Vitamin D (1, 25-OH)

General Information

1,25-dihydroxy [1,25(OH)2D] is the active form
of vitamin D and is produced primarily in the kidney by the
hydroxylation of vitamin D, 25-hydroxy [25(OH)D]. It is
tightly bound to the transport protein DBP in circulation and
has a half-life of four to six hours. Plasma levels of 1,25(OH)2D
are increased in calcium-deficient individuals, children, and
during pregnancy and lactation. Decreased levels of vitamin D
25(OH) are seen in the elderly due to malabsorption, decreased
vitamin D intake, or lack of sun exposure. Since vitamin D is
stored in the body, it may accumulate to toxic levels due to
long-term therapeutic use or excessive self-medication, but
this is rare.

Although there are many disorders associated with abnormal plasma concentrations of 25(OH)D and 1,25(OH)2D,laboratory measurements are of value in only a few clinical conditions.