Diabetes insipidus

Diabetes insipidus is an uncommon condition in which the kidneys stop holding on to water. A person loses far more fluid in the urine than they should and drinks constantly trying to make up the loss. The word «diabetes» only means «to pass through», and it stuck to this illness because of an outward resemblance: thirst and heavy urine output also occur in diabetes mellitus, but the cause here is quite different. Insulin and blood sugar have nothing to do with it; what fails is the hormone that tells the kidneys to conserve water. Because of the constant confusion, the condition has in recent years been renamed: vasopressin deficiency and vasopressin resistance. If you come across those terms in a report, they refer to the same thing.

What it feels like

The picture is made of two things, and both are hard to miss:

• there is a great deal of urine, pale, almost like water, and the urge comes round the clock, night included; in severe forms someone goes to the lavatory every twenty minutes and can lose fifteen to twenty litres in a day;
• the thirst does not let go. This is not «I fancy a drink» but a dryness that one glass or two will not settle; many people keep a bottle by the bed and wake up to drink.

Then comes everything that follows from such a rhythm: broken sleep, tiredness, irritability, trouble concentrating. Work and study suffer, less from the illness itself than from the fact that the person can neither sleep properly nor move away from a lavatory.

In children it looks different. A baby who cannot yet talk cannot complain of thirst, so parents notice other things: crying with no obvious cause that settles when a drink is offered, nappies of extraordinary weight, poor weight gain and slow growth, rises in temperature without illness, dehydration with the mildest infection. In an older child the first sign is often bedwetting returning in someone who had long stopped, along with loss of appetite and fatigue.

How it differs from other causes of passing a lot of urine

Thirst and frequent urination are a common complaint, and diabetes insipidus is nowhere near the top of that list. Behind it there are usually simpler things: a great deal of tea and coffee, diuretic medicines, poorly controlled diabetes mellitus. There is also a condition of its own — drinking enormous amounts of water out of habit or anxiety rather than from any shortage of fluid; the kidneys are healthy and simply pass on what is surplus.

The first thing ruled out is diabetes mellitus, and that is not a formality. If a child or teenager has suddenly started drinking and passing a lot of urine, is losing weight, is exhausted, is breathing deeply and quickly, and their breath smells of acetone or of overripe apples, this is almost certainly newly presenting type 1 diabetes with the threat of ketoacidosis. It develops over days and needs help immediately, not an appointment with an endocrinologist.

A yardstick for adults: most people pass urine between four and seven times in twenty-four hours. If the trips have clearly increased, if you get up more than once at night, or if a child passes urine more than ten times a day, it is worth looking into.

The hormone that fails here

Holding on to water is the job of vasopressin, also called antidiuretic hormone. It is made by nerve cells in the hypothalamus; from there it travels down to the pituitary gland, which sits at the base of the skull behind the bridge of the nose, and is stored until needed. When the body runs short of water, the pituitary releases vasopressin into the blood, and it instructs the kidneys to send water back: the urine turns dark and concentrated and its volume falls.

That chain can break in two places, and everything about treatment depends on which:

• too little hormone is produced — vasopressin deficiency, previously called cranial or central diabetes insipidus, and the commoner of the two;
• there is enough hormone but the kidneys do not hear it — vasopressin resistance, formerly nephrogenic diabetes insipidus.

The condition is rare: roughly one person in twenty-five thousand. It can begin at any age, though it more often shows itself in adults. A separate form arises in pregnancy, when the placenta breaks down the mother's vasopressin faster than usual; it generally resolves after the birth.

Why it fails

Where the hormone is lacking, the area that makes or stores it has been damaged. The commonest circumstances are surgery on or near the pituitary, a severe head injury, and a tumour pressing on the hypothalamus or the pituitary. Less often the cause is inflammation of that area in systemic diseases, meningitis and encephalitis, secondary tumours, oxygen starvation of the brain, or a rare inherited disorder in which diabetes insipidus is combined with diabetes mellitus and loss of vision. In about a third of cases no visible cause is found; the belief is that the person's own immune system damages the hormone-producing cells.

This is the part worth reading to the end for: behind newly appearing thirst and heavy urine output there is sometimes a tumour at the base of an adult's brain, and in a child a rare blood disorder affecting the pituitary. That is why vasopressin deficiency with no obvious cause is never left uninvestigated, and why an MRI scan is not ordered merely to be on the safe side.

