Vitamin D test: deficiency and when to check

Vitamin D is no longer thought of as a purely bone nutrient: we now know it helps regulate the immune system, modulates inflammation and takes part in hundreds of cellular processes. Yet its deficiency is one of the most common worldwide, and it rarely causes obvious symptoms in the early stages.

Why measuring vitamin D matters

Vitamin D deficiency is associated with:

• A higher risk of osteoporosis and fractures in adults.
• Rickets in children and osteomalacia in adults.
• Immune dysfunction and recurrent respiratory infections.
• Links with autoimmune, cardiovascular and metabolic diseases.
• Muscle weakness and falls in older adults.

Which test is ordered

The standard test to assess vitamin D nutritional status is 25-hydroxyvitamin D (25-OH-D) in blood. This metabolite reflects the combined contribution of sun exposure and diet or supplementation, and it has a half-life of 2 to 3 weeks, which makes it ideal for evaluating the body's stores.

Reference values

These are the cut-off points from the Endocrine Society's 2011 guideline, which are still used in practice to interpret 25-OH vitamin D. The same society's 2024 update did not define an optimal level for preventing disease and does not recommend routine testing in people without an established indication:

Status	Level (ng/mL)	General interpretation
Deficiency	Below 10	Increased bone risk; usually requires correction prescribed by a doctor
Insufficiency	11 – 29	Below the desirable range; assessed according to the clinical context
Recommended	Above 30	Range considered adequate for most of the population
Toxicity	Above 150	Risk of hypercalcemia; associated with excessive, unsupervised supplementation

These cut-off points may be adjusted according to the population and the laboratory. It is always best to interpret the results with your treating physician.

Why deficiency is so common

Vitamin D has a peculiarity: its main source is not food but synthesis in the skin from solar radiation. Diet (oily fish, egg yolk, fortified dairy) provides only a small share. As a result, any factor that reduces sun exposure or the skin's ability to produce it can lead to deficiency:

• Little sun exposure: working indoors, urban life, the winter months and living at latitudes far from the equator.
• Sunscreen and clothing: constant use of sunscreen and clothing that covers much of the body reduce skin synthesis.
• Age and skin color: as we age the skin produces less vitamin D, and darker pigmentation needs more sun time to synthesize it.
• Malabsorption: celiac disease, inflammatory bowel disease or bariatric surgery hinder its absorption.
• Obesity: vitamin D becomes "trapped" in fatty tissue, lowering its availability in the blood.
• Kidney or liver disease: this affects the steps needed to activate the vitamin.

Who should get their vitamin D checked?

It is not a universal routine test, but it is clearly indicated for:

• Adults over 60.
• Post-menopausal women or those with risk factors for osteoporosis.
• Pregnancy and breastfeeding.
• People with little sun exposure (working indoors, constant sunscreen use, living at high latitudes).
• Patients with chronic kidney, liver or intestinal disease (malabsorption).
• Obesity and bariatric surgery.
• Long-term use of corticosteroids, anticonvulsants or antiretrovirals.
• Children and adolescents on restrictive diets.

Vitamin D is often added to the annual check-up when one of these factors is present. If you are putting together your own work-up, it can be included in an expanded basic check-up or added to your general clinical tests; it is especially valuable for women around menopause.

How to prepare for the test

Measuring 25-OH vitamin D does not require fasting. It can be done at any time of day. If you are taking vitamin D supplements, let the laboratory know so your results can be interpreted correctly.

How deficiency is corrected

When the test shows a low level, treatment combines up to three approaches, depending on the case:

• Sensible sun exposure: a few minutes of sun on the arms and legs several times a week support synthesis, always protecting the skin and never seeking a burn.
• Diet: adding oily fish (salmon, mackerel, sardines), egg yolk and fortified dairy or foods helps, although on its own diet is rarely enough to reverse a marked deficiency.
• Doctor-prescribed supplementation: this is the main tool when levels are low. The dose depends on the baseline value and each person's condition.

One point is important: self-supplementation with high doses can lead to toxic levels and hypercalcemia. The right dose depends on the baseline value, age, clinical condition and treatment goals, which is why it should be prescribed by a doctor. Once supplementation has started, a follow-up test at 6 to 12 weeks is recommended to confirm the level has normalized and to adjust the maintenance dose.

Vitamin D at Laboratorio Aclimu

At Laboratorio Aclimu we measure 25-OH vitamin D using a high-precision method (chemiluminescence). You can add it to your routine check-up and book with or without a doctor's order, depending on your coverage. Results are available within 24 to 48 business hours through the online results portal.

This article was reviewed by the biochemistry team at Laboratorio Aclimu. The information is for general guidance and does not replace a consultation with your doctor or healthcare professional.

Frequently asked questions

What vitamin D level is considered normal?

Under the most widely used criteria, a 25-OH vitamin D level above 30 ng/mL is considered sufficient. Between 11 and 29 ng/mL is insufficiency, and below 10 ng/mL is deficiency. Cut-off points can vary slightly by scientific society and laboratory, so it is best to interpret your result with your doctor.

Do I need to fast for a vitamin D test?

No. Measuring 25-OH vitamin D does not require fasting, and the sample can be drawn at any time of day. If you are taking vitamin D supplements, let the laboratory know beforehand so that your results can be interpreted accurately.

Why is vitamin D deficiency so common?

Because its main source is synthesis in the skin from sun exposure, and many factors limit it: working indoors, sunscreen use, living at high latitudes, the winter months, skin aging and darker pigmentation. Diet contributes little on its own, so it is easy to fall below the desirable range without noticing.

Can I take vitamin D on my own?

It is not advisable. High, unsupervised doses can lead to toxic levels and hypercalcemia. The right dose depends on your baseline level, age and clinical condition, which is why supplementation should be prescribed and monitored by a doctor, with a follow-up test usually at 6 to 12 weeks.

How often should I check my vitamin D?

It is not a routine test for everyone. In people with risk factors it is usually checked once a year, and sooner if symptoms appear or a treatment is being adjusted. If you have started supplementation, it is rechecked at 6 to 12 weeks to assess the response.

References

1. Endocrine Society (Demay MB et al.). Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline (2024). Available at: pubmed.ncbi.nlm.nih.gov
2. Endocrine Society (Holick MF et al.). Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline (2011). Available at: pubmed.ncbi.nlm.nih.gov
3. USPSTF (JAMA). Screening for Vitamin D Deficiency in Adults: USPSTF Recommendation Statement (2021). Available at: pubmed.ncbi.nlm.nih.gov