PTH and vitamin D deficiency: However, this measurement is not considered very informative as vitamin D deficiency can lead to an increase in parathyroid hormone, which increases the activity of 1α hydroxylase thus promoting the conversion of bioavailable 25-hydroxyvitamin D into 1,25-dihydroxyvitamin D. Levels of 25-hydroxyvitamin D are present in nanogram/mL levels versus 1,25-dihydroxyvitamin D, which occurs in significantly lower picogram/mL concentrations.