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.
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.
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).
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.
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.