Vitamin D
Vitamin D is produced in the skin when exposed to radiation from the sun.
Vitamin D attaches to vitamin D-binding protein in the blood and carried to the
liver, where it undergoes hydroxylation into 25-hydroxyvitamin D. It is
again hydroxylated in the kidney to 1, 25 dihydroxyvitamin D. Vitamin D
enhances the efficiency of the small intestine to absorb dietary calcium and
phosphorous, and mobilizes calcium and phosphorous stores from the bone.
Serum 25-Hydroxyvitamin D
The vitamin-D metabolite 25-hydroxyvitamin D (25-OH-D) can be measured in the
serum as an indication of liver stores. Concentrations are measured by a
competitive protein-binding assay. The half-life of 25-OH-D is three weeks;
serum levels can reflect vitamin D from diet and sunlight production over a
period of several weeks to several months. Normal levels in circulation fall
between 8 and 60 ng/mL (20-150 nmol/L). Values below 10 ng/mL (25 nmol/L) are
considered to indicate vitamin-D deficiency. Toxicity of vitamin D can be seen
at concentrations greater than 150 ng/mL (375 nmol/L).
Serum vitamin-D levels may increase in summer (with increased exposure to sun),
and may decrease with old age, or as a result of malabsorption diseases.
Serum 1, 25-Dihydroxyvitamin D
The metabolite 1, 25-dihydroxyvitamin D (1,25-OH2D) is measured with a
competitive receptor-binding assay. The assay provides a direct measure of the
biological activity of 1,25 OH2D. Normal serum values fall between 16 and 60
picograms (pg) per ml (38-144 pmol/L).
Serum Alkaline Phosphatase Activity
This functional measure of vitamin D is an indirect measure of vitamin-D status.
A high level of activity of alkaline phosphatase indicates a vitamin-D
deficient state.
Urinary Calcium Excretion
Measurement of 24-hour urinary calcium excretion can be used as an estimate of
calcium absorption, but is not a very specific test of vitamin-D status.
