There are two forms of vitamin D that can be measured in the blood, 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D. If calcium is high or the person has a disease that might produce excess amounts of vitamin D, such as sarcoidosis or some forms of lymphoma, 1,25-dihydroxyvitamin D usually is ordered. Vitamin D levels may also be used to help diagnose or monitor problems with parathyroid gland functioning since PTH is essential for vitamin D activation. When vitamin D, phosphorus, calcium, or magnesium supplementation is necessary, vitamin D levels are sometimes measured to monitor treatment effectiveness. Low levels of 1,25-dihydroxyvitamin D can be seen in kidney disease and are one of the earliest changes to occur in those with early kidney failure. High levels of 1,25-dihydroxyvitamin D may occur when there is excess parathryoid hormone or when there are diseases, such as sarcoidosis or certain lymphomas, that can make 1,25-dihydroxyvitamin D outside of the kidneys.