Vitamin D testing and why levels drift

The test worth having is 25-hydroxyvitamin D, reported in nanomoles per litre, and the number it returns is best read as a seasonal position rather than a fixed trait. Vitamin D drifts more than almost any other marker on a standard panel — by month, by latitude, by skin tone, by how much of you is uncovered outdoors and by how much body fat is holding onto it.

That drift is why a single reading is so often over-interpreted in both directions, and why the month you were tested belongs next to the result.

Key takeaways

• 25(OH)D is the storage form and the correct test. 1,25-dihydroxyvitamin D answers a different, narrower question.

• Australian results are reported in nmol/L; overseas sources often use ng/mL, which is roughly 2.5 times smaller.

• Levels typically peak in late summer and bottom out at the end of winter, so timing changes the interpretation.

• Vitamin D has a long half-life, so a retest inside about three months rarely shows anything meaningful.

• Medicare rebates for vitamin D testing in Australia are restricted to defined clinical criteria.

Which vitamin D test should you ask for?

25-hydroxyvitamin D, usually written 25(OH)D. It is the circulating storage form, it has a half-life measured in weeks, and it reflects your combined intake from sunlight, diet and supplements. It is what essentially every guideline and every threshold refers to.

The alternative, 1,25-dihydroxyvitamin D, is the active hormone. It has a half-life of hours, it is tightly regulated by parathyroid hormone, and it can appear entirely normal — or even high — in someone who is deficient, because the body prioritises maintaining it. It has genuine uses in specific kidney, calcium and granulomatous conditions, but it is the wrong test for the question most people are asking.

What do the numbers mean?

The thresholds commonly used in Australia and New Zealand are set primarily around bone health, and they are expressed in nmol/L:

Two caveats matter more than the table. Because levels fall through winter, guidance is usually expressed as a winter-endpoint target — which in practice means sitting somewhat higher at the end of summer to still be adequate months later. And these thresholds are bone-health thresholds; claims about optimal levels for immunity, mood or longevity rest on considerably weaker evidence than the bone-health figures do, and higher is not reliably better — the same distance between marketing and evidence that runs through the whole supplement category. Vitamin D is fat-soluble and genuinely toxic in overdose.

Why do vitamin D levels drift so much?

• Season. The dominant factor. Levels typically peak in late summer and trough at the end of winter, and the swing can be large enough to move someone across two bands.

• Latitude. The further south you live in Australia, the less UVB reaches the ground in winter, and the longer the exposure needed to produce anything at all. Melbourne and Hobart are meaningfully different from Brisbane in this respect.

• Skin tone. Higher melanin is protective against UV damage and correspondingly slows vitamin D synthesis, so more exposure is needed for the same result.

• How you spend your time. Indoor work, covering clothing and consistent sunscreen use all reduce synthesis. Glass blocks UVB, so time by a sunny window produces none.

• Body composition. Vitamin D is fat-soluble and distributes into adipose tissue, which lowers circulating levels at a given intake.

• Age. Skin synthesises vitamin D less efficiently as we get older.

• Absorption and medication. Coeliac disease, inflammatory bowel disease, bariatric surgery and some medications all reduce absorption or accelerate breakdown.

None of this makes the test unreliable. It makes the circumstances part of the result — the same principle that applies to every marker on a panel. A result of 58 in February and a result of 58 in August describe two quite different situations.

When is a retest actually informative?

Vitamin D has a half-life of roughly two to three weeks, and stores take considerably longer than that to reach a new steady state. Around three months after a change in supplementation, sun exposure or season is the earliest point at which a repeat test tells you much. Testing at six weeks mostly measures impatience.

If you are comparing across years rather than within one, test in the same month each time. An annual panel drawn every August compares like with like; one drawn in August and then the following March does not.

What does vitamin D depend on, and what depends on it?

Vitamin D does not act alone, and a low result is rarely the whole story. Calcium and parathyroid hormone sit in the same regulatory loop, and PTH rising as vitamin D falls is a recognisable pattern. Magnesium is required for the enzymes that activate vitamin D, so magnesium status affects what a given level does. Where deficiency is unexpected or does not correct as expected, absorption becomes the question, which brings coeliac screening and gut health into view.

Low vitamin D also travels with several other findings often enough to be worth noting. It is frequently observed alongside autoimmune thyroid disease, which is one more reason to read a full thyroid panel rather than TSH alone. It overlaps with the fatigue picture that iron studies usually explain better. And it becomes more consequential through the perimenopausal years, when bone density trajectory changes.

Who should be tested?

In Australia, Medicare rebates for vitamin D testing are restricted to people meeting defined clinical criteria — which include things like signs of deficiency or bone disease, malabsorption, deeply pigmented skin, chronically low sun exposure, certain medications, and specific life stages. Testing outside those criteria is generally out of pocket, which is worth knowing before you ask.

That restriction exists because population-wide screening of people at low risk changes very few decisions. The people for whom the result genuinely alters something are the ones the criteria describe.

Frequently asked questions

What is a good vitamin D level?

Guidance in Australia is generally framed as at least 50 nmol/L at the end of winter, which implies a somewhat higher level at the end of summer to still be adequate months later. These are bone-health thresholds; claims about higher optimal ranges for other outcomes rest on weaker evidence.

What is the difference between 25(OH)D and 1,25(OH)2D?

25(OH)D is the storage form and the correct test for assessing status. 1,25-dihydroxyvitamin D is the short-lived active hormone, tightly regulated, and can look normal in a deficient person. It is reserved for specific calcium, kidney and granulomatous conditions.

Can I get enough vitamin D from sunlight in winter?

It depends heavily on latitude and skin tone. In southern Australia, winter UVB is low enough that synthesis is limited even with outdoor time, which is precisely why levels fall through the season. Sun exposure advice also has to be balanced against skin cancer risk.

How long does it take for vitamin D levels to rise?

Weeks to months. A repeat test around three months after a change is usually the earliest point at which the result reflects a new steady state.

Can vitamin D be too high?

Yes. It is fat-soluble and accumulates, and excessive intake can raise calcium to harmful levels. High-dose supplementation is not a case where more is safer, and it should be supervised.

Reading vitamin D alongside everything else

A vitamin D result is far more useful next to calcium, PTH, thyroid, iron and inflammation than it is on its own page — and next to the same result from the same month last year. That is how every DearLife 360 is written: markers weighed together, banded, and explained in plain language. If you need a comprehensive panel arranged, we can organise it, or get in touch if you are not sure where to start.