1. Clinical Overview
Calcitriol is the active hormonal form of Vitamin D3 (1,25-dihydroxycholecalciferol). It is a potent regulator of calcium and phosphate homeostasis, acting directly on target tissues (intestine, bone, kidney, and parathyroid glands) without requiring hepatic or renal activation. It is the treatment of choice for managing hypocalcemia and secondary hyperparathyroidism in patients with chronic kidney disease (CKD) on dialysis, hypoparathyroidism, and renal osteodystrophy.
| Onset | Duration | Bioavailability |
|---|---|---|
| Intestinal calcium absorption begins within 2-6 hours. Peak serum calcium effects are observed within 24-48 hours. | The biological effect on calcium homeostasis persists for 3-5 days after a single oral dose due to its genomic action and long half-life. | Approximately 70-75% after oral administration in healthy subjects. Absorption is rapid and occurs in the small intestine via passive diffusion. |
2. Mechanism of Action
Calcitriol binds to the intracellular Vitamin D Receptor (VDR), which then heterodimerizes with the Retinoid X Receptor (RXR). This complex translocates to the nucleus, binds to Vitamin D Response Elements (VDREs) in the promoter regions of target genes, and modulates their transcription. This genomic action increases intestinal absorption of calcium and phosphate, promotes bone mineralization, and suppresses parathyroid hormone (PTH) synthesis and secretion.
3. Indications & Uses
- Hypocalcemia and secondary hyperparathyroidism in patients with chronic kidney disease (CKD) on dialysis (Stage 5)
- Hypoparathyroidism (post-surgical, idiopathic, pseudohypoparathyroidism)
- Renal osteodystrophy in CKD patients
- Management of osteoporosis (in combination with calcium supplements, though less common than other Vitamin D forms)
4. Dosage & Administration
Adult Dosage: Dose must be individualized based on disease, serum calcium, phosphate, and PTH levels. Initial: 0.25 mcg once daily. May increase by 0.25 mcg/day at 4-8 week intervals. Typical maintenance: 0.5 to 1 mcg daily. In hypoparathyroidism: 0.5 to 2 mcg daily. Maximum recommended daily dose is often 3 mcg, but higher doses may be used under strict monitoring.
Administration: Administer orally, preferably with or after food to enhance absorption. Capsule/tablet should be swallowed whole with water. Administer at the same time each day. Phosphate binders (if prescribed) should be taken with meals, while calcitriol can be taken at a different time to avoid complexation.
5. Side Effects
Common side effects may include:
- Early signs of hypercalcemia: nausea, constipation, anorexia, polydipsia, polyuria
- Headache, somnolence
- Mild skin rash
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Thiazide Diuretics (e.g., Hydrochlorothiazide) | Reduce urinary calcium excretion, increasing risk of hypercalcemia. | Major |
| Digoxin | Hypercalcemia potentiates digitalis toxicity, increasing risk of cardiac arrhythmias. | Major |
| Magnesium-containing Antacids/Laxatives | Increased risk of hypermagnesemia, especially in dialysis patients. | Major |
| Ketoconazole, Itraconazole | Inhibit CYP24A1 and CYP3A4, potentially increasing calcitriol levels and toxicity. | Moderate |
| Phenytoin, Phenobarbital, Carbamazepine | Induce hepatic CYP enzymes, increasing metabolism of calcitriol, reducing its efficacy. | Moderate |
| Cholestyramine, Mineral Oil | Reduce intestinal absorption of calcitriol. | Moderate |
| Corticosteroids (e.g., Prednisolone) | Antagonize calcitriol's effect on calcium absorption. | Moderate |
7. Patient Counselling
- DO take the medication exactly as prescribed, at the same time each day.
- DO adhere to dietary recommendations regarding calcium and phosphate intake provided by your doctor/dietitian.
- DO keep all scheduled lab appointments for calcium, phosphate, and PTH checks.
- DO inform all your doctors and dentists you are taking calcitriol.
- DONT take over-the-counter calcium, vitamin D, or antacids without consulting your doctor.
- DONT change your dose without medical advice, even if you feel well.
8. Toxicology & Storage
Overdose: Manifestations of acute or chronic hypervitaminosis D: Severe hypercalcemia, hypercalciuria, hyperphosphatemia. Symptoms: Profound anorexia, intractable nausea/vomiting, constipation, pancreatitis, weakness, headache, somnolence, polydipsia, polyuria, metastatic calcification, cardiac arrhythmias, renal failure, coma.
Storage: Store below 25°C, protected from light and moisture. Keep the container tightly closed. Do not freeze. Keep out of reach of children.