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Related Subjects: | Vitamin D Replacement | Osteomalacia-Rickets-Vitamin D | Vitamin D resistant rickets | Vitamin D (25 OH D) Testing | X linked Hypophosphataemic rickets | Osteoporosis

Serum 25(OH)D concentration is an indicator of exposure to vitamin D (from skin synthesis and dietary intake). It is recommended that the serum 25(OH)D concentration of all individuals in the UK should not fall below 25 nmol/L at any time of the year. Adequate dietary calcium is necessary for a clinical response to Vitamin D

About : Always check BNF

• A group of steroid hormones regulating body levels of calcium and phosphorus
• Induction of cell differentiation, inhibition of cell growth
• Immunomodulation, and control of other hormonal systems.

Molecules

• Ergocalciferol D2 may be used as a vitamin D supplement
• Cholecalciferol D3 is generated in the skin from 7-dehydrocholesterol when exposed to ultraviolet light.
• Both are prohormones that need two hydroxylations to become active
• These are done in the liver and then the kidney

Physiology

• Vitamin D "Calciferol" (ergocalciferol-D2, cholecalciferol-D3, alfacalcidol)
• Kidney 1-hydroxylation: by 1-alpha-hydroxylase, yielding 1,25-dihydroxycholecalciferol the biologically active form [Tightly regulated by Parathyroid hormone] also called Calcitriol 1 (OH) Vitamin D
• Liver 25-hydroxylation: cholecalciferol is hydroxylated to 25-hydroxycholecalciferol by the enzyme 25-hydroxylase 1,25(OH) Vitamin D which is carried bound to vitamin D-binding protein
• Vitamin D hormone activates its cellular receptor (vitamin D receptor or VDR), which alters the transcription rates of target genes responsible for the biological responses.

Action

• Binds intracellular receptors and modifies gene expression
• Receptor has a very high affinity for 1,25(OH)2 D
• Increases intestinal absorption of calcium, phosphate and magnesium ions
• In bone provide the proper balance of calcium and phosphorus to support mineralization.
• Increases reabsorption of phosphate in the kidney

Vitamin D deficiency : Rickets/Ostemalacia

• Result is a failure of osteoid to calcify in adults is called osteomalacia
• Poor dietary intake or malabsorption
• Poor sunlight exposure
• Vitamin D binders in food
• Vitamin D resistant rickets

Investigations

• A 25-OHD = 30 nmol/L suggests deficiency
• A 25-OHD 30-50 nmol/L suggests possible deficiency

New Indications

• The new advice from PHE is that adults and children over the age of one should consider taking a daily supplement containing 10mcg of vitamin D, particularly during autumn and winter.
• People who have a higher risk of vitamin D deficiency are being advised to take a supplement all year round.
• Consider at-risk groups include people whose skin has little or no exposure to the sun, like those in care homes, or people who cover their skin when they are outside.
• People with dark skin, from African, African-Caribbean and South Asian backgrounds, may also not get enough vitamin D from sunlight in the summer. They should consider taking a supplement all year round as well.

Management

• Check BNF Cholecalciferol (D3)
  • Prevention of vitamin D deficiency: Cholecalciferol (D3)400 units daily.
  • Treatment of vitamin D deficiency:Cholecalciferol (D3) 800 units daily
  • Higher doses may be necessary for severe deficiency.
• Check BNF Ergocalciferol (D2)
  • Malabsorption or liver disease: Ergocalciferol (D2) 40,000 units daily
  • Hypocalcaemia of hypoparathyroidism to achieve normocalcaemia: Ergocalciferol (D2) 100,000 units daily.
  • Prevention of vitamin D deficiency: Ergocalciferol (D2) 400 units daily.
  • Treatment of vitamin D deficiency: Ergocalciferol (D2) 800 units daily or higher
• Check Alfacalcidol in BNF
  • Patients with severe renal impairment requiring vitamin D therapy
  • Treatment: Alfacalcidol 0.5-1 microgram OD adjusted to avoid hypercalcaemia
  • Maintenance: Alfacalcidol 0.25-1 microgram OD adjusted to avoid hypercalcaemia