Vitamin D TestThe Vitamin D test measures the level of 25 hydroxyvitamin D in your blood, which is the active form your body needs. Vitamin D is absorbed through sunlight or fortified foods and stored in fat cells until required. Your liver and kidneys then convert it into its active form, 25 hydroxyvitamin D. A deficiency in this vitamin can indicate issues like bone disease, nutritional deficiencies, or other metabolic disorders. Preparing for Your Sample