Vitamin D Testing: Who Needs It and What the Numbers Mean

It's one of the most common requests we hear in Pattaya: "Can you check my vitamin D?" It makes sense — you've moved somewhere drenched in sunshine, you feel a bit tired, and vitamin D has a reputation as the sunshine vitamin that fixes low energy. The honest answer is a little more interesting than yes or no. Deficiency really is common here, even under a tropical sun — but vitamin D is also one of the most over-ordered and over-supplemented tests in modern medicine. This is a plain-language guide to what the test actually measures, what the numbers mean, who genuinely benefits, and why "sunny country" and "healthy levels" are not the same thing. It's general education, not a diagnosis; your own result is read by a doctor who knows your history.

What the vitamin D test actually measures

When a clinic checks your vitamin D, the test you want is 25-hydroxyvitamin D, usually written as 25(OH)D. This is the storage form of vitamin D circulating in your blood, and it's the standard because it reflects everything you've taken in — from sun, food and supplements combined — over roughly the past two weeks, thanks to its longer half-life ¹².

There's a second, more active form called 1,25-dihydroxyvitamin D, but it is not the right test for ordinary screening. Its levels are tightly regulated and can look normal even when your overall stores are low, so ordering it by mistake is a classic way to get a misleading "all clear" ². If you're checking your vitamin D status, 25(OH)D is the number that matters.

What do the numbers mean?

Vitamin D results come in two units depending on the lab — nanomoles per litre (nmol/L) or nanograms per millilitre (ng/mL) — and the conversion is simply 1 ng/mL ≈ 2.5 nmol/L ⁴. That alone causes a lot of confusion when people compare reports from different countries, so always check which unit your report is using.

The commonly used thresholds look like this ²⁴:

Here's the honest complication: the definitions are not unanimous. The US National Institutes of Health and the Institute of Medicine treat 50 nmol/L (20 ng/mL) as adequate for most people, while the Endocrine Society draws the line higher and calls anything below 75 nmol/L (30 ng/mL) deficient ²⁴. That genuine disagreement is a real source of patient confusion — the very same result can be labelled "normal" by one lab and "low" by another. It's one more reason a number is read in context rather than as a verdict, much as with any blood-test result.

What does vitamin D actually do?

Strip away the hype and vitamin D's core, well-established job is about calcium and bones. Without enough of it, your gut absorbs only about 10–15% of the calcium in your diet; with adequate vitamin D, that rises to 30–40% ². That's why a serious shortfall has clear bone consequences: rickets (soft, deforming bones) in children, and osteomalacia — softening of the bones — in adults ².

This is also where honesty matters. Vitamin D is involved in far more than bone, and you'll see it linked to everything from mood to immunity, but it is clearly not a panacea. The strongest, most reliable benefit of treatment is in people who are genuinely deficient — correcting a real shortfall. For someone whose levels are already adequate, taking more is unlikely to add much, and chasing an ever-higher number is not the goal.

Should you even get tested?

This surprises people: major guidance does not recommend routinely screening everyone. The US Preventive Services Task Force reviewed the evidence and issued an "I statement" — it found insufficient evidence to assess the balance of benefits and harms of screening for vitamin D deficiency in asymptomatic, non-pregnant adults ³. In plain terms: for a healthy person with no symptoms and no risk factors, there isn't good evidence that testing everyone improves outcomes.

Why does this matter? Because over-testing has real downsides. A test on someone with no reason to be low often returns a borderline figure that triggers unnecessary supplementation, follow-up tests, and worry — cost and anxiety without a clear health gain ³. Testing makes far more sense when there's a reason: bone disease, conditions affecting absorption, certain medications, limited sun exposure, or symptoms that point that way. This is the same principle behind not ordering every test for its own sake — a focused reason beats a reflexive panel.

Sunny Thailand, and still deficient

Here's the part that catches expats off guard. You'd assume a tropical country bathed in equatorial sun would have plenty of vitamin D — yet deficiency is genuinely common across Southeast Asia, and Thailand is no exception ⁶. In Thailand's 4th National Health Examination Survey, vitamin D insufficiency (below 75 nmol/L) was found in 64.6% of people in Bangkok, 46.7% in other municipal areas, and 33.5% outside municipal areas — and it was worse in women, in younger people, and in urban dwellers ⁵.

