Vitamin D (25-Hydroxy)
The storage form of vitamin D and the correct test for vitamin D status — not 1,25-dihydroxy, which is frequently ordered by mistake.
Functions more like a hormone than a vitamin, with receptors throughout the body. Deficiency is extremely common and associated with impaired immunity, bone loss, low mood, and reduced testosterone.
The plan of attack
In this order. Most people start at step four, which is why they change five things at once and learn nothing.
- Confirm the number is real
D2 vs D3, and assay variation. Some assays measure only D3, others total. Between-lab variation in vitamin D testing is larger than most people assume. Stay with one lab. Do not chase small differences. - Read it with its partner
Retest 8–12 weeks after changing your dose — response varies enormously between individuals. Draw it alongside: Magnesium, RBC, Parathyroid Hormone & Calcium. - Work out which direction is yours
If it's high — Excess supplementation — hypercalcemia, nausea, kidney stones.
If it's low — Fatigue, frequent illness, low mood, bone loss, muscle weakness. - Fix it in this order
Nutrition. Fatty fish, egg yolks, fortified foods — but diet alone rarely corrects a real deficiency. Sensible sun exposure is the natural route.
Lifestyle. Regular midday sun exposure where feasible; fat loss improves bioavailability.
Supplements. D3, dosed to your labs. Take with a fat-containing meal — absorption is substantially better. Co-factors matter: vitamin K2 (MK-7) directs calcium to bone rather than arteries, and magnesium is required to activate vitamin D — supplementing D while magnesium-deficient blunts the benefit.
Hormones. No hormonal intervention needed — this is a straightforward supplementation fix.
Compounds. One of the highest-yield, cheapest corrections in all of bloodwork, with plausible downstream effects on testosterone and immune function.
Work down the list, not across it. Adding a compound on top of an unfixed diet is why generic protocols fail. - Retest
8–12 weeks after a dose change; then every 6–12 months. Change one thing at a time, or the retest can't tell you which thing worked.
How to fix it
📚 Endocrine Society CPG — Evaluation, Treatment and Prevention of Vitamin D Deficiency. Holick MF, NEJM 2007.
🩸 Test your Vitamin D (25-Hydroxy)
Order directly through Marek Diagnostics — no doctor's visit needed, drawn at any Quest location in the US. Code CAMERON applies 10% off automatically.
Order this test — 10% off → Browse all 102 markers →What Vitamin D (25-Hydroxy) is usually tested alongside
On its own, one marker is a data point. These panels include Vitamin D (25-Hydroxy) plus the markers that make it interpretable — each names every test, why it is on the list, and loads the whole set into your cart in one click at 10% off.
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What moves your Vitamin D (25-Hydroxy)
3 compounds and 4 supplements in the Vault have a documented effect on this marker, or are a reason to have measured it first:
Browse all 237 compounds & 350 supplements →
Would you feel it? Symptoms Vitamin D (25-Hydroxy) helps explain
People rarely search for a marker — they search for how they feel. These are the complaints where this marker is worth checking, and whether it is a first-line test or a follow-up once the obvious causes are ruled out.
Why your Vitamin D (25-Hydroxy) might be wrong
Most abnormal results are interference, not disease. These are the things that measurably move this specific marker — check them before you change anything. Each one says whether the number is wrong and should be repeated, badly timed and should be redrawn, or real with a cause worth explaining.
Levels swing substantially between late summer and late winter — the same person can differ by 20 ng/mL across the year.
Test at the same time of year when tracking, and know which end of the swing you measured.
Some assays measure only D3, others total. Between-lab variation in vitamin D testing is larger than most people assume.
Stay with one lab. Do not chase small differences.
A weekly or monthly bolus produces a peak that does not represent your steady state.
Test at least a week after a large dose.
What Vitamin D (25-Hydroxy) means in combination
A single marker tells you a little. Combinations tell you the story — these are the named patterns this marker takes part in, and what each one points at.
Female pattern shedding is very often nutritional and thyroidal before it is hormonal. Each of these alone can shed hair; together they reliably do.
Ferritin above 50 is the usual threshold for regrowth — normal is not the same as sufficient here. Fix all three and give it 3–6 months; the hair cycle is slow and nothing shows in weeks.
Multiple unrelated nutrients low at once is rarely four separate dietary problems — it points at absorption. Coeliac, inflammatory bowel disease, chronic PPI use, gastric surgery or low stomach acid all produce this.
Stop supplementing harder and find the cause. Coeliac serology is cheap and routinely skipped. Review PPI and metformin use. The Gut Health panel is built for this picture, and fixing absorption fixes all of them at once.
A well-run plant-based diet is fine, but four nutrients are genuinely harder to get and this combination shows a diet that has not been supplemented deliberately.
B12 is non-negotiable and non-optional — deficiency causes irreversible neurological damage. Add iron with vitamin C away from tea and coffee, plus vitamin D, zinc and an algal omega-3. Retest in 3 months.
When vitamin D is short, PTH rises to hold calcium steady by pulling it out of bone. Calcium looks perfectly normal on the panel while bone density is being spent to keep it that way.
Correct the vitamin D and PTH usually follows within a few months — recheck both together. Ensure adequate calcium, magnesium and vitamin K2, and load the skeleton with resistance training.
What to test next
Markers rarely answer alone. These are the ones that put Vitamin D (25-Hydroxy) in context — each with its own full breakdown.
Frequently asked questions
Deficient <20 · Insufficient 20–29 · Sufficient 30–100 ng/mL Ranges vary by laboratory and assay — always compare to the range printed on your own report.
40–60 ng/mL. Above 100 risks toxicity and hypercalcemia — more is not better.
Excess supplementation — hypercalcemia, nausea, kidney stones.
Fatigue, frequent illness, low mood, bone loss, muscle weakness.
You can order Vitamin D (25-Hydroxy) directly through Marek Diagnostics without a doctor's visit — drawn at any Quest Diagnostics location in the US. Code CAMERON applies 10% off automatically.