Makindo Medical Notes

Vitamin D is a fat-soluble vitamin that behaves like a steroid hormone. It is vital for calcium and phosphate homeostasis, bone mineralisation, and muscle function. The two main forms are cholecalciferol (D₃) from sunlight/animal sources and ergocalciferol (D₂) from plants.

☀️ Physiology & Metabolism

• UV-B light converts 7-dehydrocholesterol in skin → vitamin D₃.
• Liver: 25-hydroxylation → 25-hydroxyvitamin D (25(OH)D).
• Kidney: 1-α-hydroxylation → active 1,25-dihydroxyvitamin D (calcitriol).
• Enhances intestinal Ca²⁺ and PO₄³⁻ absorption; supports bone mineralisation.
• Downregulated by high phosphate or fibroblast growth factor-23 (FGF-23).

🇬🇧 UK Context

• Effective skin synthesis occurs only between April–September.
• From October–March, UV-B levels in the UK are too low → dietary/supplemental intake required.
• Deficiency common in elderly, dark-skinned, veiled, institutionalised, and obese individuals.

📊 Definitions (NICE / SACN)

Status	25(OH)D Level	Interpretation
Deficient	< 25 nmol/L	Associated with osteomalacia/rickets
Insufficient	25–50 nmol/L	May be inadequate for some individuals
Sufficient	> 50 nmol/L	Adequate for bone health

🧍‍♂️ Clinical Features of Deficiency

• Adults: Bone pain, proximal myopathy, fragility fractures, fatigue.
• Children: Rickets — bowing of legs, delayed milestones, craniotabes.
• Biochemical pattern: ↓ Ca²⁺, ↓ PO₄³⁻, ↑ ALP, ↑ PTH (secondary hyperparathyroidism).

🔬 Investigations

• Primary test: Serum 25(OH)D (not calcitriol).
• Also check calcium, phosphate, ALP, ± PTH and renal function.
• Testing indicated only if symptomatic or before starting potent anti-resorptive therapy.

💊 Management & Supplementation

• Prevention (Public Health England): All adults and children > 4 yrs should take 10 µg = 400 IU daily Oct–Mar.
• Year-round supplementation: For those with dark skin, veiling, housebound, or little sun exposure.
• Treatment of deficiency: Loading dose total ≈ 300 000 IU (e.g. 50 000 IU weekly × 6 weeks) → maintenance 800–2000 IU daily.
• Ensure sufficiency before anti-resorptive therapy (e.g., zoledronate, denosumab).

⚠️ Vitamin D Toxicity

• Excess supplementation → hypercalcaemia, nausea, renal stones, confusion.
• Monitor calcium and renal function in those on high-dose or long-term therapy.

🧠 Teaching Pearls

• Think vitamin D deficiency in older patients with falls or proximal weakness.
• Do not routinely test in healthy people — encourage prevention instead.
• Understand the physiology: Skin → Liver → Kidney → Bone — this explains why CKD, liver disease or immobility predispose to deficiency.
• Simple intervention, major benefit: low-cost, low-risk, high-impact in frailty and fracture prevention.

📋 Summary Table

Aspect	Key Point
Deficiency threshold	< 25 nmol/L (UK)
Routine screening	No — reserve for symptomatic or high-risk
Standard supplement	10 µg = 400 IU daily (Oct–Mar)
Treatment regimen	Loading ≈ 300 000 IU → maintenance 800–2000 IU/day
Key effect	↑ Ca²⁺ and PO₄³⁻ absorption → bone mineralisation
Toxicity	Hypercalcaemia, nephrolithiasis, confusion

🩺 Summary for Practice

In UK medicine, vitamin D deficiency is common yet easily corrected. Always check in patients with bone pain, proximal weakness, fragility fractures, malabsorption, or before giving anti-resorptive therapy. Focus on prevention, seasonal awareness, and practical dosing.