Calcitriol (1,25-Dihydroxyvitamin D) Converter
Calcitriol (1,25-Dihydroxyvitamin D) Converter
Convert 1,25-dihydroxyvitamin D between pg/mL, ng/L and pmol/L — and read the result knowing that calcitriol is a different analyte from 25-hydroxyvitamin D and is not the test for vitamin D status.
Calcitriol (1,25-Dihydroxyvitamin D) converter
Mass ⇄ molarCalcitriol 40 pg/mL, adult male
The conversion, and which vitamin D metabolite it belongs to
pg/mL = pmol/L ÷ 2.40015
because 2.40015 = 1 pg/mL ÷ 416.64 g/mol, the molecular weight of calcitriol
- pg/mL = ng/L
- a picogram per millilitre and a nanogram per litre are the same concentration, so no arithmetic is needed between them. Laboratories in the United Kingdom and Europe usually report pmol/L; pg/mL appears on most North American reports
- MW 416.64 — calcitriol
- 1,25-dihydroxyvitamin D₃, C₂₇H₄₄O₃. The hormone: made in the proximal tubule by 1-alpha-hydroxylase, acting on the vitamin D receptor to raise intestinal calcium and phosphate absorption, with a half-life of a few hours and a circulating concentration a thousandfold lower than its precursor
- MW 400.64 — 25-hydroxyvitamin D, the other test
- the storage form, and the one that measures vitamin D status. It circulates in nanograms per millilitre rather than picograms, has a half-life of two to three weeks, and converts at 2.49601 nmol/L per ng/mL. Two different molecules, two different units, two different questions — use the <a href="/vitamin-d-unit-converter/">25-hydroxyvitamin D converter</a> for that one
- the thousandfold gap
- a typical 25-hydroxyvitamin D is around 30 ng/mL and a typical calcitriol around 40 pg/mL. If a number in the tens has arrived labelled ng/mL it is the storage form; in the tens labelled pg/mL, the hormone. Mixing the two up is the commonest error this page exists to prevent
Worked example
Calcitriol 40 pg/mL, adult male
40 pg/mL = 40.0 ng/L — the same concentration written the other way
40 × 2.40015 = 96.0 pmol/L, using calcitriol's molecular weight of 416.64
96.0 pmol/L sits inside Mayo's male interval of 18–64 pg/mL, which is 43.2–153.6 pmol/L
Going back: 96.0 ÷ 2.40015 = 40.0 pg/mL
What this result does not tell you is whether the patient is vitamin D deficient. A 40 pg/mL calcitriol is entirely compatible with a 25-hydroxyvitamin D of 8 ng/mL and florid deficiency — the parathyroid drive that deficiency produces is exactly what keeps the calcitriol up
The two vitamin D tests, and which question each answers
| 25-hydroxyvitamin D | 1,25-dihydroxyvitamin D (calcitriol) | |
|---|---|---|
| What it is | The storage form — the sum of what the skin made and the diet supplied | The active hormone, made from it in the kidney |
| Typical concentration | Nanograms per millilitre — tens | Picograms per millilitre — tens. A thousandfold lower |
| Half-life | Two to three weeks | Four to six hours |
| Regulation | Essentially unregulated — it tracks supply | Tightly regulated by PTH, phosphate and FGF23 |
| Measures vitamin D status? | Yes. This is the test. | No. Never order it for this. |
| Behaviour in vitamin D deficiency | Low — that is the diagnosis | Normal or raised, because secondary hyperparathyroidism drives its production |
| Converts at | × 2.49601 ng/mL → nmol/L | × 2.40015 pg/mL → pmol/L |
The narrow list of situations where a calcitriol is the right test
| Situation | What is being asked | Expected pattern |
|---|---|---|
| Hypercalcaemia with a suppressed PTH and no malignancy found | Is there extrarenal 1-alpha-hydroxylase activity — sarcoidosis, tuberculosis or another granulomatous disease, or lymphoma? | Raised or inappropriately high-normal calcitriol with suppressed PTH and a normal or low 25-hydroxyvitamin D |
| Rickets or osteomalacia that does not fit the usual pattern | Is this 1-alpha-hydroxylase deficiency (vitamin D-dependent rickets type 1A) or vitamin D receptor resistance (type 2)? | Type 1A: low calcitriol with a normal 25-hydroxyvitamin D. Type 2: very high calcitriol, because the receptor cannot respond |
| Hypophosphataemia with renal phosphate wasting | Is FGF23 excess suppressing 1-alpha-hydroxylase — tumour-induced osteomalacia, X-linked hypophosphataemia? | Low or inappropriately normal calcitriol for the degree of hypophosphataemia |
| Chronic kidney disease, in a research or specialist setting | How much 1-alpha-hydroxylase capacity is left? | Falls as GFR falls, usually before the PTH rises visibly |
How to read a calcitriol alongside the results that give it meaning
| Calcitriol | With calcium and PTH | Reading |
|---|---|---|
| Normal or high | Hypercalcaemia, PTH suppressed | Granulomatous disease or lymphoma until proven otherwise. The macrophage 1-alpha-hydroxylase is not subject to normal feedback |
| Normal or high | Normal calcium, PTH raised, 25-hydroxyvitamin D low | Vitamin D deficiency with secondary hyperparathyroidism. The calcitriol is a red herring — treat the deficiency |
| High | Hypercalcaemia, PTH raised | Primary hyperparathyroidism; PTH is driving the 1-alpha-hydroxylase |
| Low | Low or normal calcium, PTH raised, eGFR reduced | Chronic kidney disease — mineral and bone disorder. The kidney cannot hydroxylate, whatever the PTH asks for |
| Low | Hypocalcaemia, PTH low or undetectable | Hypoparathyroidism. Without PTH there is no drive to 1-alpha-hydroxylate |
A hormone, not a status marker
1,25-dihydroxyvitamin D — calcitriol — is reported either as a mass concentration in picograms per millilitre, identical to nanograms per litre, or as an amount of substance in picomoles per litre. The bridge is its molecular weight, 416.64, so one picogram per millilitre is 2.40015 picomoles per litre. The arithmetic is straightforward. The reason this page exists is the analyte, not the arithmetic.
