What labs should be ordered for a patient with polydipsia (increased thirst)?

Laboratory Evaluation for Polydipsia (Increased Thirst)

For patients presenting with polydipsia (increased thirst), initial laboratory evaluation should include complete blood count, urinalysis, serum electrolytes (including calcium and magnesium), blood urea nitrogen, serum creatinine, fasting blood glucose (glycohemoglobin), lipid profile, liver function tests, and thyroid-stimulating hormone. 1

Initial Laboratory Tests

The following tests should be ordered for all patients with polydipsia:

• Complete blood count to assess for infection or other hematologic abnormalities 1
• Urinalysis to evaluate urine concentration, presence of glucose, protein, or signs of infection 1
• Serum electrolytes, including sodium, potassium, calcium, and magnesium to detect electrolyte disturbances 1
• Blood urea nitrogen and serum creatinine to assess renal function 1
• Fasting blood glucose and glycohemoglobin (HbA1c) to screen for diabetes mellitus 1
• Liver function tests to evaluate hepatic function 1
• Thyroid-stimulating hormone to rule out thyroid disorders 1

Specific Tests Based on Clinical Suspicion

For Suspected Diabetes Mellitus

• Fasting plasma glucose ≥126 mg/dl (7.0 mmol/l) or casual plasma glucose ≥200 mg/dl (11.1 mmol/l) with symptoms of diabetes (polyuria, polydipsia, unexplained weight loss) is diagnostic for diabetes 1
• If initial results are equivocal, consider oral glucose tolerance test (OGTT) 1

For Suspected Diabetes Insipidus

• Serum and urine osmolality to assess the kidney's concentrating ability 2, 3
• Water deprivation test to differentiate between central diabetes insipidus, nephrogenic diabetes insipidus, and primary polydipsia 2
• Consider measurement of copeptin (a surrogate marker for arginine vasopressin) 2

For Suspected Psychiatric Causes

• Serum sodium levels to evaluate for hyponatremia, which can occur in psychogenic polydipsia 4, 5
• Monitor fluid intake and output patterns 4

For Suspected Endocrine Disorders

• For suspected hypercalcemia: serum calcium, phosphorus, and parathyroid hormone 1
• For suspected Cushing's syndrome: consider specific tests for hypercortisolism 3
• For suspected acromegaly or other hormone excess states: specific hormone testing based on clinical suspicion 1

For Suspected Heart Failure

• B-type natriuretic peptide (BNP) or NT-proBNP levels if heart failure is suspected 1
• Electrocardiogram and chest radiograph 1

Monitoring and Follow-up Testing

• For patients with confirmed diabetes mellitus, regular monitoring of glycemic control with HbA1c 1
• For patients with psychogenic polydipsia, regular monitoring of serum sodium levels 4, 5
• For patients with diabetes insipidus, monitoring of fluid balance and electrolytes 2

Potential Pitfalls and Caveats

• Low urine specific gravity may be the only abnormality in primary disorders of water balance (central diabetes insipidus, nephrogenic diabetes insipidus, and psychogenic polydipsia) 3
• Hyponatremia in patients with polydipsia can progress to water intoxication with symptoms including confusion, lethargy, psychosis, and seizures 4, 5
• Excessive water intake in health-conscious individuals (dipsogenic diabetes insipidus) is an emerging cause of polydipsia that should be considered in non-psychiatric patients 6, 2
• Medications can affect water balance and contribute to polydipsia, particularly psychiatric medications 4, 5

By systematically evaluating patients with polydipsia using these laboratory tests, clinicians can identify the underlying cause and implement appropriate treatment to improve patient outcomes.