Signs of vitamin D deficiency, and why most adults are told to skip the routine test

Most adults with an ordinary diet and some daylight do not need a vitamin D supplement, and do not need their blood level checked. That is not a shortcut opinion — it is the position of the committee that set the reference intakes in the first place, and of a large clinical guideline published in 2024. Genuine deficiency is real, has recognisable signs, and clusters in specific groups. For everyone else, the routine 25-hydroxyvitamin D test that has become common is one the same authorities suggest against [s2] [s3].

What deficiency actually looks like

Vitamin D lets the body absorb calcium and keep muscles, nerves and the immune system working normally, and a sustained shortage shows up in bone [s2]. In adults the recognised signs are bone pain, muscle weakness or aches, and — when deficiency is severe and prolonged — soft or deformed bones (osteomalacia) and fractures [s2]. In children the classic deficiency disease is rickets, a disorder of bone growth [s2]. These are the symptoms that would prompt a clinician to order a test; tiredness alone, the reason many people ask for one, is not on that list [s2].

A test measures 25-hydroxyvitamin D, the form the liver makes and the most accurate marker of status [s2]. The trouble is what counts as "low." The 2011 Institute of Medicine review found that a blood level of 20 nanograms per millilitre is enough for good bone health in practically all individuals, and warned that the number of people labelled deficient in North America may be overestimated because many laboratories use cut-points far higher than the evidence supports [s1]. A single person can be called deficient or sufficient depending on which lab runs the sample [s1].

Why most people are not short

The committee assumed minimal sun exposure and still concluded that 600 International Units a day meets the needs of almost everyone, rising to 800 IU for people over 70 [s1]. Against that standard, national surveys showed that average blood levels sat above the 20 ng/mL mark — evidence that sunlight contributes meaningfully and that a majority of the population is meeting its needs [s1]. In other words, the widely repeated claim of a deficiency epidemic rests heavily on where the line is drawn.

The 2024 Endocrine Society guideline reached the same practical place from trial evidence. Its panel suggested against empiric supplementation above the reference intake to lower disease risk in healthy adults younger than 75, and — finding no trial support for it — suggested against routine 25(OH)D testing in all the populations it considered, including people with obesity or darker skin [s3].

Who genuinely is at higher risk

Deficiency concentrates in identifiable groups, and for them testing or supplementation can make sense [s2]. The IOM flagged people who are older and living in institutions, and those with dark skin pigmentation, as more likely to fall short [s1]. MedlinePlus lists breastfed infants — breast milk is low in vitamin D — people who rarely expose skin to sunlight, those with malabsorption from Crohn's disease, ulcerative colitis or coeliac disease, people who have had weight-loss surgery, and those with kidney or liver disease that impairs activation of the vitamin [s2]. Certain medicines, including some statins, steroids and weight-loss drugs, can lower levels too [s2].

The 2024 guideline is not a blanket "no." It suggested empiric supplementation for children and adolescents aged 1 to 18 to prevent rickets and possibly reduce respiratory infections, for adults aged 75 and older, for pregnancy, and for people with high-risk prediabetes — each on the strength of specific trial signals rather than a general deficiency worry [s3].

The other direction: more is not better

The reference intakes come with a ceiling that supplement marketing rarely mentions. The IOM judged that the risk of harm begins to rise once daily intake passes 4,000 IU, and that very high intakes above 10,000 IU a day are known to cause kidney and tissue damage [s1]. Its blunt conclusion was that higher levels of vitamin D have not been shown to confer greater benefits and have been linked to other health problems, "challenging the concept that more is better" [s1].

What to take from this

If you have bone pain, muscle weakness, a malabsorption condition, very little sun exposure, or a young child at risk of rickets, vitamin D status is a reasonable thing to raise with a clinician [s2] [s3]. If you are an otherwise healthy adult reaching for a test or a high-dose pill because of general fatigue, the evidence points the other way [s3]. This piece describes reference intakes and guidelines and is not medical advice; dosing and testing decisions belong with a clinician who knows your history.