Calcitriol

Vitamin D Analog Calcium-Regulating Hormone

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO Initial: 2.5-5 ng/kg/day; Maintenance: 2.5-20 ng/kg/day, titrated to maintain serum calcium and PTH within normal limits Once daily (q24h) Duration: Long-term, often lifelong for CKD or hypoparathyroidism
Notes: Monitor serum calcium, phosphorus, and PTH levels regularly. Adjust dose based on response. Administer with food to enhance absorption.
Cat PO Initial: 2.5-5 ng/kg/day; Maintenance: 2.5-20 ng/kg/day, titrated to maintain serum calcium and PTH within normal limits Once daily (q24h) Duration: Long-term, often lifelong for CKD or hypoparathyroidism
Notes: Monitor serum calcium, phosphorus, and PTH levels regularly. Adjust dose based on response. Administer with food to enhance absorption.
Horse PO Not established; experimental doses may range from 0.5-2 mcg/kg/day, but not recommended Not established Duration: Not established
Notes: Not commonly used; use only under expert guidance and with careful monitoring.
Cattle PO Not established; experimental doses may range from 0.5-2 mcg/kg/day, but not recommended Not established Duration: Not established
Notes: Not commonly used; use only under expert guidance and with careful monitoring.
Small Ruminants PO Not established; not recommended Not established Duration: Not established
Notes: No established indications.
Rabbit PO Not established; may use 0.05-0.1 mcg/kg/day, but limited evidence Once daily Duration: Variable
Notes: Use with caution; monitor serum calcium closely.
Bird/Poultry PO Not established; not recommended Not established Duration: Not established
Notes: No established indications.
Exotic/Other PO Not established; may use 0.05-0.1 mcg/kg/day in reptiles, but limited evidence Once daily Duration: Variable
Notes: Use with caution; monitor serum calcium closely.

Clinical Indications & Species Uses

General Indications
  • Management of hypocalcemia due to various causes
  • Control of secondary hyperparathyroidism in chronic kidney disease
Dog (Canine)
  • Treatment of renal secondary hyperparathyroidism associated with chronic kidney disease (CKD)
  • Management of hypocalcemia (e.g., post-thyroidectomy, primary hypoparathyroidism)
  • Treatment of nutritional secondary hyperparathyroidism (in conjunction with dietary management)
  • Adjunctive therapy for certain types of cancer-associated hypercalcemia (though not primary use)
Cat (Feline)
  • Treatment of renal secondary hyperparathyroidism in chronic kidney disease (CKD)
  • Management of hypocalcemia (e.g., post-thyroidectomy, primary hypoparathyroidism)
  • Treatment of nutritional secondary hyperparathyroidism (in conjunction with dietary management)

Pharmacology & Mechanism of Action

Drug Class: Vitamin D Analog | Pharmacological Group: Calcium-Regulating Hormone

Mechanism of Action: Calcitriol is the active form of vitamin D3 (1,25-dihydroxycholecalciferol). It binds to the vitamin D receptor (VDR) in the nucleus of target cells, modulating gene transcription. In the intestine, it increases the absorption of calcium and phosphate by upregulating calcium-binding proteins (calbindin) and the calcium channel TRPV6. In bone, it stimulates osteoblasts to produce RANKL, which activates osteoclasts to resorb bone, releasing calcium and phosphate into the circulation. In the kidney, it enhances distal tubular reabsorption of calcium and phosphate. It also suppresses parathyroid hormone (PTH) synthesis and secretion via a negative feedback mechanism.

Pharmacodynamics: Calcitriol increases serum calcium and phosphate levels by promoting intestinal absorption, renal reabsorption, and bone resorption. It also has direct effects on parathyroid glands to reduce PTH secretion, which is beneficial in managing renal secondary hyperparathyroidism. The onset of action after oral administration is typically within 2-6 hours, with peak effect on serum calcium in 24-48 hours. The duration of effect is approximately 3-5 days.

⚡ Pharmacokinetics Summary

Absorption: Rapidly absorbed from the gastrointestinal tract after oral administration. Peak serum concentrations occur within 2-6 hours. Absorption is enhanced by bile salts and dietary fat.
Distribution: Widely distributed throughout the body. It is highly protein-bound (approximately 99.9%) to vitamin D-binding protein (DBP) and albumin. It crosses the placenta and is distributed into milk.
Metabolism: Metabolized primarily in the kidney and liver via hydroxylation and side-chain oxidation. The main metabolic pathway involves 24-hydroxylation to form calcitroic acid, which is inactive.
Excretion: Excreted primarily in the bile and feces. A small amount is excreted in the urine. Enterohepatic recirculation may occur.
Half-Life: Dog: approximately 10-12 hours; Cat: approximately 15-20 hours; Human: 4-6 hours (for reference).
Bioavailability: Oral bioavailability is approximately 60-80% in dogs and cats, but may be variable depending on gastrointestinal health and food intake.
Protein Binding: Approximately 99.9% bound to vitamin D-binding protein (DBP) and albumin.

