Thirst or Dry Mouth
The complaint of thirst rings alarm bells in clinicians and patient alike. Diabetes clearly needs to be excluded but the differential may need to be extended beyond this in the light of negative initial tests. Dry mouth tends to create less concern but can sometimes herald significant pathology and may be a serious nuisance to the patient.
Differential diagnosis
Common Diagnoses
- Diabetes Mellitus
- Dehydration
- Medication (e.g. Tricyclic Antidepressants, Antihistamines)
- Mouth Breathing (Usually Through Nasal Blockage)
- Anxiety
Occasional Diagnoses
- Normality (Children sometimes Presented because ‘they are Always Thirsty’)
- Smoking
- Excess Alcohol
- Sjögren’s Syndrome
- Hypercalcaemia
- Chronic Kidney Disease (CKD)
Rare Diagnoses
- Diabetes Insipidus
- Pregnancy (Common Condition but Rarely Causes Significant Thirst)
- Compulsive Water Drinking
- Sickle-Cell Disease
- Previous Head/Neck Irradiation
Ready reckoner
Key distinguishing features of the most common diagnoses
| Diabetes Mellitus | Dehydration | Medication | Mouth Breathing | Anxiety | |
|---|---|---|---|---|---|
| Polyuria/Frequency | Yes | No | No | No | Possible |
| Generally Unwell | Possible | Yes | Possible | No | Possible |
| Difficulty Breathing Through Nose | No | No | No | Yes | No |
| Symptom Intermittent | No | No | Possible | Possible | Possible |
| Clinically Dehydrated | Possible | Yes | No | No | No |
Possible investigations
Likely: Urinalysis, fasting blood glucose or HbA1c.
Possible: FBC, ESR/CRP, U&E, calcium, RA factor/anti-CCP and other autoantibody screen.
Small Print: Serum and urine osmolality, sickle-cell screen.
- Urinalysis: Glycosuria in diabetes, specific gravity raised in dehydration and reduced in diabetes insipidus and compulsive water drinking, may be proteinuria and/or microscopic haematuria in CKD.
- Fasting blood glucose or HbA1c: To definitively diagnose diabetes.
- FBC/ESR: Hb may be reduced and ESR elevated in Sjögren’s linked to connective tissue disorder; Hb may also be reduced in CKD.
- U&E: May suggest dehydration or CKD.
- Calcium: Elevated in hypercalcaemia.
- RA factor/anti-CCP and other autoantibodies: Sjögren’s may be linked to rheumatoid arthritis, SLE or other connective tissue disease.
- Serum and urine osmolality: Serum osmolality raised and urine osmolality low in diabetes insipidus; in compulsive water drinking, serum osmolality low.
- Sickle-cell screen: To detect sickle-cell anaemia.
Top Tips
- The assessment of thirst does not stop at the exclusion of diabetes mellitus – consider other causes.
- Intermittent dry mouth in an anxious individual also reporting episodic perioral paraesthesiae is likely to be caused by anxiety – perhaps aggravated by certain medications the patient might be taking for the problem.
- Do not underestimate the complaint of dry mouth, especially in the elderly – it can cause significant distress.
- Remember hypercalcaemia, particularly in palliative care patients – this is a potentially remediable cause of troublesome thirst.
- Children who are ‘always thirsty’, have always been like this and are otherwise well will not have diabetes – though the parents may feel short-changed if this isn’t tested for.
Red Flags
- In the acute presentation of thirst, it is essential to exclude diabetes immediately – a very high glucose level with ketonuria will require admission.
- Beware the thirsty elderly patient with an acute illness, particularly if the patient is on ACE inhibitors – he or she may be significantly dehydrated and developing renal failure.
- Dry eyes and joint swellings in association with a dry mouth may indicate Sjögren’s.
- Beware that the elderly with diabetes may complain of dry mouth rather than thirst.