Calcitriol is not the vitamin D test. 25-hydroxyvitamin D is the storage form, it circulates at concentrations a thousand times higher, it has a half-life of two to three weeks, and it tracks supply — which is precisely what makes it a measure of status. Calcitriol is the hormone made from it in the proximal tubule by 1-alpha-hydroxylase. It has a half-life of a few hours and it is held where the body wants it by parathyroid hormone, phosphate and FGF23. A tightly regulated hormone with a short half-life is, almost by definition, a poor measure of the size of the store it is drawn from.
The clinically dangerous consequence is that calcitriol is commonly normal, and can be frankly raised, in genuine vitamin D deficiency. As 25-hydroxyvitamin D falls, calcium absorption falls with it, the parathyroids respond, and the resulting secondary hyperparathyroidism drives what substrate remains hard through 1-alpha-hydroxylase. A patient with a 25-hydroxyvitamin D of 8 ng/mL and obvious osteomalacia can return a mid-normal calcitriol. If that number is the one that was ordered, the deficiency is reported as excluded, and the patient is not treated. Ordering the wrong vitamin D test is not a neutral error.
The legitimate uses are narrow and specific. The first is hypercalcaemia with a suppressed parathyroid hormone and no malignancy identified, where a raised calcitriol points to extrarenal 1-alpha-hydroxylase in granulomatous tissue — sarcoidosis, tuberculosis, other granulomatous disease — or in lymphoma. That enzyme is not subject to the feedback that restrains the renal one, so it keeps producing hormone into a hypercalcaemic patient. The second is rickets or osteomalacia that does not behave: a low calcitriol with a normal 25-hydroxyvitamin D suggests 1-alpha-hydroxylase deficiency, and a very high one suggests vitamin D receptor resistance. The third is renal phosphate wasting, where FGF23 excess suppresses the enzyme. In each of those the calcitriol is read with the calcium, the PTH, the phosphate and the renal function — never alone.
Frequently asked questions
How do you convert calcitriol from pg/mL to pmol/L?
Multiply by 2.40015, which is one picogram per millilitre divided by calcitriol’s molecular weight of 416.64 g/mol. A calcitriol of 40 pg/mL is 96.0 pmol/L. Divide by the same factor to go back. pg/mL and ng/L are numerically identical, so no conversion is needed between those two.
Is calcitriol the same as vitamin D?
No. Calcitriol is 1,25-dihydroxyvitamin D, the active hormone. Vitamin D status is measured as 25-hydroxyvitamin D, the storage form, which circulates about a thousand times more concentrated, has a half-life of two to three weeks rather than a few hours, and is reported in ng/mL or nmol/L rather than pg/mL or pmol/L. They are different molecules answering different questions.
Why can calcitriol be normal in vitamin D deficiency?
Because it is regulated rather than passive. As 25-hydroxyvitamin D falls, calcium absorption falls, parathyroid hormone rises, and that secondary hyperparathyroidism drives 1-alpha-hydroxylase to convert the remaining substrate. The result is a normal or even raised calcitriol in a patient who is severely deficient. A normal calcitriol therefore never excludes vitamin D deficiency — check the 25-hydroxyvitamin D.
When is it worth measuring calcitriol?
Three situations, broadly. Hypercalcaemia with a suppressed PTH and no malignancy found, where a high calcitriol suggests granulomatous disease such as sarcoidosis or tuberculosis, or lymphoma. Rickets or osteomalacia that does not fit the usual pattern, where 1-alpha-hydroxylase deficiency or vitamin D receptor resistance is suspected. And renal phosphate wasting driven by FGF23. Outside those, it is rarely indicated.
What does a high calcitriol mean?
It depends entirely on the calcium and the PTH beside it. With hypercalcaemia and a suppressed PTH it suggests extrarenal 1-alpha-hydroxylase activity in granulomatous tissue or lymphoma. With hypercalcaemia and a raised PTH it is primary hyperparathyroidism. With a normal calcium, a raised PTH and a low 25-hydroxyvitamin D it means vitamin D deficiency with secondary hyperparathyroidism — the opposite of what a raised active hormone appears to say.
Related calculators
References
- Mayo Clinic Laboratories. Test ID: DHVD — 1,25-Dihydroxyvitamin D, Serum. Reference values: males ≥16 years 18–64 pg/mL, females ≥16 years 18–78 pg/mL, under 16 years 24–86 pg/mL. Liquid chromatography tandem mass spectrometry.
- Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2011;96(7):1911–1930. doi:10.1210/jc.2011-0385
- Tebben PJ, Singh RJ, Kumar R. Vitamin D-mediated hypercalcemia: mechanisms, diagnosis, and treatment. Endocr Rev. 2016;37(5):521–547. doi:10.1210/er.2016-1070
- Bikle DD. Vitamin D metabolism, mechanism of action, and clinical applications. Chem Biol. 2014;21(3):319–329. doi:10.1016/j.chembiol.2013.12.016
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.