Vitamin D is removed from bloodstream into various tissues Biologic half-life is ~ 60 days Eventually converted to 25-hydroxy-vitamin D in the hepatocytes Vitamin D3 is synthesized in the skin from UV Vitamin D2 is derived from irradiation of certain fungi Both Vitamin D2 and Vitamin D3 create 1,25-dixydroxyvitamin D (the active form) In Canada, most Vitamin supplements consist of Vitamin D3. High-dose preparations are Vitamin D2 (prescription) The serum concentration of 25-hydroxyvitamin D3 is the best indicator of the nutritional and functional status of vitamin D Many techniques are widely available for measuring 25-hydroxyvitamin D Immunoassays – most reliable for clinical decisions Serum 25-hydroxyvitamin D level indicates the effectiveness of vitamin D therapy Its half-life in the body is 15-20 days With standard dose supplementation, 25-hydroxyvitamin D plateaus after three to four months Monitoring should take place at least three months after start of
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