Excessive Thirst (Polydipsia): Causes and When to Test Glucose

The first step is to clarify what exactly is going on. Thirst is the need to drink fluids; dry mouth (xerostomia) is reduced saliva production, which can give the impression of thirst although the body is not dehydrated; polyuria is passing an excessive amount of urine, which in practice usually means frequent, copious urination and getting up at night. These three phenomena can occur separately or together and point to different causes.

When thirst is secondary to a large loss of water in urine (diabetes, diabetes insipidus, hypercalcaemia, some medicines), water is 'drained out' of the body and the thirst is a response to its loss. When, on the other hand, one drinks excessively in the first place (primary polydipsia), there is a lot of urine because a lot of fluid is drunk. In both situations the symptoms look similar and telling them apart requires tests, which is why a conversation about the symptoms with a doctor and a diary of drinking and urination can be useful.

When the blood glucose level is high, it exceeds the kidneys' capacity to reabsorb it and glucose passes into the urine, drawing water with it (osmotic diuresis). The body loses fluid and thirst builds. That is why thirst, frequent urination, tiredness and weight loss are the classic symptoms of hyperglycaemia.

An important conclusion: even in newly diagnosed type 2 diabetes, thirst and polyuria occurred in only about half of patients and some had atypical symptoms, so the absence of thirst does not exclude diabetes and its presence does not settle it. This is a small case series from one centre, so it cannot be used to estimate frequency in the population. Type 1 diabetes usually develops faster and with clearer symptoms, including rapid weight loss, and needs urgent assessment. We discuss the diagnostic criteria for glucose and HbA1c in separate articles, and for suspected prediabetes the article on how to reverse it is helpful.

Diabetes insipidus is unrelated to diabetes mellitus: the name is similar, the mechanism different. It involves a disturbance of the action of vasopressin (antidiuretic hormone), which normally makes the kidneys concentrate urine. When it is lacking or the kidneys do not respond to it, large amounts of dilute urine are lost and the thirst is enormous. To avoid confusion with diabetes mellitus, the names were changed in 2022: central diabetes insipidus is now arginine vasopressin (AVP) deficiency, and nephrogenic diabetes insipidus is AVP resistance.

For the reader the conclusion is practical. Distinguishing the three causes of the polyuria-polydipsia syndrome requires specialised tests carried out in a clinical setting (the hypertonic saline test, copeptin measurement), which are not done on your own or 'as a trial' without supervision. In particular, do not restrict drinking on your own or use medicines that affect water balance, because in true diabetes insipidus this can quickly lead to dehydration, and in primary polydipsia to dangerous sodium disturbances.

In primary polydipsia it is the excessive drinking that is primary and does not result from water loss: a high fluid intake physiologically lowers vasopressin secretion, which mirrors diabetes insipidus. Primary polydipsia covers several syndromes of differing background, including psychogenic polydipsia (often in people with mental illness), habitual polydipsia and polydipsia linked to felt thirst, and its basis is usually not homeostatic regulation but non-homeostatic influences.

The most dangerous complication is hyponatraemia from excess water. A systematic review of 590 patients with hyponatraemia associated with excessive water intake (177 papers, 88.7% case reports) showed a severe picture: mean sodium at presentation was about 118 mmol/L, features were severe (seizures, coma) in 53% of patients, and the motivation for drinking was most often psychogenic polydipsia (55%), but also iatrogenic factors (13%), exercise (12%) and habitual polydipsia (7%). The authors stress the low quality and high risk of bias of the studies, and intake must exceed population recommendations substantially. Ordinary 'drinking plenty of water' for health rarely leads to such states, but it is worth knowing about them. More about low sodium in the article on hyponatraemia.

Medicines are a common cause. The best-described example is lithium, used in bipolar disorder, which can cause nephrogenic diabetes insipidus (renal resistance to vasopressin). In a prospective observational study of 217 people treated with lithium, 21% had polyuria (over 3 litres a day), and a higher daily lithium dose was linked to a higher vasopressin level and a larger urine volume, independently of other factors.

The practical significance: a person taking lithium who develops marked thirst and polyuria should discuss it with the prescribing doctor and should not change the dose or stop the drug on their own; sudden withdrawal of lithium carries its own risks. Limiting salt and protein in the diet might have helped reduce polyuria in this study, but decisions about management belong to the doctor. Besides lithium, dry mouth and thirst are affected, among others, by anticholinergic medicines, and increased urine output by diuretics; we do not base these statements in this text on studies from our list, so we treat them as a pointer for a medicine review, not as evidence.

Other causes mentioned include hypercalcaemia, discussed below, chronic kidney disease (the kidneys concentrate urine less well), poorly controlled diabetes after changes of treatment, and simply a large intake of salty foods, alcohol or caffeine.

A raised blood calcium level can make it harder for the kidneys to concentrate urine, leading to polyuria and thirst, alongside constipation, weakness, tiredness and nausea. The most common cause of hypercalcaemia in the outpatient setting is primary hyperparathyroidism, resulting from an autonomous excess of parathyroid hormone; according to an educational resource it is the leading cause of hypercalcaemia outside hospital, most often in postmenopausal women. The course is now more often asymptomatic, and a raised calcium is often found by chance in screening tests.

For this reason calcium is one of the basic tests for unexplained thirst and polyuria. We describe the causes, symptoms and further work-up of high calcium in the article on hypercalcaemia and in the entry on total and ionised calcium in the blood.

Thirst that begins suddenly, grows and has no obvious cause deserves a few simple tests, not patient waiting. Start with glucose and HbA1c, then check sodium, calcium and the kidneys; that explains the great majority of cases. If these results are normal and the symptom persists, an endocrinologist can help, because telling diabetes insipidus from primary polydipsia requires specialised tests. Until then, avoid both playing it down and restricting fluids on your own or 'hydrating in advance' in amounts that may disturb sodium balance.