Excessive thirst: causes, red flags and when to seek help

Thirst is the brain responding to a small rise in the concentration of the blood or a fall in its volume. After sweating, a salty meal, alcohol or a short illness with vomiting or diarrhoea it is entirely expected, and it eases once you drink. Excessive thirst is thirst that is out of proportion to your circumstances, keeps returning soon after drinking, or wakes you at night.

It almost always goes together with passing large volumes of urine. The question for the doctor is which came first: does the body lose too much fluid and make you thirsty, or do you drink too much and therefore pass too much urine?

Dehydration from hot weather, illness or inadequate fluid intake is the commonest and is usually obvious. Among longer-lasting causes, diabetes mellitus tops the list.

• Type 2 diabetes: gradual thirst, passing more urine, tiredness, weight loss, blurred vision, slow healing and recurrent thrush, according to the NHS symptom list.

• Type 1 diabetes: similar symptoms that tend to come on quickly in children and young adults.

• Medicines: water tablets (diuretics), lithium and some antipsychotics or steroids can raise thirst or urine volume.

• High calcium in the blood, or low potassium, which can disturb the kidneys concentrating urine.

• Diabetes insipidus, a rare condition of the water-regulating hormone system, not related to diabetes mellitus.

• A dry mouth from mouth breathing, some medicines or Sjogren-type conditions, which feels like thirst without increased urine.

• Primary polydipsia, a habit or psychiatric pattern of drinking very large volumes.

The NHS says the symptoms of type 2 diabetes may develop gradually and not everyone gets them, and that risk is higher if you are white and over 40 or over 25 from a South Asian, Chinese, Black African or Black Caribbean background, have a parent, brother or sister with it, are overweight and not very active, or have a condition such as high blood pressure.

The NHS describes diabetes insipidus as a rare condition where you pee a lot and often feel thirsty, with extreme thirst (polydipsia) and peeing a lot, even at night (polyuria) as the two main symptoms. It is caused by problems with arginine vasopressin (AVP, also called antidiuretic hormone), which helps the kidneys retain water; in most cases the body does not make enough AVP, and in rarer cases the kidneys do not respond to it. The NHS says it affects about 1 in 25,000 people and that some medicines, particularly lithium, can cause the kidney form.

Extreme thirst with passing a lot of urine, plus nausea, vomiting, abdominal pain, fast deep breathing, a fruity smell on the breath, drowsiness or confusion may signal diabetic ketoacidosis in someone with type 1 diabetes or undiagnosed diabetes. This is a medical emergency; go to A&E or call 999. The NHS stomach-ache guidance also lists vomiting in someone with diabetes among reasons to call 999 or go to A&E.

Severe thirst with confusion, fainting, a very dry mouth, a fast pulse and passing almost no urine suggests serious dehydration and needs urgent assessment. New thirst with unexplained weight loss, marked tiredness or blurred vision should prompt a prompt GP appointment and blood glucose test rather than waiting.

The NHS says to see your GP if you pee more than normal, noting that most healthy adults pass urine 4 to 7 times in 24 hours, or if you need to pee small amounts at frequent intervals; for children it says to seek advice if a child pees more than 10 times a day.

You will be asked how long the thirst has lasted, how much you drink and what, how often you pass urine including at night, whether you have lost weight, felt tired, had blurred vision, thrush or slow-healing cuts, what medicines you take, and about family history of diabetes. The examination covers weight, hydration, blood pressure and sometimes a urine dipstick for glucose and ketones, plus a finger-prick glucose.

The NHS says diabetes is diagnosed with a blood test that checks glucose levels, and that tests confirm whether it is type 1 or type 2. Typically that means an HbA1c or a fasting glucose, with kidney function, electrolytes and calcium on the same sample.

If these are normal but the thirst and urine volume are persistent, a specialist may arrange urine concentration tests, a water-deprivation test or measurement of the hormone copeptin, together with a pituitary MRI where diabetes insipidus is suspected. A diary of fluid intake and urine output over a few days is a simple and informative aid.

Drink to thirst with water rather than sugary drinks, which can make high glucose worse. Do not restrict fluids, because that risks dehydration if the cause is diabetes or diabetes insipidus. The NHS advises that mild AVP deficiency may need nothing more than drinking enough water to replace what is lost in urine. Keep a note of weight, symptoms and fluid intake to bring to your appointment.