Vitamin D tests are used to see if bone weakness, bone malformation, or abnormal metabolism of calcium (reflected by abnormal calcium, phosphate, or PTH tests) is occurring as a result of a deficiency or excess of vitamin D.
Since vitamin D is a fat-soluble vitamin and is absorbed from the intestine like a fat, vitamin D tests are sometimes used to monitor and assure individuals with diseases that interfere with fat absorption, such as cystic fibrosis and Crohn's disease that they have adequate amounts of vitamin D. vitamin D tests are also are used to determine the effectiveness of treatment when vitamin D, calcium, phosphate, and/or magnesium supplementation is prescribed
Either 25 hydroxy-vitamin D or 1,25 dihydroxy-vitamin D tests may be requested when a patient has an abnormal blood calcium, phosphate and/or magnesium level or evidence of bone disorders.
If calcium concentration is low or the patient has symptoms of vitamin D deficiency, such as bone malformation in children (rickets) and bone weakness, softness, or fracture in adults (osteomalacia), the 25 hydroxy-vitamin D test usually is requested to identify a possible deficiency in vitamin D.
If calcium concentration is high or the patient has a disease that might produce excess amounts of 1,25 dihydroxy-vitamin D, such as sarcoidosis or some forms of lymphoma, the calcitriol test may also be requested.
Vitamin D tests also may be used to help diagnose or monitor problems with parathyroid gland functioning since parathyroid hormone is essential for vitamin D activation. When vitamin D, calcium, phosphorus, or magnesium supplementation is necessary, vitamin D levels are sometimes measured to monitor treatment effectiveness.