In an adult with a serum 25‑hydroxy‑vitamin D level of 17 ng/mL, what vitamin D supplementation regimen should be prescribed?

Treatment for Vitamin D Level of 17 ng/mL

For an adult with a vitamin D level of 17 ng/mL, prescribe ergocalciferol (vitamin D2) or cholecalciferol (vitamin D3) 50,000 IU once weekly for 8-12 weeks, followed by maintenance therapy with 800-2,000 IU daily. 1

Understanding the Deficiency Severity

A serum 25-hydroxyvitamin D level of 17 ng/mL represents frank vitamin D deficiency (defined as <20 ng/mL), not merely insufficiency. 1, 2 This level is associated with:

• Elevated parathyroid hormone and increased bone turnover 1
• Significantly increased fracture risk 1
• Greater severity of secondary hyperparathyroidism 1
• Reduced bone mineral density 2

The target level after treatment should be at least 30 ng/mL for optimal anti-fracture efficacy and musculoskeletal health. 3, 1

Loading Phase Protocol

Standard Regimen (Preferred)

Prescribe 50,000 IU of vitamin D2 (ergocalciferol) or D3 (cholecalciferol) once weekly for 8-12 weeks. 3, 1, 4

• For deficiency in the 10-20 ng/mL range: 8 weeks is typically sufficient 1
• For severe deficiency (<10 ng/mL): extend to 12 weeks 1
• Since this patient has 17 ng/mL, 8 weeks is appropriate 1

Vitamin D2 vs D3 Selection

Cholecalciferol (D3) is strongly preferred over ergocalciferol (D2) because it maintains serum levels longer and has superior bioavailability, particularly with intermittent (weekly) dosing schedules. 1 However, both are acceptable and effective for the loading phase. 1

Expected Response

Using the rule of thumb that 1,000 IU daily raises 25(OH)D by approximately 10 ng/mL, the weekly 50,000 IU regimen (equivalent to ~7,000 IU daily) should increase levels by approximately 40-70 ng/mL over 8-12 weeks, bringing the patient from 17 ng/mL to a target range of 28-40 ng/mL or higher. 1

Maintenance Phase

After completing the loading phase, transition to maintenance therapy with 800-2,000 IU daily (or 50,000 IU monthly, which equals approximately 1,600 IU daily). 3, 1, 4

• For patients ≥65 years: minimum 800 IU daily, though 700-1,000 IU daily more effectively reduces fall and fracture risk 1
• For younger adults: 800-1,000 IU daily is typically sufficient 1
• Higher maintenance doses (up to 2,000 IU daily) may be needed for patients with malabsorption, obesity, or other risk factors 1

An alternative maintenance regimen is 50,000 IU once monthly, which provides equivalent steady-state levels. 1

Essential Co-Interventions

Ensure adequate calcium intake of 1,000-1,500 mg daily from diet plus supplements if needed. 1 Calcium is necessary for the clinical response to vitamin D therapy and optimal bone health. 1

• Calcium supplements should be taken in divided doses of no more than 600 mg at once for optimal absorption 1
• Four portions of calcium-rich dairy foods daily can help achieve both calcium and protein goals 1

Monitoring Protocol

Recheck serum 25(OH)D levels 3 months after initiating treatment to confirm adequate response and guide ongoing therapy. 1, 2 This timing allows vitamin D levels to plateau and accurately reflect the response to supplementation, given vitamin D's long half-life. 1

• If using weekly dosing, measure just prior to the next scheduled dose 1
• Also check serum calcium and phosphorus every 3 months during treatment 1
• Once target levels (≥30 ng/mL) are achieved and stable, annual reassessment is sufficient 1, 2

Discontinue all vitamin D therapy immediately if:

• Serum corrected total calcium exceeds 10.2 mg/dL (2.54 mmol/L) 1
• Serum phosphorus exceeds 4.6 mg/dL (1.49 mmol/L) despite phosphate binders 1

Safety Considerations

Daily doses up to 4,000 IU are generally safe for adults, with some evidence supporting up to 10,000 IU daily for several months without adverse effects. 3, 1 The upper safety limit for serum 25(OH)D is 100 ng/mL. 3, 1

Avoid single very large doses (>300,000 IU) as they may be inefficient or potentially harmful, particularly for fall and fracture prevention. 1

Vitamin D toxicity is rare and typically only occurs with prolonged daily doses exceeding 10,000 IU or serum levels above 100 ng/mL. 3, 1

Critical Pitfalls to Avoid

Never use active vitamin D analogs (calcitriol, alfacalcidol, doxercalciferol, paricalcitol) to treat nutritional vitamin D deficiency. 1 These bypass normal regulatory mechanisms, do not correct 25(OH)D levels, and carry higher risk of hypercalcemia. 1

Do not rely on standard multivitamins, which typically contain only 400 IU of vitamin D—insufficient for treating deficiency. 2

Do not recommend sun exposure for vitamin D repletion due to increased skin cancer risk. 1

Verify patient adherence before increasing doses for inadequate response, as poor compliance is a common reason for treatment failure. 1

Special Population Considerations

Chronic Kidney Disease (CKD Stages 3-4)

For patients with GFR 20-60 mL/min/1.73m², use the same standard nutritional vitamin D replacement (ergocalciferol or cholecalciferol), not active vitamin D analogs. 1 CKD patients are at particularly high risk due to reduced sun exposure, dietary restrictions, and urinary losses. 1

Malabsorption Syndromes

For patients with inflammatory bowel disease, post-bariatric surgery, celiac disease, or pancreatic insufficiency who fail oral supplementation, intramuscular vitamin D 50,000 IU is the preferred route, resulting in significantly higher 25(OH)D levels and lower rates of persistent deficiency. 1 When IM is unavailable, substantially higher oral doses (4,000-5,000 IU daily) are required. 1

Obesity

Patients with higher BMI may require higher doses to achieve target levels, as vitamin D is sequestered in adipose tissue. 1, 5 Consider checking response at 3 months and adjusting accordingly. 1