Thirst or Dry Mouth
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 been like that for as long as the parents can remember 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.