What should be checked before using Thiazide (thiazide diuretics)?

What to Check Before Using Thiazide Diuretics

Before initiating thiazide diuretic therapy, electrolyte levels (especially potassium, sodium, and calcium) and renal function (eGFR) must be checked, with follow-up testing within 4 weeks of starting treatment or after dose escalation. 1

Pre-Initiation Assessment

Laboratory Tests

• Baseline electrolytes:
  • Potassium (risk of hypokalemia)
  • Sodium (risk of hyponatremia)
  • Calcium (thiazides decrease calcium excretion)
  • Magnesium (risk of hypomagnesemia)
• Renal function:
  • eGFR (glomerular filtration rate)
  • Serum creatinine
• Other important parameters:
  • Uric acid (risk of hyperuricemia/gout)
  • Glucose (risk of glucose intolerance)

Clinical Assessment

• Blood pressure measurement - establish baseline for monitoring efficacy
• Volume status evaluation - avoid excessive diuresis before starting treatment 1
• Weight measurement - baseline for monitoring fluid status changes

Patient-Specific Considerations

Renal Function

• For patients with eGFR <30 mL/min/1.73 m², consider:
  • Chlorthalidone may be more effective than hydrochlorothiazide in advanced CKD 1
  • Thiazides should not automatically be discontinued when eGFR decreases below 30 mL/min/1.73 m² 1

Medication Review

Check for concurrent medications that may interact with thiazides:

• ACE inhibitors/ARBs - increased risk of renal dysfunction
• NSAIDs - may reduce diuretic effectiveness 2
• Corticosteroids/ACTH - increased risk of hypokalemia 2
• Lithium - increased risk of lithium toxicity 2
• Digitalis/digoxin - hypokalemia may increase digitalis toxicity 2

Risk Factors for Adverse Effects

• Age >65 years - increased risk of hyponatremia 2
• Female gender - higher risk of hyponatremia 3
• Low body mass - increased risk of adverse effects 3
• History of gout or hyperuricemia - thiazides increase uric acid levels 1
• Diabetes or impaired glucose tolerance - thiazides may worsen glycemic control 4

Dosing Considerations

• Start with low doses, especially in elderly patients 2
• For hypertension, lower doses (hydrochlorothiazide ≤25 mg daily) have fewer adverse glycemic effects 4
• Chlorthalidone is likely superior to hydrochlorothiazide, particularly in advanced CKD 1

Follow-Up Monitoring Plan

• Check electrolytes and renal function within 4 weeks of initiation 1
• Monitor for symptoms of electrolyte disturbances:
  • Hypokalemia: muscle weakness, cramps
  • Hyponatremia: confusion, lethargy, seizures
  • Hypercalcemia: constipation, fatigue, cognitive changes
• Instruct patients to monitor weight and report significant changes
• Advise patients to hold or reduce medication during decreased oral intake, vomiting, or diarrhea 1

Common Pitfalls to Avoid

1. Failure to check baseline electrolytes - can miss pre-existing abnormalities
2. Not monitoring potassium levels - hypokalemia increases risk of arrhythmias and glucose intolerance
3. Overlooking drug interactions - especially with ACE inhibitors, NSAIDs, and lithium
4. Excessive dosing in elderly patients - increased risk of adverse effects
5. Ignoring early signs of hyponatremia - can progress to severe neurological symptoms
6. Automatically excluding patients with eGFR <30 mL/min/1.73 m² - thiazides may still be effective, especially chlorthalidone 1

Thiazide diuretics remain valuable antihypertensive agents with proven cardiovascular benefits, but proper pre-treatment assessment and monitoring are essential to minimize risks and optimize outcomes.