NU 237 · NursingSection 5.4 · Digestive and endocrine disorders
Section 20 of 25
Posterior pituitary disorders: diabetes insipidus and SIADH
Explain what vasopressin does, then use that one hormone to explain the two opposite disorders of the posterior pituitary: diabetes insipidus, in which too little vasopressin action produces large volumes of dilute urine, and SIADH, in which too much vasopressin dilutes the blood sodium — and state the nursing monitoring each disorder demands.
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Why this matters
Two patients can arrive on the same ward with exactly opposite water problems, and both look ordinary until you read the numbers. In diabetes insipidus the body is losing water it cannot keep, and the danger is dehydration with a sodium that rises; in SIADH the body is holding water it should have lost, and the danger is a sodium that falls and makes the brain swell. One hormone explains both. Because the nurse is the person who measures the urine, weighs the patient and sees the confusion first, this section turns a single hormone into the bedside reasoning that keeps both patients safe.
What you need to know first
You need only the everyday idea that the pituitary is a small gland at the base of the brain and that the kidneys make urine. Vasopressin, the posterior pituitary and the vocabulary of diuresis are all defined below, so no previous endocrinology course is assumed.
Words to know (5)
Vasopressin (antidiuretic hormone)
The hormone made in the hypothalamus and released by the posterior pituitary that tells the kidney to keep water instead of losing it in the urine. When vasopressin is high, the patient passes a small amount of concentrated urine.
Diabetes insipidus
A shortage of vasopressin action, either because too little is released (central) or because the kidney cannot answer it (nephrogenic), so large volumes of dilute urine are passed. The patient passes several litres of pale urine a day and is constantly thirsty.
Polyuria and polydipsia
Passing abnormally large volumes of urine (polyuria) and drinking abnormally large volumes of fluid because of thirst (polydipsia); the two usually appear together. The nurse records six litres of urine in 24 hours and the patient empties the water jug twice.
Nephrogenic
Caused by the kidney itself rather than by the hormone: the kidney receives vasopressin but its tubules cannot respond to it. Lithium treatment damages the kidney’s response and produces a nephrogenic diabetes insipidus.
SIADH
Syndrome of inappropriate antidiuretic hormone secretion: too much vasopressin is present, so the kidney keeps water and the blood sodium falls by dilution. A patient started on an antidepressant becomes confused and has a serum sodium of 122 mmol/L.
One idea at a timeIdea 1 / 5
In one sentence Vasopressin is released from the posterior pituitary and tells the kidney to keep water, so the urine becomes small in volume and concentrated.
One hormone decides how much water the kidney keeps
The posterior lobe of the pituitary does not manufacture its own hormones; it stores and releases hormones that are made in the hypothalamus, the part of the brain just above it. The one that matters here is vasopressin, also called antidiuretic hormone. When the body senses that water is short or that the blood is becoming too concentrated, vasopressin travels in the blood to the kidney and reopens the water channels in the collecting ducts. Water leaves the urine and returns to the blood, so only a small, concentrated volume is passed. When the body holds too much water, vasopressin release falls, those channels close, and a large volume of dilute urine is passed.
This one switch explains both disorders in this section, because the trouble can lie in either direction.
For example
After a long walk in the heat without drinking, the urine is dark and scanty because vasopressin is high. After drinking two litres of water, it becomes pale and abundant because vasopressin has fallen.
Pause and check yourself
What does vasopressin make the kidney do, and what happens when its release falls?
Show the answer
It reopens the water channels in the collecting ducts, so water returns to the blood and a small concentrated urine is passed; when release falls, the channels close and a large volume of dilute urine is lost.
Read detailed explanations
How vasopressin is released, and how the collecting duct answers it
The trigger for vasopressin is not the blood volume alone but the concentration of the plasma, and the sensor is exquisitely sensitive: osmoreceptor cells in the hypothalamus detect a rise in plasma osmolality of only one or two per cent, which is why a dry mouth and a mild water deficit are enough to change the urine within minutes.
The hormone itself is synthesised in the supraoptic and paraventricular nuclei of the hypothalamus and travels down the axons of those nerve cells to the posterior lobe, where it is stored in granules and released into the bloodstream on demand.
