Vitamins, minerals and amino acids
Vitamine D (1,25-dihydroxy)
The active form of vitamin D; not the measure for a vitamin D deficiency, but for calcium and kidney problems.
Everything about this testWhat it measures
Vitamin D from sunlight and food is first converted in the liver into 25-hydroxyvitamin D, the storage form. In the kidneys this is turned into 1,25-dihydroxyvitamin D (calcitriol), the active form that regulates the absorption of calcium from the gut. Parathyroid hormone (PTH) controls this last step. The active form is present in the blood at a concentration a thousand times lower than the storage form and disappears from it again within a few hours.
- A cause of too much calcium in the blood, for example with sarcoidosis or an overactive parathyroid gland
- A disturbed conversion of vitamin D with severe kidney damage
- Rarely: a hereditary disorder in the conversion or action of vitamin D, which usually already causes symptoms in childhood
High
A raised active vitamin D occurs when extra is made outside the kidneys. This happens in diseases with small foci of inflammation in tissue, such as sarcoidosis, and rarely with a malignant disease of the lymph nodes. An overactive parathyroid gland also raises the value. A raised value often goes together with too much calcium; symptoms that may go with it are great thirst, frequent urination, constipation, nausea and tiredness. A supplement with ordinary vitamin D mainly raises the storage form and not so much the active form.
Low
A low active vitamin D mainly fits severe kidney damage, because the kidneys can no longer carry out the conversion properly. An underactive parathyroid gland and a low magnesium can also lower the value. Important: a low value is not a good measure of a vitamin D deficiency. With a vitamin D deficiency, parathyroid hormone rises, so the active form can actually be normal or even raised while the storage form is low. For vitamin D status, 25-hydroxyvitamin D is therefore the better measure.
If your result is abnormal
With an abnormal value, the GP assesses the result together with calcium, phosphate, parathyroid hormone and kidney function. With a raised calcium together with a raised active vitamin D, referral to an internist (specialist in internal medicine) usually follows to find the cause; with severe kidney damage, care is managed by the nephrologist.
Reliability
This test is not a measure of vitamin D status; the storage form 25-hydroxyvitamin D is. The active form is mainly informative with too much calcium, with severe kidney damage or when a rare hereditary disorder is suspected. The concentration is very low and fluctuates from hour to hour under the influence of parathyroid hormone, phosphate and calcium, so a single value cannot be interpreted without those other tests.
Limitations
- The half-life is short (hours), so the value fluctuates strongly; the result can only be assessed together with calcium, phosphate, parathyroid hormone and kidney function.
- A vitamin D deficiency can give a normal or raised result; the test therefore does not rule out a deficiency.
- The measurement is technically difficult because of the very low concentration; methods and reference values differ per laboratory.
- Medicines that affect the liver, such as some antiepileptic drugs, and a low magnesium can disturb the result.
How common is this
A vitamin D deficiency is common in the Netherlands, but it is measured with the storage form and not with this test. Conditions in which the active form is abnormal, such as sarcoidosis and severe kidney damage, are much rarer; sarcoidosis affects about 1 to 2 in 10,000 people.
Who is this useful for
Useful with a confirmed high calcium in the blood, with severe kidney damage and when sarcoidosis or a rare hereditary disorder of vitamin D metabolism is suspected. This test is not suitable for finding out whether you have enough vitamin D; 25-hydroxyvitamin D is measured for that. In adults without symptoms the result cannot be interpreted.
Downsides and risks
- The result is easily confused with the ordinary vitamin D test, so a deficiency can be missed or wrongly assumed.
- An abnormal value without abnormal calcium easily leads to unnecessary further testing.
Sources
Always discuss an abnormal result with your GP. If a result is strongly abnormal, we try to reach you as soon as possible. Read your result yourself as well and contact your GP if in doubt. A test does not make a diagnosis. Read more about risks and results