Vitamin D levels explained: reading your 25-hydroxyvitamin D result

What the test measures

Vitamin D from sunlight, food and supplements is converted in the liver to 25-hydroxyvitamin D, which circulates for weeks and reflects your overall supply. That is the form measured in the standard blood test. The kidneys then make the active hormone, but its level is tightly regulated and is not a good guide to stores.

Check the units on your report. Results are reported either in ng/mL or nmol/L, and 1 ng/mL is about 2.5 nmol/L, so a result of 50 means very different things depending on the unit.

Where the cutoffs sit

The NIH Office of Dietary Supplements summarizes the thresholds most often used.

• Below 12 ng/mL (30 nmol/L): deficiency, linked to softening of the bones in adults
• 12 to under 20 ng/mL (30 to under 50 nmol/L): generally considered inadequate for bone and overall health
• 20 ng/mL (50 nmol/L) or more: adequate for most people
• Above 50 ng/mL (125 nmol/L): linked to potential adverse effects, especially above 60 ng/mL

Why experts disagree on the target

Some organizations and clinicians use 30 ng/mL as a target, while the NIH fact sheet treats 20 ng/mL as sufficient for most people. The disagreement arises because the evidence linking specific levels to specific outcomes is limited. The Endocrine Society's 2024 guideline notes that outcome-specific thresholds have not been established in clinical trials and suggests against routine screening of vitamin D levels in healthy adults.

The U.S. Preventive Services Task Force likewise concluded that evidence is insufficient to recommend for or against screening adults without symptoms. Testing makes most sense when you have risk factors or a condition that vitamin D affects.

Why levels often fall after 45

Several factors make low vitamin D more likely with age.

• The skin makes less vitamin D from sunlight as you get older
• Many adults spend more time indoors
• In northern parts of the U.S., winter sunlight is too weak for much skin production
• Darker skin produces less vitamin D from the same sun exposure
• Higher body fat holds vitamin D in fat tissue, lowering blood levels
• Conditions that reduce fat absorption, and weight-loss surgery

Where the evidence is strong, and where it isn't

Correcting genuine deficiency is clearly worthwhile for bone and muscle health. The picture is different for people whose levels were not low to begin with. The large VITAL trial, in which adults took 2,000 IU of vitamin D3 daily for about five years, found no significant reduction in heart attacks, strokes, overall cancer incidence or depression, according to the ODS summary. An ancillary analysis found no reduction in fractures.

One reason for the mismatch with earlier observational studies is that low vitamin D often travels with other health problems, such as less outdoor activity and higher body weight, so it may be partly a marker of poorer health rather than its cause.

Vitamin D and tiredness

Low vitamin D is often blamed for fatigue. Severe deficiency can cause bone pain and muscle weakness, but the evidence that correcting a mild shortfall restores energy is weak. If you are tired, it is worth checking iron stores, B12, thyroid function, blood sugar, sleep and mood as well, rather than stopping at vitamin D.

Repletion and safety

If your level is low, a clinician will suggest a dose and a recheck, usually after a couple of months. The recommended daily intake is 600 IU for most adults and 800 IU over 70, and the upper limit for routine use without supervision is 4,000 IU a day. Vitamin D is fat-soluble and accumulates; toxicity, almost always from excessive supplements, raises calcium and can damage the kidneys and heart.

The Endocrine Society does suggest routine supplementation for adults 75 and older. Combination products with magnesium or vitamin K2 are popular, but the evidence for adding them is thinner than marketing suggests. HealifyMD's Comprehensive Lab Panel is $199, confirmed at your eligibility review.