Calcitriol (1,25-dihydroxyvitamin D) test: what it shows (1,25(OH)₂D)

1,25-dihydroxyvitamin D, also called calcitriol, is the biologically most active form of vitamin D in the body. It is formed in two steps: in the liver, vitamin D is converted into 25-hydroxyvitamin D (this is what the usual, most commonly performed vitamin D test measures), and then, in the kidneys, the enzyme 1-alpha-hydroxylase converts this form into active calcitriol. Calcitriol acts on the lining of the gut so that more calcium is absorbed from food, and together with parathyroid hormone (PTH) it regulates the amount of calcium and phosphorus in the blood and the mineralisation of bone. Because calcitriol production depends on kidney function, this test is not used for everyday assessment of vitamin D sufficiency — the usual 25-OH vitamin D test is enough for that — but for narrower situations: to monitor how kidney disease affects vitamin D metabolism, or to work out the cause of an unclear calcium level in the blood, MedlinePlus states.

• When there is a need to assess how chronic kidney disease affects vitamin D metabolism and the calcium–phosphorus balance
• When an unusual (raised or lowered) amount of calcium has been found in the blood and the cause is being looked for
• When disorders of parathyroid hormone (PTH) regulation are suspected — the test is then done together with PTH, calcium and phosphate
• In rare cases, when the usual 25-OH vitamin D test does not give enough information about a disorder of vitamin D metabolism

We do not publish a reference range for 1,25-dihydroxyvitamin D on this page: laboratories use their own units and their own reference limits, so compare your result with the range printed on your own report. A single result on its own decides nothing — it is assessed together with calcium, phosphate and parathyroid hormone, and the final cause is determined by a doctor.

MedlinePlus states that the 1,25-dihydroxyvitamin D test is most often done not to assess vitamin D sufficiency, but to monitor kidney function disorders or to establish the cause of an unclear raised calcium level in the blood. A raised result is usually assessed together with calcium, phosphate and parathyroid hormone (PTH), and the final cause is determined by a doctor. The causes considered:

• Disorders of calcium metabolism whose cause is being looked for with this very test (for example, a raised calcium concentration in the blood)
• Disorders of parathyroid hormone (PTH) function — PTH and calcitriol together regulate the amount of calcium in the blood

What usually comes next:

• Calcium, ionised calcium and phosphate tests together — they help assess the overall picture
• A parathyroid hormone (PTH) test — it is closely linked to calcitriol production
• Assessment of kidney function (creatinine, GFR), because calcitriol is produced in the kidneys

A low result: what it can mean

A low amount of 1,25-dihydroxyvitamin D is most often linked to kidney function disorders, because the last step of vitamin D activation takes place precisely in the kidneys. MedlinePlus states that this test is used specifically to monitor kidney problems and to establish the cause of calcium metabolism disorders. The causes considered:

• Kidney function disorders that reduce calcitriol production
• Disorders of parathyroid hormone (PTH) function that affect the formation of calcitriol

Symptoms listed alongside a low result:

• Bone pain
• Muscle weakness or pain
• Soft or deformed bones
• Weak bones and fractures

What usually comes next:

• Kidney function tests (creatinine, GFR)
• Calcium, phosphate and PTH tests together
• Discussing with your doctor whether the usual 25-OH vitamin D test is needed