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

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 MellitusDehydrationMedicationMouth BreathingAnxiety
Generally UnwellPossibleYesPossibleNoPossible
Difficulty Breathing Through NoseNoNoNoYesNo
Symptom IntermittentNoNoPossiblePossiblePossible
Clinically DehydratedPossibleYesNoNoNo

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.

Published: 2nd August 2022 Updated: 10th April 2024

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