This test is used to diagnose intestinal malabsorption and vitamin D deficiency or intoxication and to monitor therapeutic response in patients being treated for vtamin D-related disorders.
25-Hydroxyvitamin D (25OHD) is the major circulating form of vitamin D and the precursor of the active form (1,25-dihydroxyvitamin D). Because of its long half-life, 25OHD measurements are useful for assessing vitamin D status in patients.
Vitamin D occurs in 2 forms: vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Vitamin D3 is obtained from foods of animal origin and from ultraviolet light-stimulated conversion of 7-dehydrocholesterol in the skin, whereas small amounts of vitamin D2 are obtained from foods of plant origin. Both forms of the vitamin are used to fortify various foods and in over-the-counter supplements, and vitamin D2 is used in a high potency (50,000 IU) formulation for treating severe vitamin D deficiency.