Available Formulations & Strengths

Oral Capsule 0.25 mcg, 0.5 mcg (PO)
Oral Solution 1 mcg/mL (PO)
Injectable Solution 1 mcg/mL, 2 mcg/mL (IV)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to calcitriol or any component of the formulation
  • Hypercalcemia or hypercalciuria
  • Vitamin D toxicity
  • Evidence of metastatic calcification
  • Use in animals with known malabsorption syndromes (oral form)
Warnings & Clinical Precautions:
  • Use with caution in animals with renal disease, as hypercalcemia may worsen renal function.
  • Monitor serum calcium, phosphorus, and PTH levels regularly (initially weekly, then monthly).
  • Avoid concurrent use of calcium-containing supplements unless specifically indicated; adjust calcium intake to avoid hypercalcemia.
  • Use with caution in pregnant or lactating animals; safety not established.
  • Do not administer intravenously unless specifically indicated; rapid IV administration may cause acute hypercalcemia.
  • In animals with CKD, calcitriol may increase serum phosphorus; use with phosphate binders if needed.
  • Discontinue if hypercalcemia develops; treat with IV fluids, diuretics, and corticosteroids if severe.

Adverse Effects & Reactions

Common:

  • Hypercalcemia (mild to moderate)
  • Hyperphosphatemia
  • Increased thirst and urination (polyuria/polydipsia)
  • Gastrointestinal upset (vomiting, diarrhea, anorexia)

Serious / Severe:

  • Severe hypercalcemia leading to soft tissue calcification (nephrocalcinosis, vascular calcification)
  • Acute renal failure
  • Cardiac arrhythmias
  • Seizures (due to extreme hypercalcemia)

Rare:

  • Allergic reactions (urticaria, angioedema)
  • Bone pain
  • Metastatic calcification

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Calcium-containing supplements or antacids Increased risk of hypercalcemia High
Thiazide diuretics Increased risk of hypercalcemia due to decreased renal calcium excretion High
Corticosteroids May reduce the efficacy of calcitriol by antagonizing its effects on calcium absorption Moderate
Phenytoin or phenobarbital May increase metabolism of calcitriol, reducing its effectiveness Moderate
Cholestyramine or colestipol May reduce intestinal absorption of calcitriol Moderate
Digoxin Hypercalcemia may increase the risk of digitalis toxicity High
Magnesium-containing antacids May cause hypermagnesemia, especially in renal failure Moderate

Overdose & Toxicity Management

Signs of Toxicity:

  • Hypercalcemia (elevated serum calcium)
  • Anorexia, vomiting, constipation
  • Polyuria, polydipsia
  • Weakness, lethargy
  • Muscle twitching or tremors
  • Seizures
  • Cardiac arrhythmias
  • Soft tissue calcification (chronic overdose)

Emergency Treatment Protocol: Immediate discontinuation of calcitriol and calcium supplements. Administer intravenous fluids (0.9% NaCl) to promote calciuresis. Use loop diuretics (e.g., furosemide) to enhance calcium excretion. Corticosteroids (e.g., prednisone) may be used to reduce intestinal calcium absorption and inhibit bone resorption. In severe cases, calcitonin or bisphosphonates (e.g., pamidronate) may be considered. Monitor serum calcium, renal function, and electrocardiogram. Provide supportive care as needed.

Food Animal Withdrawal Times

Calcitriol is not approved for use in food-producing animals. Withdrawal times are not established. If used off-label in food animals, consult with a veterinary toxicologist and follow regulatory guidelines.

Storage, Handling & Regulatory Information

Storage Temperature: Store at room temperature (15-30°C or 59-86°F). Protect from light and moisture.

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Keep in tightly closed container. Do not freeze. Keep out of reach of children and animals.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use (Rocaltrol).

Extra-Label (Off-Label) Use: In the US, calcitriol is not FDA-approved for veterinary use, but it is commonly used off-label in dogs and cats. Extra-label use in food animals is prohibited due to lack of withdrawal times and potential for residues. In the EU, similar regulations apply under the cascade.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Calcitriol is a potent vitamin D analog used primarily in dogs and cats for managing renal secondary hyperparathyroidism associated with chronic kidney disease (CKD). It is also used to treat hypocalcemia due to primary hypoparathyroidism or following thyroidectomy. Dosing must be individualized based on serum calcium, phosphorus, and PTH levels. Start with a low dose and titrate upward slowly to avoid hypercalcemia. Regular monitoring is essential, especially during the initial phase. In CKD patients, calcitriol can help reduce PTH levels and slow the progression of renal disease, but it may increase serum phosphorus; therefore, dietary phosphorus restriction and phosphate binders are often necessary. Calcitriol should be administered with food to enhance absorption. It is contraindicated in hypercalcemic animals. Overdose can cause severe hypercalcemia, which is a medical emergency. Due to its narrow therapeutic index, careful client education and follow-up are crucial. In food animals, calcitriol is not recommended due to lack of withdrawal data.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)