Vitamin D test: what your 25(OH)D level means, who needs testing, and deficiency symptoms

What a vitamin D test measures

Vitamin D status is checked with 25(OH)D (25-hydroxyvitamin D, or calcidiol). Your liver turns vitamin D from skin, food and supplements into this form. It has a half-life of about 15 days in the blood, so the test reflects your intake over recent weeks rather than one day.

On a request form or home test kit it may be called:

• “25-OH vitamin D” or “25(OH)D”;
• “vitamin D, 25-hydroxy”;
• “calcidiol”.

“1,25-dihydroxyvitamin D” is a different test. NIH ODS says it’s a poor guide to vitamin D status: it lasts only hours in the blood and falls only when deficiency is severe.

Vitamin D levels: nmol/L and ng/mL

Labs report results in one of two units. UK labs usually report nmol/L, while US labs use ng/mL. If an NHS report says 50 nmol/L and a US-style report says 20 ng/mL, that’s the same level. Converting is simple:

• ng/mL × 2.5 = nmol/L;
• nmol/L × 0.4 = ng/mL.

The cut-offs used by NIH ODS, based on the US National Academies:

Some labs and private tests print a range starting at 75 nmol/L (30 ng/mL). That’s a higher target used by some clinics. NIH ODS considers 50 nmol/L or more enough for most people. In its 2024 guideline, the US Endocrine Society chose not to set cut-offs for “sufficiency” or “deficiency” at all.

UK guidance starts lower. In 2016 the Scientific Advisory Committee on Nutrition (SACN) concluded that 25(OH)D shouldn’t fall below 25 nmol/L at any time of year. It worked out that about 10 mcg (400 IU) a day keeps most people above that line in winter, and that’s where the NHS dose comes from. UK osteoporosis guidance uses bands that match the NIH table: below 30 nmol/L deficient, 30–50 insufficient, above 50 adequate.

Lab methods differ too. NIH ODS warns that a result can read falsely high or low depending on the assay and the lab. A spring test from your GP and an autumn kit from a private lab can’t be compared fairly, so stick to one lab.

Who needs a vitamin D test

In the UK, the NHS advises everyone to consider 10 mcg (400 IU) of vitamin D a day in autumn and winter, and all year for some groups, without testing first. The US Endocrine Society also advises against routine testing in healthy people.

A test makes sense if:

• you have symptoms that could be osteomalacia: bone pain, muscle weakness and aches;
• you have a condition that affects fat absorption: coeliac disease, Crohn’s disease, ulcerative colitis, cystic fibrosis or some liver diseases;
• you’ve had gastric bypass surgery;
• your GP is treating a deficiency or has prescribed high doses;
• you’ve taken more than 4,000 IU a day for a long time without monitoring.

Low vitamin D is more common in people who rarely get sun on their skin, people with darker skin, older adults and people with obesity. Even then, decide on testing with your GP or pharmacist.

Vitamin D levels in women and in pregnancy

The 25(OH)D cut-offs are the same for women and men, and pregnancy doesn’t change them. The recommended intake in pregnancy is the same as for other adults: 10 mcg (400 IU) a day according to the NHS.

After the menopause, vitamin D is often paired with calcium for bone health. How to combine them and whether you need K2 is covered in our guide to calcium and vitamin K2. For a broader overview, see supplements for women.

Vitamin D deficiency symptoms

According to NIH ODS, severe deficiency leads to:

• in adults, osteomalacia: soft bones with bone pain, plus muscle weakness and aches;
• in children, rickets: bone deformities.

Mild deficiency usually has no clear symptoms. Tiredness, frequent colds and low mood have many possible causes, from poor sleep to low iron. You can’t judge your vitamin D level by how you feel.

What to do with your result

First get the number into the unit you need. The converter below also shows the NIH ODS cut-offs.

Retest no sooner than 2–3 months after changing your dose. With a half-life of about 15 days, the level settles after 4–5 half-lives.

If your bottle gives IU and your GP gave the dose in micrograms, use our IU to mcg converter.

Vitamin D in winter: why your result depends on the season

Your skin makes vitamin D from summer sunlight, but at northern latitudes it makes little or none in winter. In the UK that’s roughly October to March. So the same person can get noticeably different results in September and in March.

• Compare tests taken at the same time of year.
• A result from late summer may drop by the end of winter.
• Our vitamin D sunlight calendar shows which months the sun does little for vitamin D where you live.

Who should be careful with vitamin D

Don’t raise your dose without medical advice if you:

• have kidney disease or kidney stones;
• take thiazide diuretics: with vitamin D they can push blood calcium too high;
• take steroids such as prednisolone, or orlistat: they lower vitamin D levels, so agree the dose with your GP;
• are pregnant or breastfeeding and want more than 4,000 IU a day.

Levels above 375 nmol/L (150 ng/mL), almost always from large supplement doses, cause nausea, weakness, confusion, kidney stones and an irregular heartbeat. The NHS says adults shouldn’t take more than 100 mcg (4,000 IU) a day. For children, follow NHS guidance and your health visitor or GP.

How to choose vitamin D on iHerb

1. If you have a reason to test, check your 25(OH)D first.
2. Choose D3 (cholecalciferol): it raises blood levels better than D2.
3. Check the dose per softgel or drop in mcg and IU: 25 mcg = 1,000 IU.
4. For maintenance, the NHS dose is 10 mcg (400 IU) a day; many adults take up to 25 mcg (1,000 IU). Higher doses only on medical advice.
5. Count vitamin D in multivitamins and calcium combinations.
6. Take it with a meal that contains some fat.

Other common tests, such as ferritin levels, are collected in tests and deficiencies.