At the kidney it binds V2 receptors on the basolateral membrane of the collecting duct, raises cyclic AMP inside the principal cells, and causes vesicles loaded with aquaporin-2 water channels to fuse with the apical membrane. The duct becomes permeable to water, water follows the salty medullary gradient, and urine osmolality can climb to about 1200 mosmol/kg; when the hormone is switched off, the channels are withdrawn and urine can be diluted to roughly 50 mosmol/kg.
Several everyday and clinical influences act on this same point: alcohol and atrial natriuretic peptide suppress release, while pain, nausea, stress and nicotine stimulate it. In the nephrogenic form the fault is downstream — lithium enters the principal cells and disrupts the signalling cascade, while hypercalcaemia and hypokalaemia flatten the medullary gradient — which is why hydrochlorothiazide, a diuretic, can paradoxically reduce urine volume by causing mild volume depletion, so that more sodium and water are reabsorbed in the proximal tubule and less filtrate ever reaches the collecting duct.
Separating the causes: the supervised deprivation test and its traps
Because polyuria has several causes that look alike, the diagnosis is made with a supervised water deprivation test, not with a single urine sample. The patient takes no fluid while the team measures body weight, urine volume, urine osmolality and plasma osmolality at fixed intervals, and the test is stopped if the weight falls by three to five per cent or the plasma osmolality exceeds about 300 mosmol/kg.
A healthy person concentrates the urine steadily; a patient with central diabetes insipidus stays dilute, and the picture changes when desmopressin is given, because the kidney can still respond. A patient with the nephrogenic form remains dilute after desmopressin, and a patient with primary polydipsia — chronic excessive drinking, sometimes called psychogenic polydipsia — also stays dilute during deprivation, because years of over-drinking have washed out the medullary gradient, but then concentrates after desmopressin.
Two traps catch the unwary. First, an osmotic diuresis produced by glycosuria, mannitol, radiocontrast or a high urea load gives a large urine volume with a normal or even high osmolality, so it is not a water-loss disorder at all and must be excluded by checking the glucose and reviewing the drugs. Second, the test is not free of risk: it can dehydrate the patient, so weight and plasma osmolality are watched during it, and a measured copeptin level, a stable fragment of the vasopressin precursor used to estimate how much hormone the pituitary is releasing, or imaging of the pituitary when a tumour is suspected, may be added to shorten the assessment.
Correcting the sodium: why the speed matters more than the number
In SIADH the sodium is low, but the danger depends less on the value than on how fast it fell and what the patient is doing. When the plasma sodium drops quickly, water enters brain cells faster than they can adapt, producing cerebral oedema with headache, vomiting, confusion, convulsions, coma and, at worst, herniation of the brainstem. When it falls slowly over days, the cells have time to extrude solute and the patient may look surprisingly well until the sodium is very low, which is why a chronic value of 120 mmol/L can cause only mild symptoms while an acute fall to the same value causes a seizure.
Treatment therefore follows the symptoms. A patient with severe neurological signs is treated with hypertonic three per cent saline in a monitored setting, with serial sodium measurements and neurological checks, because the correction is titrated rather than guessed. The rate is deliberately limited to roughly 8 to 10 mmol/L in the first 24 hours and no more than about 18 mmol/L in 48 hours; a faster rise risks osmotic demyelination syndrome, a permanent neurological injury with weakness, swallowing difficulty and disturbed consciousness that appears days after an over-rapid correction.
For the stable patient without severe symptoms, the first and safest measure remains fluid restriction of about 800 to 1000 mL a day, supported by daily weight, strict intake and output and repeated electrolytes, with demeclocycline or a vasopressin receptor antagonist such as tolvaptan reserved for selected chronic cases.
The mirror image matters too: a patient with diabetes insipidus who receives an excessive dose of desmopressin can stop excreting water altogether and swing into water intoxication with a falling sodium, so the nurse watches for headache, nausea and weight gain after each dose just as carefully as for polyuria.
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One hormone, two opposite water disorders
The same collecting duct is shown under three levels of vasopressin. When the hormone is present in the right amount, water channels open briefly, water returns to the blood and a small concentrated urine is passed. When it is missing, the duct stays watertight, water is lost and the blood concentrates. When it is in excess, the channels stay open, water is retained and the blood sodium is diluted. Reading the three columns side by side shows why the treatments are opposites.
