Hydrochlorothiazide

Hydrochlorothiazide is a thiazide diuretic used for selected fluid-retaining and urinary disorders in veterinary medicine. Its applications include additional diuresis in refractory congestion and reduction of urinary calcium loss in some recurrent calcium oxalate stone formers. Other roles include selected hypertension, nephrogenic diabetes insipidus and postpartum udder edema in cattle.

Group

Thiazide diuretic

Mechanism of Action

Hydrochlorothiazide inhibits the sodium–chloride cotransporter in the distal convoluted tubule, increasing urinary sodium and water excretion. Its distal action can complement loop-diuretic blockade at a different nephron segment. Enhanced calcium reabsorption reduces urinary calcium loss. In nephrogenic diabetes insipidus, mild volume contraction increases proximal fluid reabsorption and can paradoxically reduce the volume delivered to the distal nephron.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Diuresis	2–4 mg/kg	PO	Every 12 h
Hypertension	1 mg/kg	PO	Every 12 h
Nephrogenic diabetes insipidus	2 mg/kg	PO	Every 12 h	Low-sodium diet; evidence includes a case report.
Nephrogenic diabetes insipidus	2.5–5 mg/kg	PO	Every 12 h
Hypertension	1 mg/kg	PO	Every 12–24 h	Second-choice adjunct rather than reliable feline monotherapy.
Reduction of urinary calcium	2 mg/kg	PO	Every 12 h
Reduction of urinary calcium	2.2 mg/kg	PO	Every 12 h
Reduction of urinary calcium	2–4 mg/kg	PO	Every 12 h
Refractory congestive heart failure	2 mg/kg	PO	Every 12 h	When adding to high-dose furosemide, one protocol reduces furosemide by 50%; monitor renal function and electrolytes.
Right-sided-heart-failure ascites	Reference only	PO		2 mg/kg initially every other day with furosemide, spironolactone, ACE inhibition and dietary sodium restriction; adjust electrolytes and renal function.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Diuresis	2–4 mg/kg	PO	Every 12 h
Hypertension	1 mg/kg	PO	Every 12 h
Reduction of urinary calcium	1 mg/kg	PO	Every 12 h
Urine alkalinisation	2 mg/kg	PO	Every 12 h
Congestive heart failure	1–2 mg/kg	PO	Every 12–24 h
Hypertension	1 mg/kg	PO	Every 12–24 h	Second-choice adjunct rather than reliable feline monotherapy.

Cattle

Use / stage	Dose	Route	Schedule	Instructions
Postparturient udder oedema	125–250 mg/animal	IV, IM	Every 12–24 h

Horses

Use / stage	Dose	Route	Schedule	Instructions
Hyperkalaemic periodic paralysis adjunct	0.5–1 mg/kg	PO	Every 12 h	When dietary adjustment does not control episodes.

Indications

Adjunctive diuresis in selected dogs with persistent congestion despite loop-diuretic treatment.
Reduction of hypercalciuria and recurrent calcium oxalate urolith formation in selected dogs; feline evidence is more limited.
Treatment of selected systemic hypertension in veterinary patients.
Reduction of polyuria in selected patients with nephrogenic diabetes insipidus.
Adjunctive treatment of postpartum udder edema in cattle with an indicated product.
Management of selected equine hyperkalemic periodic paralysis cases when a thiazide regimen is appropriate.

Contraindications

Hypercalcaemia
Anuria or sulfonamide/thiazide hypersensitivity; intestinal absorptive hypercalciuria can worsen hypercalcaemia.

Interactions

Other diuretics and antihypertensives have additive effects.
Corticosteroids or amphotericin B increase hypokalaemia. Electrolyte depletion increases digoxin toxicity; lithium concentrations and insulin requirements may rise. Calcium or vitamin D increases hypercalcaemia risk.

Adverse effects

Hypokalaemia and other electrolyte disturbances
Dehydration

Toxicity

Overuse may cause hypovolaemia and electrolyte depletion; reassess renal function and electrolytes.

Preparations

Preparation	Form	Strength	Route
Hydrochlorothiazide 25 mg tablet	Tablet	25 mg / tablet	PO
Hydrochlorothiazide 50 mg tablet	Tablet	50 mg / tablet	PO
Hydrochlorothiazide 100 mg tablet	Tablet	100 mg / tablet	PO
Hydrochlorothiazide 10 mg/mL oral solution	Oral Liquid	10 mg / mL	PO
Hydrochlorothiazide 100 mg/mL oral solution	Oral Liquid	100 mg / mL	PO
Hydrochlorothiazide 25 mg/mL injection Cattle injection.	Injection	25 mg / mL	IV, IM

Monitoring and Administration

Monitor hydration, electrolytes, calcium and renal function.

The oral maintenance drug mentioned for cattle after injectable treatment is chlorothiazide, a different medicine. Do not substitute its bolus dose here.

Check urine 2-4 weeks after starting calcium-oxalate prevention and electrolytes within several weeks of initiation or within 2 weeks of changing dose. Aldactazide ascites regimens are based on the spironolactone component, not hydrochlorothiazide mass.