Constant thirst

Feeling thirsty after exercise, in hot weather or after a salty meal is normal. This is about something else: thirst that does not settle however much you drink, that gets you up at night to drink and to pass urine, and that lasts for weeks. That kind of thirst almost always has a specific cause, and the first one to exclude — because it is the commonest and the most important — is diabetes.

What counts as abnormal

• you drink more than three litres a day and are still thirsty;
• you wake at night to drink;
• you pass a lot of urine, frequently, including at night;
• your mouth is constantly dry;
• the thirst does not settle after drinking;
• it has appeared relatively recently and persists.

Causes

• diabetes mellitus: the commonest cause. High glucose draws water into the urine and the body asks for it back. It usually comes with tiredness, weight loss and blurred vision;
• diabetes insipidus: the kidney does not concentrate the urine, either through lack of antidiuretic hormone or because the kidney does not respond to it. Several litres of very pale urine are passed;
• dehydration: from heat, exertion, vomiting, diarrhoea or fever;
• medicines: diuretics, lithium, antidepressants and antihistamines with anticholinergic effects, some antipsychotics;
• high blood calcium, often from hyperparathyroidism;
• chronic kidney disease;
• Sjögren's syndrome: dry mouth and eyes, with joint pain;
• mouth breathing, snoring and sleep apnoea: these give a dry mouth rather than true thirst;
• anaemia;
• hyperthyroidism;
• alcohol and caffeine in quantity;
• pregnancy;
• psychogenic polydipsia: drinking excessively out of habit or anxiety, which in turn perpetuates the sensation.

When to see a doctor

• intense thirst has lasted more than a week or two;
• you are passing far more urine than usual, particularly at night;
• you have lost weight without trying;
• you are very tired or your vision has changed;
• there are repeated skin, gum or genital thrush infections;
• your mouth and eyes are dry and your joints ache;
• the thirst started after beginning a medicine;
• there is nausea, constipation and confusion — calcium may be high;
• a child has started drinking and passing urine excessively, or has started wetting the bed again after being dry: this should be seen promptly.

Urgent care: intense thirst with vomiting, abdominal pain, deep rapid breathing, fruity-smelling breath, drowsiness or confusion. This may be diabetic ketoacidosis.

How it is investigated

The initial work-up is simple and follows an order:

• fasting glucose and HbA1c: always first;
• urine testing, including glucose, specific gravity and osmolality;
• sodium, potassium, calcium and creatinine;
• thyroid function;
• full blood count;
• if diabetes mellitus is excluded and the polyuria persists, a water deprivation test to investigate diabetes insipidus, always in hospital;
• investigation for Sjögren's syndrome if the eyes and mouth are dry;
• a complete medication review.

What to bring to the appointment: a three-day record of how much you drink and how much urine you pass in 24 hours. That single figure is more informative than any description.

What to do in the meantime

• drink water according to thirst, without forcing or restricting;
• cut sugary drinks, juice, alcohol and caffeine;
• reduce salt;
• humidify the bedroom if your mouth dries at night;
• look after oral hygiene: a dry mouth encourages decay;
• sugar-free sweets or artificial saliva for dryness;
• treat nasal congestion if you breathe through your mouth;
• note the accompanying symptoms rather than putting everything down to the weather.

Common questions

How much water should I drink? There is no universal figure. Thirst is a good guide in a healthy person; what is abnormal is thirst that does not settle.

Are dry mouth and thirst the same? No. A dry mouth can come from mouth breathing or medication without any true thirst.

Could it just be stress? It may contribute, but the medical causes need excluding first.

Is it always diabetes? No, but that is what gets checked first, because it is common and treatable.

Is drinking a lot of water harmful? In excess and continuously it can dilute the blood sodium, which does have consequences.

Online consultation

In an online consultation the doctor works out how much you are really drinking and passing, excludes diabetes first, reviews the medicines that cause thirst, arranges the necessary tests and explains what to do if the initial work-up comes back normal.

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