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01 / 4Normal vasopressin release
If the drawing is wider than this panel, scroll it sideways. Labels stay readable; the numbered explanations below show the same ideas.
Normal vasopressin release
Vasopressin is made in the hypothalamus and released by the posterior pituitary when plasma osmolality rises by only one or two per cent. It binds V2 receptors on the collecting duct, inserts aquaporin-2 water channels into the duct wall, and lets water follow the salty medullary gradient back into the blood. The result is a small volume of concentrated urine, normally one to two litres a day, with the serum sodium held between 135 and 145 mmol/L.
Schematic comparison, not anatomy: the three columns summarise one collecting duct under three levels of vasopressin, and the figures are typical adult ranges rather than the values of any one patient. Urine volume varies widely between individuals and usually falls with age.
Key points to remember
✓The posterior pituitary stores and releases vasopressin, the hormone that tells the kidney to keep water, so urine becomes small and concentrated.
✓Too little vasopressin action gives diabetes insipidus: large volumes of dilute urine, intense thirst, and — when the patient cannot drink — a rising sodium and dehydration.
✓Central and nephrogenic diabetes insipidus look the same but are not: only the central form responds to replacement hormone, because the kidney is normal and simply lacks the signal.
✓SIADH is too much vasopressin: water is retained, the sodium falls by dilution, and headache, nausea, confusion and personality change follow because the brain swells first.
✓Both disorders are followed with intake and output, a daily weight on the same scale and the serum sodium — and their treatments are opposites: fluids and hormone replacement for one, fluid restriction for the other.
Go deeper
Worked example
Six litres of urine after a head injury. A 34-year-old man is in his third day in hospital after a fall with a head injury. The fluid chart shows six litres of urine in 24 hours, all pale and watery, with a specific gravity of 1.002. He complains of intense thirst and drinks whenever he is awake, but he has been drowsy and has missed several drinks. His serum sodium is 152 mmol/L, his blood glucose is normal, and his weight has fallen by two kilograms since admission.
Start with the hormone, not the urine. The posterior pituitary releases vasopressin after it has been made in the hypothalamus, and both structures sit close to the site of a head injury. A fall severe enough to cause a head injury can interrupt that release, so a sudden failure of water conservation is the first hypothesis, and the central form of the disorder is the likely cause.
Read the urine on its own terms. Six litres in a day is more than double the usual adult output, and a specific gravity as low as 1.002 means the urine is almost water. The kidney is excreting water it should have reclaimed rather than filtering poorly. Because the glucose is normal, this is not the osmotic diuresis of diabetes mellitus, in which sugar in the tubule drags water out with it.
Read the sodium and the weight together. A sodium of 152 mmol/L means the blood is concentrated, and a fall of two kilograms confirms that water loss has exceeded intake. The drowsiness is the part that changes the plan: a patient who cannot reliably drink can no longer defend his own sodium, so the situation will worsen on its own.
Confirm which form it is. Desmopressin is given as prescribed and the response is observed: if the urine output falls and the urine concentrates, the kidney was able to answer the hormone and the fault is central. If the output continues unchanged, the fault is in the kidney itself and the diagnosis is the nephrogenic form — which would change the whole treatment plan.
Act and monitor. Fluid is replaced as prescribed, water is kept within reach when he is awake, and the record of intake and output together with a daily weight on the same scale becomes the measure of whether the replacement is keeping up. The nurse reports the sodium and any increase in drowsiness at once, because the patient who stops drinking is the patient who becomes dehydrated.
The case is read in a fixed order: what the hormone should be doing, what the urine shows, what the sodium and the weight show, which form the disorder is, and only then what to do. The single most important bedside observation is not the sodium result but the amount the patient actually drinks, because that is what stands between a large urine volume and dehydration. A patient with this disorder who is awake and drinking is usually safe; the same patient becoming drowsy needs the nurse to notice before the next blood test does.
