Vitamin D (25 OH) Blood Test

Vitamin D controls how your body absorbs calcium and phosphate. It’s vital for keeping your bones, teeth, and muscles healthy, and plays a role in immune function and mental health. Vitamin D is particularly important for pregnant women, breastfed babies, and children under five.

Symptoms of vitamin D deficiency can be vague. They include feeling low in energy, having more aches and pains than usual, depression (especially in winter), and frequent illnesses. Even if you don't have symptoms, it’s worth testing your vitamin D level, as a high proportion of adults in the UK are deficient or have insufficient levels.

If your results are outside the normal range, we'll let you know what steps you can take to improve your vitamin D level. For more information, have a look at our guide to vitamin D deficiency.

Vitamin D is a fat-soluble vitamin. It’s also known as the sunshine vitamin because we produce most of what we need through exposure to the sun. 

Before your body can use vitamin D produced by sun exposure (known as vitamin D3), it must be converted it into another form called 25 hydroxycholecalciferol (25 OH). Vitamin D (25 OH) is the major circulating form of vitamin D, and so your vitamin D (25 OH) level is considered the most accurate indicator of vitamin D supply to your body. 

You also get some vitamin D from food, but it’s difficult to get all your requirement from food alone. Sources of vitamin D include oily fish, mushrooms, egg yolks, and vitamin D-fortified foods.

The normal range for vitamin D levels in the blood varies between laboratories. But, generally, a level below 50nmol/L is considered insufficient. The following table, based on figures from NICE guidance and other sources, is given as a guide. There is currently no agreement on an ‘optimal’ vitamin D level, which may be as high as 70-80 nmol/L.

Vitamin D deficiency has many causes. You may be more prone to a vitamin D deficiency if you're over 65 or have dark skin, which makes it harder to produce vitamin D through sunlight exposure. Certain medical conditions can affect your body’s ability to absorb vitamin D from your food, such as coeliac and Crohn’s disease. Lifestyle factors including your diet and how long you spend outdoors can also play a part. You can read more about vitamin D deficiency in our Vitamin D Guide.

We make most of the vitamin D we need with the help of sunlight. So, it’s not surprising that levels can be affected by the changing seasons. Your risk of a deficiency is greater during autumn and winter, when it’s harder to get adequate amounts of vitamin D from time spent outdoors. And as some areas of the UK are sunnier than others, where you live can also affect your vitamin D level. To find out more, read our blog about how the British weather can affect your vitamin D levels.