What this test measures
Vitamin D has two key blood markers. 25-hydroxyvitamin D (25-OH D) reflects total body stores and is the test for routine deficiency screening. 1,25-dihydroxyvitamin D (calcitriol) is the active hormone — made by the kidneys from 25-OH D — and reflects current PTH activity, kidney function, and granulomatous activation.
Because 1,25(OH)2D has a short half-life and is tightly controlled by PTH, it can be normal or even high in vitamin D deficiency (PTH compensates by upregulating activation). It is NOT the right test to screen for vitamin D deficiency — 25-OH D is. Calcitriol is reserved for specific clinical questions.
Why it matters
In India, the right vitamin D test for routine deficiency assessment is 25-OH vitamin D (extremely common to be low). 1,25-dihydroxyvitamin D is occasionally needed in: chronic kidney disease (where reduced renal 1-α-hydroxylase causes low calcitriol, driving secondary hyperparathyroidism), suspected granulomatous diseases (sarcoidosis, TB, lymphoma — macrophages activate 1,25(OH)2D outside the kidney, causing hypercalcaemia), inherited rickets (vitamin D-dependent rickets type 1 and 2), and oncogenic osteomalacia.
Ordering calcitriol for "vitamin D deficiency screening" is a common but costly mistake — it can be normal or high in deficiency.
How to prepare
No fasting strictly required. Avoid active vitamin D analogues (calcitriol, alfacalcidol, paricalcitol) for at least 24–48 hours before — they directly raise the measured value. Mention sun exposure, vitamin D supplements, and any kidney or parathyroid disease.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| 1,25-Dihydroxyvitamin D (pg/mL)[1][2] | 18 – 72 (method-dependent — confirm with lab) | Low calcitriol — chronic kidney disease (failure of renal 1-α-hydroxylase — the dominant cause), severe vitamin D deficiency with depleted substrate, vitamin D-dependent rickets type 1 (1-α-hydroxylase deficiency), oncogenic osteomalacia (FGF-23 mediated), hypoparathyroidism. | High calcitriol — primary hyperparathyroidism (PTH-driven), granulomatous disease (sarcoidosis, TB, lymphoma — macrophages make 1,25(OH)2D), vitamin D-dependent rickets type 2 (receptor resistance — high circulating hormone), William's syndrome, vitamin D toxicity from active analogue use. In granulomatous disease, hypercalcaemia results — calcitriol explains why. |
25-OH D vs 1,25(OH)2D — when to order each
| Test | Best use | Notes |
|---|---|---|
| 25-OH Vitamin D | Routine vitamin D deficiency screening — the right test for almost everyone | Reflects body stores; long half-life; cheap |
| 1,25-Dihydroxyvitamin D | Investigating hypercalcaemia of granulomatous disease (sarcoid, TB, lymphoma); CKD evaluation; inherited rickets | Short half-life; tightly regulated; expensive |
| Total Calcium + PTH + 25-OH D | Standard "vitamin D / bone" workup | Bundle to interpret correctly |
| 1,25(OH)2D in suspected vitamin D deficiency | Wrong test — often normal or high | Use 25-OH D instead |
Frequently asked questions
Is this the right test for vitamin D deficiency?
No. 25-hydroxyvitamin D (25-OH D) is the right test for routine vitamin D deficiency screening. 1,25-dihydroxyvitamin D is for specific clinical situations — granulomatous disease, CKD, inherited rickets, oncogenic osteomalacia.
Do I need to fast?
No fasting required. Hold active vitamin D analogues (calcitriol, alfacalcidol) for 24–48 hours before — they raise the measured value.
Why might calcitriol be high in TB or sarcoid?
Granulomas contain macrophages that have 1-α-hydroxylase activity outside the kidney — they activate vitamin D without the normal PTH controls, raising 1,25(OH)2D and causing hypercalcaemia. This is a useful diagnostic clue in sarcoidosis and active TB.
Why is calcitriol low in CKD?
The kidney is where 25-OH D becomes calcitriol (via 1-α-hydroxylase). When the kidney fails, calcitriol production falls, driving secondary hyperparathyroidism and CKD-MBD. KDIGO recommends targeted treatment with active vitamin D analogues (calcitriol, alfacalcidol, paricalcitol) — not plain vitamin D — once advanced CKD develops.
My 25-OH D is low but 1,25 vitamin D is normal — should I be worried?
No — this is the expected physiology. As 25-OH D falls, PTH rises to maintain calcium homeostasis; PTH upregulates 1-α-hydroxylase, keeping 1,25(OH)2D normal. The 25-OH D value is what guides treatment.
How is hypercalcaemia of sarcoidosis treated?
Hydration, glucocorticoids (which suppress macrophage 1-α-hydroxylase activity), reducing dietary calcium and vitamin D intake, and limiting sun exposure. Treating the underlying disease is essential.
Is calcitriol used as a supplement?
Yes — as a prescription drug in advanced CKD, hypoparathyroidism, and inherited rickets. It bypasses the failing kidney's ability to activate vitamin D. It is not used for ordinary vitamin D deficiency (where plain vitamin D2/D3 is preferred).
Related Vitamins / Nutrition tests
Tests commonly ordered alongside VITAMIN D 1,25 - DIHYDROXY, or that help interpret an unexpected result.
Sources & references
- Endocrine Society — Vitamin D Clinical Practice Guideline · accessed 2026-05-30T00:00:00.000Z
- NCBI StatPearls — Calcitriol · accessed 2026-05-30T00:00:00.000Z
- Mayo Clinic Laboratories — 1,25-Dihydroxyvitamin D · accessed 2026-05-30T00:00:00.000Z
- NIH Office of Dietary Supplements — Vitamin D · accessed 2026-05-30T00:00:00.000Z
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