In a female patient with a serum 25‑hydroxy‑vitamin D level of 9 ng/mL, what loading dose of vitamin D3 should be prescribed and for how many weeks before transitioning to a maintenance dose?

Vitamin D Repletion for Severe Deficiency (9 ng/mL)

For a female patient with a vitamin D level of 9 ng/mL, prescribe ergocalciferol (vitamin D₂) or cholecalciferol (vitamin D₃) 50,000 IU once weekly for 12 weeks, then transition to a maintenance dose of 800–2,000 IU daily.

Rationale for 12-Week Loading Phase

Your patient has severe vitamin D deficiency (defined as <10–12 ng/mL), which places her at significant risk for osteomalacia, secondary hyperparathyroidism, muscle weakness, and fractures 1, 2. At a level of 9 ng/mL, she may already have subclinical bone disease 2.

• The standard evidence-based loading regimen is 50,000 IU once weekly for 8–12 weeks 1, 2, 3.
• For severe deficiency (<10 ng/mL), the full 12-week course is recommended to ensure adequate repletion and achieve target serum 25(OH)D levels ≥30 ng/mL 1, 2.
• This cumulative dose of 600,000 IU over 12 weeks reliably raises serum 25(OH)D by approximately 40–70 ng/mL (16–28 ng/mL), bringing her from 9 ng/mL to the target range of 30–40 ng/mL 1.

Vitamin D₃ vs. D₂: Which to Prescribe

• Cholecalciferol (vitamin D₃) is strongly preferred over ergocalciferol (vitamin D₂) because it maintains serum concentrations longer and has superior bioavailability, particularly with weekly dosing 1, 2.
• Both forms are acceptable for the loading protocol, but D₃ is more effective for sustained levels 1, 2.

When to Transition to Maintenance Dosing

• After completing the 12-week loading phase, transition immediately to a maintenance dose 1, 2, 3.
• Do not wait for repeat lab testing before starting maintenance; the goal is to sustain the achieved level 1.

Maintenance Dose Selection

• Prescribe 800–2,000 IU vitamin D₃ daily after the loading phase 1, 3.
• For most women, 1,000–2,000 IU daily is optimal to maintain levels ≥30 ng/mL and provide anti-fracture and anti-fall benefits 1, 4.
• An alternative is 50,000 IU once monthly (equivalent to ~1,600 IU daily), which is convenient and effective 1.

Monitoring Protocol

• Recheck serum 25(OH)D at 3 months after completing the loading phase (i.e., at week 12–16) to confirm achievement of target levels ≥30 ng/mL 1, 4.
• If using weekly dosing, measure just before the next scheduled dose 1.
• Target serum level is ≥30 ng/mL for optimal bone health, fracture prevention, and fall reduction 1, 4, 5.
• Once stable, recheck annually 1, 4.

Essential Co-Interventions

• Ensure adequate calcium intake of 1,000–1,500 mg daily from diet plus supplements if needed 1, 2, 3.
• Calcium is required for vitamin D to exert its full bone-protective effect; vitamin D alone is insufficient 1, 2.
• Calcium supplements should be taken in divided doses of ≤600 mg for optimal absorption 1.

Safety Monitoring

• Check serum calcium and phosphorus at baseline and every 3 months during high-dose therapy to detect hypercalcemia early 1, 2.
• Discontinue all vitamin D immediately if serum calcium rises above 10.2 mg/dL (2.54 mmol/L) 1, 2.
• The upper safety limit for serum 25(OH)D is 100 ng/mL; toxicity is rare below this threshold 1, 4, 6.
• Daily doses up to 4,000 IU are completely safe for adults; the 50,000 IU weekly regimen (equivalent to ~7,000 IU daily) is well within safe limits 1, 4, 6.

Critical Pitfalls to Avoid

• Do not use active vitamin D analogs (calcitriol, alfacalcidol, doxercalciferol, paricalcitol) to treat nutritional vitamin D deficiency; they bypass normal regulation and dramatically increase hypercalcemia risk 1, 2.
• Avoid single mega-doses ≥300,000 IU, which are inefficient and may paradoxically increase fall and fracture risk 1, 4.
• Do not prescribe maintenance doses <800 IU daily; doses <400 IU are ineffective for achieving target levels and preventing fractures 1, 4.
• Do not measure 25(OH)D levels before 3 months after starting or adjusting therapy; levels need time to plateau 1, 4.

Special Considerations

• If she has malabsorption (e.g., inflammatory bowel disease, celiac disease, post-bariatric surgery), consider intramuscular vitamin D 50,000 IU or escalate oral dosing to 50,000 IU 2–3 times weekly 1, 2.
• If she has chronic kidney disease (CKD stages 3–4), use the same loading regimen with standard nutritional vitamin D (not active analogs), but monitor calcium and phosphorus more frequently 1, 2.
• If she is elderly (≥65 years), ensure maintenance doses are at least 800–1,000 IU daily to reduce fall and fracture risk 1, 4.

Expected Outcomes

• With this regimen, her serum 25(OH)D should rise from 9 ng/mL to 30–40 ng/mL by week 12 1, 2.
• Achieving ≥30 ng/mL reduces non-vertebral fractures by ~20%, hip fractures by ~18%, and falls by ~19% 1.
• Symptoms of deficiency (muscle weakness, bone pain, fatigue) should improve within weeks to months 3, 7.