Vitamin D deficiency: symptoms, testing and how to correct

Vitamin D is the only vitamin the body can make on its own — but only under ultraviolet light of a particular wavelength. At mid and northern latitudes that sunlight is absent for most of the year, so deficiency here is not exotic but widespread. The problem is that you cannot recognise it by how you feel: it has no specific symptoms.

Why symptoms will not identify it

Everything usually listed — fatigue, heaviness in the morning, muscle and bone pain, calf cramps, hair loss, frequent infections, low mood — occurs in a dozen other conditions. The same complaints come from low ferritin, reduced thyroid function, B12 deficiency, poor sleep or plain chronic stress.

The practical conclusion: vitamin D is not inferred from signs, it is measured. And it is measured not instead of the rest but alongside it — because in practice deficiencies almost always arrive as a group.

Which test to take

You need 25(OH)D — 25-hydroxyvitamin D. This is the transport form, and it is what shows how much vitamin your body has stored.

People often ask about 1,25(OH)₂D, the active form. It is not tested routinely, and here is why: in deficiency the body compensates by keeping the active form normal or even raising it, so it can look excellent while the store is empty. A doctor orders it in specific situations — kidney disease, sarcoidosis, calcium metabolism disorders — not for screening.

Fasting is not required. A dose taken the day before does not meaningfully change the result, but if you take high doses it is sensible to pause and discuss timing with your doctor.

What counts as deficiency

Units differ: some laboratories print ng/mL, others nmol/L (nmol/L ≈ ng/mL × 2.5). That is the first thing to check on the report, otherwise it is easy to panic or to relax for no reason.

The commonly used landmarks are roughly: pronounced deficiency below 20 ng/mL, insufficiency 20–30, a target range of about 30–60, and values above 100 ng/mL considered excessive. Your laboratory's exact limits are printed on the report, and your result should be read against those.

What to check alongside

Vitamin D does not act alone — it is part of calcium and phosphate metabolism. So with a pronounced deficiency, and certainly before a long course of high doses, a doctor usually checks calcium (total and albumin-corrected), phosphorus, parathyroid hormone and creatinine to assess kidney function.

If the complaints are fatigue and hair loss, it makes sense to close the neighbouring causes at the same time: ferritin with an iron panel, B12, TSH. Checking them one at a time over months is the most common waste of time in this story.

Who is at risk

Essentially anyone who gets little sun: indoor workers, residents of northern regions, people who cover their body, those using SPF constantly. Separately: people with darker skin (melanin reduces synthesis), older adults (skin synthesises less well), people with obesity (the vitamin is stored in fat tissue and is less available), those with bowel disease or after stomach surgery (impaired fat absorption), and people taking certain medicines such as anticonvulsants or glucocorticoids.

How it is corrected

Sunlight does not work as the main corrective tool: the ultraviolet dose needed for synthesis is uncomfortably close to the dose that damages skin, and in winter at northern latitudes synthesis essentially stops. Diet closes the gap poorly too — oily fish, cod liver, egg yolk and fortified foods contribute, but they cannot fix a pronounced deficiency on their own.

So correction means a supplement, and the dose is chosen by a doctor based on your starting level, weight and other conditions. The regimens differ fundamentally: a loading dose for pronounced deficiency and a maintenance dose once you reach the target range. Vitamin D is fat-soluble, it accumulates, and excess is not harmless: it raises calcium with everything that follows. That is exactly why «taking extra just in case» is a bad idea.

Absorption improves when taken with a meal containing fat. A follow-up test usually makes sense after 2–3 months — earlier than that the picture has not settled.

What to do with the result

If the value is below target, that is a reason to talk to a doctor rather than self-treat: they will set the regimen and decide whether calcium and parathyroid hormone should be checked alongside. If the result is normal but the complaints remain, then the cause lies elsewhere — and that is a useful answer too.

A finished report is best read together with the rest of your metabolic markers: vitamin D almost always turns out to be part of a wider picture rather than an isolated finding.