lacking, you pass litres of dilute urine and are always thirsty — a condition confusingly named 'diabetes insipidus' that has nothing to do with sugar. The story of ADH ties water balance, a simple replacement drug, and a neat naming puzzle into one tidy lesson.
The back part of the pituitary releases just a couple of hormones, and the important one for pharmacology is antidiuretic hormone (ADH, also called vasopressin). Its job is exactly what the name says: 'anti-diuretic' means it stops you making too much urine. When you're dehydrated, ADH is released and tells the kidney to reabsorb water back into the body, so you produce a smaller amount of concentrated urine and hold onto your water. When you're well-hydrated, ADH falls and the kidney lets water go. It's the body's water-conservation hormone — and when it goes wrong, water balance goes wrong in dramatic ways.
Diabetes insipidus — the great water leak
Too little ADH means the kidney can't hold onto water. If ADH is lacking, or the kidney can't respond to it, water conservation fails: the kidney pours out huge volumes of dilute, watery urine, and the person becomes intensely thirsty, drinking constantly to keep up. This is diabetes insipidus. The name causes endless confusion, so it's worth untangling: the word 'diabetes' simply means 'passing large amounts of urine', which is why both this and diabetes mellitus (the sugar disease) share it. But they're completely different — diabetes mellitus is sweet urine full of glucose ('mellitus' = honeyed), while diabetes insipidus is tasteless, dilute, sugar-free urine ('insipidus' = tasteless). Diabetes insipidus comes in two flavours: cranial (central), where the pituitary doesn't make enough ADH, and nephrogenic, where the kidney can't respond to ADH even though it's present. That distinction decides the treatment.
Desmopressin — replacing the hormone
For cranial diabetes insipidus, where the problem is a shortage of ADH, the treatment is simply to replace it with desmopressin — a manufactured, longer-acting version of ADH. It restores the kidney's ability to conserve water, so urine volume falls back to normal and the relentless thirst settles. (Nephrogenic diabetes insipidus, where the kidney can't respond, won't improve with desmopressin and is managed differently, by addressing the cause and using other measures.) Desmopressin has a couple of other neat uses worth knowing: it's used for bed-wetting in children (reducing overnight urine production) and can help in some bleeding disorders, because ADH/vasopressin also boosts certain clotting factors. The one hazard to respect is the mirror image of the disease: give too much desmopressin and the body retains too much water, diluting the blood's sodium and causing hyponatraemia (dangerously low sodium) — so fluid intake needs sensible limits. Replace the missing hormone, but don't overshoot into water overload.
- ADH (vasopressin) makes the kidney reabsorb water — the body's water-conservation hormone.
- Too little ADH → diabetes insipidus: huge volumes of dilute urine + intense thirst.
- 'Diabetes' just means excess urine; insipidus = tasteless/dilute (NOT the sugar disease).
- Cranial DI (ADH lacking) is treated with desmopressin; nephrogenic DI (kidney can't respond) is not.
- Over-treatment causes water retention and low sodium (hyponatraemia) — limit fluids sensibly.
The 'diabetes' naming puzzle is worth getting straight once and for all, because it trips up so many people. Long ago, 'diabetes' just meant a disease of passing large amounts of urine — that's the only thing the two diseases share. Then physicians tasted the urine (yes, really) to tell them apart: one was sweet with sugar, earning the name 'mellitus' (honeyed); the other was tasteless and watery, earning 'insipidus' (bland). So diabetes mellitus is the sugar disease you spent this whole chapter on, and diabetes insipidus is a completely unrelated water-balance disorder from a lack of ADH. Same first name, entirely different families — a reminder that in medicine, an old name can hide a totally different mechanism.
- Confusing diabetes insipidus with diabetes mellitus — different diseases; check the glucose.
- Giving desmopressin for nephrogenic DI — the kidney can't respond to ADH.
- Not limiting fluids on desmopressin — risk of water overload and low sodium.
- Missing intense thirst + huge dilute urine as a sign of diabetes insipidus.
A patient passes large volumes of dilute urine with normal blood glucose and is very thirsty. Cranial diabetes insipidus is treated with…
- ADH (vasopressin) conserves water; too little → diabetes insipidus (dilute urine + thirst).
- Diabetes insipidus is NOT the sugar disease — 'insipidus' means tasteless/dilute urine.
- Cranial DI (ADH lacking) → desmopressin; nephrogenic DI won't respond to it.
- Beware over-treatment → water retention and low sodium; desmopressin also treats bed-wetting.
- Katzung BG. Basic & Clinical Pharmacology — Hypothalamic & Pituitary Hormones (posterior pituitary).
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Vasopressin & desmopressin.
- Guyton & Hall. Textbook of Medical Physiology — Antidiuretic hormone & water balance.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Posterior pituitary hormones.
