Related Subjects:
|Vitamin D Replacement
|Osteomalacia-Rickets-Vitamin D
|Vitamin D resistant rickets
|Vitamin D (25 OH D) Testing
|X linked Hypophosphataemic rickets
|Osteoporosis
☀️ Vitamin D testing is mainly used to screen for osteomalacia and rickets.
👉 Replacement is appropriate if deficiency <30 nmol/L.
👉 Those with slightly higher levels may also benefit if symptomatic or at risk.
⚠️ Routine testing of Vitamin D is NOT recommended (new guidance).
🔹 About Vitamin D
- Most assays measure 25-(OH)-Vitamin D (inactive form) - the major circulating storage form.
- 1,25-(OH)₂-Vitamin D (active metabolite) has a half-life of only 4–6 hours → unreliable for status assessment.
- Deficiency: <20 ng/mL (<30 nmol/L).
Insufficiency: 21–29 ng/mL (30–50 nmol/L).
Adequate: ≥30 ng/mL (>50 nmol/L).
🚫 Do not measure 1,25-(OH)₂-D for deficiency:
- Short half-life, tiny concentration (~1/1000 of 25(OH)D), and often falsely normal/high.
✅ Always measure 25(OH)D for clinical assessment.
🔎 When to Check Vitamin D
- Clinical features suggestive of deficiency:
- Bone pain, tenderness, or pseudo-fractures
- Children: deformity, growth delay, refusal to weight bear
- Proximal muscle weakness (difficulty rising from chair, waddling gait)
- Fractures (e.g. femoral neck, pubic rami, ribs, vertebrae)
- Myalgia ± raised CK, or statin-associated myalgia
- Risk factors:
- 🌍 Darker skin tone, ethnic minority background
- 🏠 Elderly, housebound, or institutionalised
- 💊 Medications: cholestyramine, rifampicin, glucocorticoids, antiretrovirals, anticonvulsants (check every 2–5 yrs per NICE)
- 🌱 Vegan/vegetarian diet, liver or renal disease
- Malabsorption: coeliac, IBD, bariatric surgery, CF, radiation enteritis
- Covering of skin (veils, sunscreen use)
- Pregnancy, lactation, obesity (BMI >30)
- Exclude other mimics:
- Myeloma, Rheumatoid arthritis
- Polymyalgia rheumatica, Hypothyroidism
- Other clinical indications:
- Older patients with falls
- Bone disease improved by vitamin D or requiring optimisation before therapy
- Before Zoledronic acid or Denosumab therapy
- Unexplained ↑ ALP, ↓ phosphate/calcium, or ↑ PTH
- Before thyroidectomy or bariatric surgery
- At diagnosis of multiple sclerosis
- Sample requirements:
- Avoid high-dose biotin interference: collect sample ≥8h after last dose if >5 mg/day.
- Adults: blood sample in 5mL gold-top tube.
📊 Interpretation of 25(OH)D
| Vitamin D (nmol/L) | Status |
|---|
| > 50 | ✅ Adequate |
| 30 – 50 | ⚠️ Insufficiency |
| < 30 | ❌ Deficiency |
💡 Teaching Pearl:
Think: “<30 = treat, 30–50 = consider, >50 = fine.”
Always optimise vitamin D before starting antiresorptives (bisphosphonates, Denosumab) or major bone surgery.
