Calcitriol
Calcitriol is 1,25-dihydroxycholecalciferol, the active hormonal form of vitamin D. It regulates calcium and phosphorus balance and is used for selected hypocalcemic disorders, often together with calcium supplementation. Because it is already active, it does not require the renal activation needed by some precursor forms of vitamin D.
Another veterinary application is suppression of renal secondary hyperparathyroidism in carefully selected chronic kidney disease patients, although evidence of benefit is variable. It is a potent regulator of mineral metabolism, not a routine nutritional vitamin supplement or an immediate substitute for injectable calcium in an emergency.
Group
Active vitamin D analogue
Mechanism of Action
Calcitriol binds the intracellular vitamin D receptor, which regulates expression of proteins involved in mineral transport and metabolism. In the intestine, this increases the capacity to absorb calcium and phosphorus, helping restore calcium availability when dietary supply and absorption can support the response.
It also influences renal mineral handling and bone turnover in conjunction with parathyroid hormone and other regulators. The net effect depends on the patient’s mineral status and endocrine environment rather than simply moving all calcium in one direction.
At the parathyroid gland, vitamin D receptor signaling suppresses parathyroid hormone synthesis and secretion and can limit glandular stimulation. This provides the rationale for selected renal secondary hyperparathyroidism treatment. Because both calcium and phosphorus exposure can rise, the same mechanism can promote hypercalcemia or an undesirable calcium–phosphorus burden when treatment is unsuitable.
Dose Rates
Dogs
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Renal secondary hyperparathyroidism — Initial | 0.0025 mcg/kg | PO | Every 24 h | Equivalent to 2.5 ng/kg. Titrate to calcium and PTH. |
| Renal secondary hyperparathyroidism — Maintenance | 0.0035 mcg/kg | PO | Every 24 h | Increase only when PTH remains high without hypercalcemia; increments up to 0.005 mcg/kg per day are described. |
| Intermittent renal secondary hyperparathyroidism treatment | Reference only | PO | Alternative 0.01 mcg/kg orally twice weekly when daily dosing is impractical. This is a weekly schedule, not a fixed hourly interval. | |
| Renal secondary hyperparathyroidism | 0.0025–0.0035 mcg/kg | PO | Every 24 h | Equivalent to 2.5–3.5 ng/kg; monitor ionized calcium, phosphorus and PTH. |
| Refractory renal secondary hyperparathyroidism | Reference only | PO | Historical escalation up to 6 ng/kg daily, with a stated 6.6 ng/kg/day ceiling unless ionized calcium is monitored. This differs from the lower ceiling in the other monograph; use a specialist titration plan rather than merging them into one range. | |
| Pulsed renal secondary hyperparathyroidism treatment | Reference only | PO | An alternative historical pulse is approximately 20 ng/kg orally twice weekly at bedtime on an empty stomach, reserved for patients needing high daily doses and close ionized-calcium monitoring. | |
| Hypoparathyroidism — Initial | 0.02–0.03 mcg/kg | PO | Every 24 h | Up to 4 days; Initial 3–4 days; then use the separate maintenance regimen. |
| Hypoparathyroidism — Maintenance | 0.005–0.015 mcg/kg | PO | Every 24 h | Adjust to calcium response; not the CKD regimen. |
| Divided-dose hypocalcemia alternative | Reference only | PO | A separate historical course gives 20–30 ng/kg TOTAL DAILY divided twice daily for 3–4 days, then 5–15 ng/kg TOTAL DAILY divided twice daily. It must not be confused with the once-daily regimen. | |
| Primary idiopathic seborrhea | 0.01 mcg/kg | PO | Every 24 h | Historical use especially in spaniels; give apart from the main meal. |
| Long-term hypoparathyroidism alternative | Reference only | PO | 0.03–0.06 mcg/kg TOTAL DAILY is described with oral calcium; the amount is a distinct historical alternative and must be individualized by calcium monitoring. |
Cats
| Use / stage | Dose | Route | Schedule | Instructions |
|---|---|---|---|---|
| Renal secondary hyperparathyroidism — Initial | 0.0025 mcg/kg | PO | Every 24 h | Equivalent to 2.5 ng/kg. Titrate to calcium and PTH. |
