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# Vitamin D
## Overview
- **Classification**: Fat-soluble vitamin, prohormone.
- **Mechanism**: Converted to active calcitriol (1,25-dihydroxyvitamin D), which regulates calcium/phosphate homeostasis, bone mineralization, and various cellular functions.
## Primary Indications
1. **Vitamin D Deficiency/Insufficiency**: Treatment and prevention to achieve optimal serum levels.
2. **Osteoporosis**: Adjunctive therapy to improve bone mineral density and reduce fracture risk.
3. **Rickets/Osteomalacia**: Treatment and prevention of bone softening due to severe deficiency.
## Adult Dosing
### Standard Dosing
**Vitamin D Deficiency (Target 25(OH)D >30 ng/mL)**
- **Dose**: **50,000 IU** (Ergocalciferol [D2] or Cholecalciferol [D3])
- **Frequency**: Once weekly
- **Route**: Oral
- **Duration**: 8-12 weeks, then recheck levels.
**Vitamin D Maintenance (Post-repletion or Prevention)**
- **Dose**: **600-2000 IU** (D3 preferred, or D2)
- **Frequency**: Once daily
- **Route**: Oral
- **Duration**: Long-term
**Osteoporosis (Adjunctive Therapy)**
- **Dose**: **800-2000 IU** (D3 preferred)
- **Frequency**: Once daily
- **Route**: Oral
- **Considerations**: Often combined with calcium supplementation.
### Dose Adjustments
- **Renal Impairment**: Standard D2/D3 may be less effective in severe impairment; active forms (calcitriol) may be needed. Consult nephrology.
- **Hepatic Impairment**: No specific adjustments for D2/D3 are generally required.
- **Elderly Patients**: May have reduced skin synthesis; daily needs often recommended at the higher end of the maintenance range (**800-2000 IU/day**).
## Pediatric Dosing
### Neonates (0-28 days)
- **Indication**: Prevention of vitamin D deficiency/rickets in breastfed infants.
- **Dose**: **400 IU**
- **Frequency**: Once daily
- **Route**: Oral (liquid drops)
- **Maximum**: **400 IU/day**
- **Special Notes**: Supplementation should begin soon after birth.
### Infants (1-12 months)
- **Indication**: Prevention of deficiency/rickets.
- **Dose**: **400 IU**
- **Frequency**: Once daily
- **Route**: Oral (liquid drops)
- **Maximum**: **1000 IU/day** (UL for <6 months), **1500 IU/day** (UL for 6-12 months)
- **Special Notes**: Continue daily supplementation, especially for breastfed infants.
### Children (1-12 years)
- **Indication**: Prevention of deficiency.
- **Dose**: **600 IU**
- **Frequency**: Once daily
- **Route**: Oral (liquid, chewable, capsule)
- **Maximum**: **2500 IU/day** (1-3 yrs), **3000 IU/day** (4-8 yrs).
- **Treatment of deficiency**: **2000 IU** daily for 6 weeks, or **50,000 IU** weekly for 6 weeks.
### Adolescents (13-18 years)
- **Indication**: Prevention of deficiency.
- **Dose**: **600 IU**
- **Frequency**: Once daily
- **Route**: Oral
- **Maximum**: **4000 IU/day**.
- **Treatment of deficiency**: Similar to adult dosing, e.g., **50,000 IU** weekly for 8-12 weeks.
## Safety Information
### Contraindications
- **Absolute**: Hypercalcemia (elevated blood calcium levels).
- **Absolute**: Vitamin D toxicity (due to excessive intake).
- **Absolute**: Malabsorption syndromes with hypercalcemia risk (e.g., sarcoidosis).
### Common Adverse Effects (Typically only with very high doses/toxicity)
- **Very Common (>10%)**: None at recommended daily doses.
- **Common (1-10%)**: Nausea, vomiting, constipation, polyuria, polydipsia, weakness, fatigue.
- **Serious but Rare**: Nephrolithiasis (kidney stones), nephrocalcinosis, renal failure, cardiac arrhythmias (all due to severe hypercalcemia).
### Key Drug Interactions
- **Thiazide diuretics**: May increase the risk of hypercalcemia; monitor calcium levels closely.
- **Digoxin**: Hypercalcemia can increase the risk of digoxin toxicity; monitor closely.
- **Orlistat, Cholestyramine**: May impair vitamin D absorption; separate administration times or adjust dose.
- **Phenytoin, Barbiturates**: May increase vitamin D metabolism, potentially requiring higher vitamin D doses.
## Monitoring & Follow-up
- **Before Treatment**: Baseline 25-hydroxyvitamin D [25(OH)D] level, serum calcium, phosphate.
- **During Treatment**: Recheck 25(OH)D and serum calcium after 8-12 weeks of high-dose repletion.
- **Clinical Signs**: Monitor for symptoms of hypercalcemia: excessive thirst/urination, nausea, vomiting, constipation.
## Clinical Pearls
- 💡 **Tip 1**: Cholecalciferol (D3) is generally preferred over ergocalciferol (D2) as it may be more effective at raising and maintaining 25(OH)D levels.
- 💡 **Tip 2**: Vitamin D is a fat-soluble vitamin; best absorbed when taken with a fatty meal.
- 💡 **Tip 3**: Many factors affect vitamin D status, including sun exposure, diet, age, and kidney/liver function.
- 💡 **Tip 4**: For deficiency, a target 25(OH)D level of **>30 ng/mL** is generally recommended.
> **⚠️ Important**: This information is for educational purposes only. Always consult current prescribing information, local guidelines, and clinical judgment before prescribing.
