Makindo Medical Notes

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

💡 Teaching Pearl: Think: “<30 = treat, 30–50 = consider, >50 = fine.” Always optimise vitamin D before starting antiresorptives (bisphosphonates, Denosumab) or major bone surgery.