Vitamin D — 25-Hydroxyvitamin D
The most commonly deficient nutrient in northern-latitude adults. Optimal range 40–60 ng/mL — most labs report 'normal' at 20+. What moves it and why toxicity is real above 100 ng/mL.
The 25-hydroxyvitamin D test measures the storage form of vitamin D — reflecting sun exposure, diet, and supplement intake — and is the most commonly deficient nutrient in northern-latitude adults, with implications for bone, immune, and mood pathways.
The biomarker
- Name: Vitamin D — 25-Hydroxyvitamin D (25-OH)
- Units: ng/mL
- Standard reference range: 20–80 (general); deficient < 20
- Optimal range: 40–60
How to read your result
| Value (ng/mL) | Interpretation |
|---|---|
| < 20 | Deficient — supplementation recommended |
| 20–30 | Insufficient |
| 30–40 | Adequate |
| 40–60 | Optimal |
| 60–80 | High — monitor |
| ≥ 100 | Toxic range — stop supplementing, recheck |
What moves the needle
- Sunlight. 10–30 min of midday sun on bare limbs without sunblock is the most efficient natural source; latitude, season, and skin tone all affect conversion.
- Cofactors. Vitamin K2 and magnesium are required for vitamin D metabolism; deficiency in either can blunt the effect of supplementation.
- Supplement dose. Appropriate dose varies widely by baseline status and body weight — discuss with a clinician before starting. Retest after 3 months of any dose change.
- Diet. Fatty fish (salmon, mackerel) and egg yolks are the most meaningful food sources; fortified foods add modest amounts.
Why this test is worth asking for
- Deficiency (< 20 ng/mL) is the norm, not the exception, in populations with limited sun exposure — yet it is invisible without testing.
- The optimal range (40–60) sits well above the standard "non-deficient" cut-off of 20 used by many labs; a result reported as "normal" may still be insufficient for bone and immune function.
- Toxicity from supplementation is real — levels ≥ 100 ng/mL cause hypercalcaemia. Testing prevents over-correction.
Testing first is the whole point — a finger-prick kit gets you the number before anything changes.
Vitamin D (25-OH) Home Test
A finger-prick home test for 25-hydroxyvitamin D, the marker clinicians use to judge vitamin-D status.
See it in the shop →Illustrative photo. We may earn a commission — the affiliate link and the full disclosure live on our shop page.
Related protocols
- Vitamin B12 + Methylmalonic Acid (MMA)
- Zinc — Immune + Hormone Cofactor
- Urine ACR — Early Kidney Marker
Sources
AgeGen lab guides are educational only. We do not provide medical diagnosis, prescribe brands, or recommend specific doses. Talk to a licensed clinician before changing your supplement or medication routine.