| Renal secondary hyperparathyroidism — Maintenance | 0.0035 mcg/kg | PO | Every 24 h | Increase only when PTH remains high without hypercalcemia; increments up to 0.005 mcg/kg per day are described. |
| Intermittent renal secondary hyperparathyroidism treatment | Reference only | PO | Alternative 0.01 mcg/kg orally twice weekly when daily dosing is impractical. This is a weekly schedule, not a fixed hourly interval. | |
| Post-parathyroidectomy hypocalcemia | 0.25 mcg/animal | PO | Every 48 h | |
| Post-parathyroidectomy hypocalcemia | 0.01–0.04 mcg/kg | PO | Every 24 h | |
| Pulsed renal secondary hyperparathyroidism treatment | Reference only | PO | An alternative historical pulse is approximately 20 ng/kg orally twice weekly at bedtime on an empty stomach, reserved for patients needing high daily doses and close ionized-calcium monitoring. | |
| Renal secondary hyperparathyroidism | 0.0015–0.0035 mcg/kg | PO | Every 24 h | Give separately from meals; monitor calcium and PTH. |
| Long-term hypoparathyroidism alternative | Reference only | PO | 0.03–0.06 mcg/kg TOTAL DAILY is described with oral calcium; the amount is a distinct historical alternative and must be individualized by calcium monitoring. |
Indications
- Long-term management of hypocalcemia associated with hypoparathyroidism, commonly with oral calcium support.
- Prevention or treatment of selected post-parathyroidectomy hypocalcemia, according to the anticipated or measured deficit.
- Selected renal secondary hyperparathyroidism in dogs or cats with chronic kidney disease when mineral status permits treatment; benefit, especially in cats, is not uniformly established.
- Reported treatment of primary idiopathic seborrhea is a less established application rather than a routine reason to administer active vitamin D.
- Calcitriol is not intended for indiscriminate vitamin D supplementation and does not provide immediate calcium replacement during an acute hypocalcemic crisis.
Contraindications
- Hypercalcemia or substantial risk of hypercalcemia.
- Human capsule strengths may greatly exceed a small animal dose.
- Vitamin D toxicity, abnormal vitamin D sensitivity or uncontrolled hyperphosphatemia; calcium oxalate stone risk requires caution.
Interactions
- Thiazides increase hypercalcemia risk.
- Calcium-containing phosphate binders increase hypercalcemia risk.
- Digoxin-treated patients are especially sensitive to hypercalcemia; verapamil also requires closer monitoring.
- Phenytoin, barbiturates and primidone may increase vitamin D analogue metabolism; corticosteroids can oppose its effects.
Adverse effects
- Hypercalcemia
- Soft-tissue mineralization
Toxicity
Stop treatment when ionized calcium rises above the laboratory reference interval; resume at a lower dose only after reassessment.
Preparations
| Preparation | Form | Strength | Route |
|---|---|---|---|
| Calcitriol 0.25 mcg capsule | Capsule | 0.25 mcg / capsule | PO |
| Calcitriol 0.5 mcg capsule | Capsule | 0.5 mcg / capsule | PO |
| Rocaltrol oral solution 1 mcg/mL | Oral Liquid | 1 mcg / mL | PO |
Monitoring and Administration
Adjust using ionized calcium, phosphorus, PTH and creatinine. Control phosphorus; a target below 6 mg/dL is described for CKD treatment.
In cats, avoid giving with a meal; evening administration before feeding is described. Benefit for slowing CKD is uncertain and not consistent in all patients.
Dilution must preserve microgram accuracy: 1 part of 1 mcg/mL solution plus 9 parts suitable oil gives 0.1 mcg/mL. A separately described 0.03 mcg/mL oil preparation uses antioxidant diluent and amber storage, with a six-month beyond-use period specific to that recipe.
Keep formulations tightly closed, protected from light at room temperature.
Assess calcium and phosphorus at baseline and about one week after starting, then every 2–4 weeks while titrating. Recheck PTH around 4–6 weeks in PTH-directed treatment. Monitoring intervals differ once stable; do not apply a long follow-up interval during initiation.
For canine hypoparathyroidism, recheck calcium approximately 4–5 days after a calcitriol adjustment because response is delayed.