Vitamin D (25-hydroxy)

UK labs print bands rather than a single range (commonly below 25, 25 to 50, and above 50 nmol/L), and values are lowest at the end of winter in the general population; use the bands the lab printed.

Vitamin D is made in the skin in sunlight and comes in smaller amounts from oily fish, eggs and fortified foods. The liver converts it to 25-hydroxy vitamin D, the storage form that circulates in the blood, and this is what the test measures. It is the standard measure of the body's vitamin D store, which is needed to absorb calcium and keep bone healthy.

Doctors request it when looking into bone pain, muscle aches, thinning of the bones, low calcium or a raised alkaline phosphatase or PTH, and in people with little sun exposure, darker skin, or gut conditions that reduce absorption. It is also one of the most popular tests in private health screens.

• Sun exposure and the summer months
• Vitamin D supplements and fortified foods
• Oily fish in the diet
• A recent high-strength vitamin D course

• Winter and little time outdoors, the commonest reason in the UK general population
• Darker skin, which makes less vitamin D from the same sunlight
• Covering the skin or using high-factor sunscreen consistently
• Being overweight, as vitamin D is stored in fat
• Gut conditions that reduce fat absorption, and weight-loss surgery
• Some epilepsy and HIV medicines, and long-term liver or kidney disease

• Values at the end of winter are commonly below 50 in the UK general population
• Different assays give different numbers for the same sample
• The lab's bands describe the general population, not the person

No fasting needed. The season matters a great deal; a value from late summer and one from late winter are not comparable.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.