Common misconception
“Diabetes insipidus” sounds like diabetes mellitus, so many beginners assume the two are related and that the patient must have a sugar problem. They are different diseases driven by different hormones: diabetes mellitus is a problem of insulin and glucose, while diabetes insipidus is a problem of vasopressin and water. In diabetes insipidus the blood glucose is normal and the urine contains no sugar; the urine is abundant only because water is not being reclaimed. The shared word comes from the Greek for “siphon”, and it dates from a time when physicians told the two apart by tasting the urine — a useful reminder that the name describes the urine, not the cause.
Check your reasoning
A patient with SIADH has a serum sodium of 120 mmol/L and is becoming sleepy. Using vasopressin as your starting point, explain why the sodium has fallen, why the brain is affected, and what the nurse must do and must not do.
Reveal an answer
Vasopressin is being released although the body does not need it, so the kidney keeps water that should have left in the urine. The total amount of salt in the body has not changed, but it is now dissolved in more water, so the measured sodium falls: this is a dilution, not a loss. Sodium is what holds water in the space around the cells, so as it falls water moves into the cells; the brain sits in a rigid skull and cannot expand, so headache, nausea, confusion, drowsiness and personality change appear first. The nurse must carry out the prescribed fluid restriction, measure intake and output, weigh the patient daily, keep the mouth comfortable, check the sodium as ordered and report any increase in drowsiness at once. The nurse must not encourage extra drinks to “flush the kidneys”, must not give fluids freely because the patient says he is thirsty, and must not allow a sodium correction to be rushed: raising the sodium too quickly is itself dangerous and can injure the brain permanently.
Where this comes from
Endocrine System Pathologies lecture notes (supplied), Level 200 nursing · “Disorder of the posterior pituitary gland”, lines 222–329: diabetes insipidus defined by polyuria, polydipsia and dilute urine; its central causes (head trauma, brain tumour, surgical ablation or irradiation of the pituitary, meningitis, encephalitis, tuberculosis, metastatic disease) and nephrogenic causes (failure of the renal tubules to respond, hypokalaemia, hypercalcaemia, lithium, demeclocycline); its diagnosis (normal blood glucose, low urine specific gravity, raised BUN, rising sodium and chloride, vasopressin challenge, pituitary or hypothalamic tumour on MRI), treatment (replacement ADH such as desmopressin, hydrochlorothiazide, low-salt diet, increased fluid intake) and nursing care (fluid and electrolyte balance, intake and output, daily weight on the same scale at the same time in similar clothing, daily medication, medical alert jewellery); and SIADH (excess ADH, its causes, hyponatraemic headache, nausea, confusion and personality change, diagnosis by hyponatraemia, treatment by saline and the underlying cause, nursing care by electrolyte monitoring, fluid restriction, daily weight and intake and output).
Corrections and additions to the supplied notes · Corrections labelled as such: the notes say that “patients with diabetes insipidus will note a decrease in output and thirst” after a vasopressin challenge, which is true only of the central form — the kidney cannot answer the hormone in the nephrogenic form, so its urine stays dilute; the notes define diabetes insipidus as a deficiency of ADH secretion and then list renal unresponsiveness among its causes, so the two mechanisms are kept distinct here; “hyrdrochlorothiazide” is corrected to hydrochlorothiazide, and the notes do not say that its use is confined to the nephrogenic form, where its effect is paradoxical; “2 to 1o litter” is read as 2 to 10 litres. Beyond the supplied notes: osmoreceptor threshold (a rise of one to two per cent in plasma osmolality, about 285 mosmol/kg), synthesis in the supraoptic and paraventricular nuclei, aquaporin-2 insertion via V2 receptors and cyclic AMP, urine osmolality reference values (about 50 to 1200 mosmol/kg), the supervised water deprivation test with its stopping rules, copeptin measurement, the cause of primary polydipsia, correction limits for hyponatraemia (about 8 to 10 mmol/L in 24 hours and 18 mmol/L in 48 hours) and the risk of osmotic demyelination, and the use of tolvaptan in selected chronic cases. These are standard, uncontroversial values and mechanisms that the supplied notes do not state.
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About this course and its sources
The urinary and endocrine lessons build on the supplied Level 200 nursing notes (course instructor Bakume Jessey). Disease mechanisms, blood, lymphatic, cardiovascular and digestive lessons use cited external references. Diagrams are original educational schematics. Sections that go beyond the supplied notes say so in their sources. Specialist clinical and French-language editorial review remain pending.