Where the kidneys fail to respond to the hormone, the causes are different. The commonest acquired one is lithium, used in bipolar disorder: taken over long periods it damages the cells of the kidney tubules. Stopping it often restores sensitivity, but not always, which is why kidney function and blood calcium and sodium are checked regularly during treatment. Stopping lithium on your own initiative is not acceptable under any circumstances — that decision belongs to the doctor. Other causes are excess calcium, persistent lack of potassium, a past kidney infection, a prolonged obstruction to the flow of urine such as a stone, and inherited forms. One of those is passed on through the X chromosome, so it mostly affects boys, while women can be carriers with no symptoms.

How it is worked out

The diagnosis is made by an endocrinologist, and nothing here is taken on trust: the choice of treatment depends on telling the two forms apart, and the treatment for one does not work for the other.

• Blood and urine tests: glucose, sodium, calcium, potassium, kidney function and how dilute the urine is. In diabetes insipidus it stays watery where a healthy body would concentrate it.
• The water deprivation test is the main investigation. Drinking is withheld for several hours and the urine is watched to see whether it starts to concentrate. It is carried out only in hospital, with weighing and sodium monitoring: repeating it at home is dangerous.
• Desmopressin, the artificial version of the hormone, is then given. If the volume of urine drops and it becomes concentrated, hormone was lacking — that is deficiency. If nothing changes, the kidneys are deaf to the hormone — that is resistance.
• A blood test for copeptin is increasingly used: it is released alongside vasopressin, reflects its production more accurately and allows the test to be simplified.
• Once deficiency is confirmed, an MRI scan of the brain is done to look at the hypothalamus and the pituitary. If a tumour is found, that will need treating too.

How it is treated

The aim is straightforward: to reduce water loss to a volume compatible with ordinary life and to prevent dehydration. Where the hormone is lacking, it is replaced with desmopressin. This is more stable than natural vasopressin and lasts longer; it comes as tablets, in a form that melts under the tongue, and as a nasal spray. The preparation and the schedule are worked out individually, so there are no doses on this page and there should not be: that is the doctor's business. Mild cases sometimes need no medicine at all — it is enough to drink as much as the body asks for and keep water within reach.

Desmopressin has a flip side. If more is taken than needed, or a lot of fluid is drunk alongside it, water is retained and the blood sodium falls. Warning signs are a headache that keeps building, nausea and vomiting, confusion, unusual drowsiness and fits. If those appear, the medicine is stopped and the doctor contacted at once; with fits or altered consciousness call an ambulance — in Europe the single number is 112. Doctors usually advise letting the effect wear off completely once a day, so that surplus water has time to leave.

Where the kidneys do not respond to the hormone, desmopressin is useless. Here the work goes into the cause: replacing, where possible, the medicine that brought it on, and correcting calcium and potassium. Cutting salt and protein in the diet helps, but that change is made with a doctor. In more severe cases thiazide diuretics are prescribed, which sounds paradoxical but genuinely reduces the volume of urine; sometimes a non-steroidal anti-inflammatory drug is added, and with it a medicine to protect the stomach.

What should put you on alert

With treatment the condition does not stand in the way of an ordinary life. The danger arises when a person, for whatever reason, cannot drink while the water keeps going.

Seek help immediately if there is:

• confusion, marked drowsiness, slurred speech;
• vomiting that makes it impossible to keep any fluid down;
• sunken eyes, a dry tongue, no urine for many hours on end;
• a fit;
• in a child, listlessness, refusing drinks, no tears and dry nappies.

A few rules that take away most of the risk:

• water always within reach — in the car, at work, by the bed, on a journey;
• carry a card or a bracelet with the diagnosis and the name of the medicine; if you are admitted unconscious it may be decisive;
• before any operation, endoscopy or test requiring fasting, tell the doctor: stretches without drinking need a plan of their own, not a pause in treatment;
• with vomiting, diarrhoea or hot weather, contact the doctor before you start feeling unwell;
• do not skip the sodium checks, particularly after a change of dose.

Online consultation

The diagnosis rests on blood tests and a hospital investigation, so it will not be made over a video call. But an online appointment saves several steps. The doctor will go through your account and say what to check first: behind thirst and heavy urine output there are far more ordinary things, and that is where to start. They will suggest which tests are worth having before the face-to-face visit and explain the ones you already have. Online is also a good place to manage life with the diagnosis in place: how you feel after a dose change, what to do in hot weather, on holiday and during an illness, how to prepare for an operation, questions about pregnancy. And, most importantly, to work out in time when the answer is not a conversation but an ambulance. If the picture calls for urgent action, the doctor will say so plainly, and that is the right outcome of the consultation.

This material is for information only and does not replace medical advice.