The reasons make sense once you see them ⁶²:

• Skin and melanin. More melanin (skin types IV–V, common across the region) is natural sun protection — which also means it reduces the skin's vitamin D production for the same sun exposure.
• Sunscreen. It works, including against vitamin D synthesis: sunscreen at SPF 15 cuts the UVB-driven production of vitamin D in the skin by over 98%.
• Lifestyle. Air-conditioned offices, covering clothing, and a sensible aversion to the fierce midday heat mean many people — especially urban professionals and expats — get very little direct skin-on-sun time precisely when UVB is strongest.

Put together, this is the spine of the whole article: a sunny address tells you almost nothing about your levels. The office-working expat who commutes by car, works indoors, and avoids the midday sun can be just as deficient in Pattaya as they were in a grey northern winter.

What we see at the clinic

In practice, two patterns come up again and again with the people we see in Pattaya. The first is the new arrival who expects their vitamin D to be high because they've moved to the tropics, and is genuinely surprised to find it low — almost always one of the indoor, sun-avoiding lifestyles above. The second is the opposite: someone already taking a large supplement "just in case," who doesn't actually need the dose and has never had a reason to check. We don't reflexively add vitamin D to every panel, and we don't push supplements — where there's a clear reason to test we test, we read the number against your history and your other blood work, and if you're genuinely low we talk through correcting it sensibly. The aim is an honest baseline, not a bigger number for its own sake.

Common questions

Which test should I ask for — and does the unit matter? Ask for 25-hydroxyvitamin D, or 25(OH)D; that's the storage form used for screening, not the active 1,25 form ¹². The unit does matter when you compare reports: 1 ng/mL ≈ 2.5 nmol/L, so a result of 20 ng/mL is the same as 50 nmol/L ⁴.

My level is "low normal" — should I take a supplement? Not automatically. The clearest benefit of treatment is in people who are genuinely deficient, and a borderline figure in someone with no symptoms is often a reason to talk it through rather than to start a daily pill ³. Vitamin D is not a cure-all, and more isn't better once you're adequate.

Can I get too much vitamin D? Not from sunlight — your skin self-regulates, so the sun essentially can't push you into toxicity ¹². Toxicity comes only from prolonged high-dose supplements: blood levels above 150 ng/mL (375 nmol/L) can cause hypercalcaemia — too much calcium in the blood — which is why mega-dosing "to be safe" is the wrong instinct ¹².

Why do two labs disagree about whether I'm deficient? Because expert bodies use different cut-offs — the NIH/IOM call 50 nmol/L (20 ng/mL) adequate, while the Endocrine Society uses a higher 75 nmol/L (30 ng/mL) line ²⁴. Ranges and units also vary by lab, so the safest comparison is the same lab over time, read by a doctor.

I live in Pattaya — surely I get enough sun? Not necessarily. Across sunny Southeast Asia, deficiency is common because darker skin, sunscreen, and indoor lifestyles all reduce how much vitamin D your skin actually makes ⁵⁶. A sunny country is not a guarantee — which is exactly why the test exists for people with a reason to check.

Key takeaway

Vitamin D testing is genuinely useful — but used thoughtfully, not reflexively. Ask for the right test (25(OH)D), know that the common cut-offs are deficient below 30 nmol/L, adequate at 50 nmol/L and up, and possibly excessive above 125 nmol/L, and remember that even experts don't fully agree on where the line sits ²⁴. Despite the tropical sun, deficiency is common in Thailand — especially for indoor, sun-avoiding expats — so a sunny address doesn't settle the question ⁵⁶. But vitamin D is clearly not a panacea: the real benefit lies in correcting a genuine shortfall, not in chasing a higher number or supplementing without a reason ³. If you have grounds to wonder about yours, it's a simple, sensible thing to check with your doctor.