Calcitriol
Calcitriol is the active, hormonal form of vitamin D. In veterinary medicine, it is primarily utilized to manage **hypocalcemia** (often secondary to hypoparathyroidism) and to suppress **secondary hyperparathyroidism** in chronic kidney disease (CKD). It may also have benefits in treating primary idiopathic seborrhea in certain dog breeds. > **Clinical Pearl:** In healthy animals, the kidneys produce the enzyme 1-alpha-hydroxylase, which converts inactive vitamin D into its active form (calcitriol). In chronic renal failure, the loss of functional renal mass leads to a deficiency of this enzyme. Supplementing with calcitriol bypasses this metabolic defect, directly suppressing parathyroid hormone (PTH) synthesis and preventing or treating renal secondary hyperparathyroidism (renal osteodystrophy).
Mechanism: Calcitriol binds to the intracellular **Vitamin D Receptor (VDR)** in target tissues (intestine, bone, kidney, and parathyroid gland) to regulate calcium and phosphorus homeostasis. * **Intestine:** Upregulates the synthesis of calcium-binding proteins (e.g., calbindin) → significantly enhances the GI absorption of calcium and phosphorus. * **Kidney:** Promotes renal tubular reabsorption of calcium. * **Parathyroid Gland:** Directly binds to VDRs on the parathyroid gland → inhibits the transcription of the **PTH** gene, reducing PTH synthesis and secretion. * **Bone:** Works synergistically with PTH to stimulate osteoclast activity, mobilizing calcium and phosphorus into the extracellular fluid. Unlike other forms of vitamin D (like cholecalciferol or ergocalciferol), calcitriol does **not** require hepatic or renal activation, resulting in a rapid onset of action (approximately 1 day) and a short half-life.
Dosing by species
- To suppress secondary hyperparathyroidism in CRF · 1.5-3.5 nanograms/kg PO daily given separately from meals (or 2.5-3.5 nanograms/kg PO once daily) · PO · q24h · Long-term · Remove oil from capsule, dilute in corn oil, then give the appropriate volume.
- Long-term maintenance in animals with hypoparathyroidism · 0.03-0.06 micrograms/kg/day · PO · q24h · Long-term · Combine with oral calcium to reduce vitamin D dose requirements.
- To suppress secondary hyperparathyroidism in CRF · 2.5-3.5 nanograms/kg/day PO (Dogs with refractory hyperparathyroidism may require up to 6 nanograms/kg/day). Alternatively, pulsed-dosing: 20 nanograms/kg twice weekly PO at bedtime on an empty stomach. · PO · q24h or twice weekly · Long-term · Confirm CRF (creatinine >2 mg/dL) and reduce hyperphosphatemia to <6 mg/dL before starting.
- Subacute and chronic maintenance treatment of hypocalcemia · Initially, 20-30 nanograms/kg/day PO divided twice a day for 3-4 days, then 5-15 nanograms/kg/day divided twice a day · PO · q12h · Initial 3-4 days, then maintenance
- Long-term maintenance in animals with hypoparathyroidism · 0.03-0.06 micrograms/kg/day · PO · q24h · Long-term · Combine with oral calcium to reduce vitamin D dose requirements.
- Primary idiopathic seborrhea (especially in spaniel breeds) · 10 nanograms/kg PO once daily · PO · q24h · Give as far away from the main meal as possible.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- Hypercalcemia
- Hyperphosphatemia (Calcium x Phosphorus product > 70)
- Vitamin D toxicity
- Malabsorption syndromes
- Abnormal sensitivity to vitamin D effects
Adverse effects
- Hypercalcemia
- Hypercalciuria
- Hyperphosphatemia
- Polydipsia
- Polyuria
- Anorexia
- Tissue mineralization (if given with hyperphosphatemia)
Drug interactions
- Calcium-containing phosphorus binding agents (e.g., calcium carbonate) · Use with calcitriol may induce hypercalcemia.
- Corticosteroids · Can nullify the effects of vitamin D analogs.
- Digoxin · Patients are highly sensitive to the arrhythmogenic effects of hypercalcemia; intensified monitoring is required.
- Verapamil · Patients are sensitive to the effects of hypercalcemia; intensified monitoring is required.
- Phenytoin, Barbiturates, Primidone · May induce hepatic enzyme systems and increase the metabolism of Vitamin D analogs, thus decreasing their activity.
- Thiazide diuretics · May cause hypercalcemia when given in conjunction with Vitamin D analogs.
Monitoring
- Serum calcium (baseline, 1 week, then every 2-4 weeks/6 months depending on protocol)
- Serum phosphorus
- Serum creatinine
- Serum PTH levels (especially in cats or refractory cases)
- Clinical efficacy (improved appetite, activity level, slowed progression of disease)
Overdose
Overdosage can cause **hypercalcemia**, **hypercalciuria**, and **hyperphosphatemia**. * **Acute Ingestion:** Manage using established protocols for GI decontamination. Orally administered mineral oil may reduce absorption and enhance fecal elimination. * **Chronic Overdosage:** Hypercalcemia secondary to chronic dosing should be treated by first temporarily discontinuing (not just dose reduction) calcitriol and exogenous calcium therapy. * **Severe Hypercalcemia:** May require treatment with furosemide, calcium-free IV fluids (e.g., normal saline), urine acidification, and corticosteroids.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.