What is polydipsia?
Polydipsia is the medical term for abnormal, persistent thirst despite adequate fluid intake. It often accompanies polyuria (excessive urination), as the body loses more fluid than usual. The most important cause to exclude is diabetes mellitus, where high blood sugar draws water out of cells and into urine, causing constant thirst and frequent urination. However, other conditions including diabetes insipidus, high calcium, kidney disease and certain medications can also cause polydipsia.
Blood tests for excessive thirst
| Test | Why ordered | Abnormal result suggests |
|---|---|---|
| Fasting blood glucose | Screen for diabetes | ≥126 mg/dL fasting = diabetes |
| HbA1c | Average blood sugar (3 months) | ≥6.5% = diabetes; 5.7–6.4% = pre-diabetes |
| Serum calcium | Hypercalcaemia causes thirst and polyuria | >10.5 mg/dL = hypercalcaemia |
| Sodium | Hypernatraemia or diabetes insipidus | >145 mEq/L = hypernatraemia |
| Serum osmolality | Concentration of blood | High = concentrated blood / fluid deficit |
| Urine osmolality | Kidney concentrating ability | Low urine osmolality + high serum osmolality = diabetes insipidus |
| Potassium | Hypokalaemia causes polyuria and thirst | <3.5 mEq/L |
| Creatinine / eGFR | Kidney disease | Elevated creatinine = impaired kidneys |
Causes of excessive thirst
Diabetes mellitus (Type 1 and Type 2)
High blood glucose draws water out of cells by osmosis, into the urine (osmotic diuresis), causing dehydration and intense thirst. This is called polydipsia-polyuria. Type 1 diabetes can present acutely with thirst, frequent urination, weight loss and lethargy. Type 2 develops more gradually. A simple fasting glucose or HbA1c diagnoses diabetes.
Diabetes insipidus
Not related to sugar diabetes. Caused by deficiency of ADH (antidiuretic hormone) from the pituitary (central DI) or kidney resistance to ADH (nephrogenic DI). Patients produce enormous volumes of dilute urine (5–20 litres per day) and drink compulsively to compensate. Serum sodium and osmolality are high; urine osmolality is paradoxically low. A water deprivation test with desmopressin distinguishes the types.
Hypercalcaemia (high blood calcium)
High calcium interferes with kidney concentrating ability, causing polyuria and compensatory polydipsia. Associated with kidney stones, constipation, nausea and confusion (bones, groans, stones and moans). Serum calcium is elevated. Investigate with PTH.
Psychogenic polydipsia
Compulsive water drinking without a physiological driver — more common in people with psychiatric conditions, particularly schizophrenia. Can cause dangerously low sodium (dilutional hyponatraemia). Blood glucose and calcium are normal. Serum sodium may be low.
Medications
Lithium (causes nephrogenic diabetes insipidus), antipsychotics (dry mouth mimics thirst), diuretics (fluid loss stimulates thirst), decongestants and some antidepressants.
Questions to ask your doctor
- Do I have diabetes?
- What is my HbA1c?
- Should I have a calcium level checked?
- Could I have diabetes insipidus?
- Is a medication causing my thirst?