What is vitamin D?

Vitamin D is a fat-soluble vitamin with an important role in aiding the absorption of calcium, magnesium and phosphates. It also contributes to bone and musculoskeletal health. Vitamin D in the blood can be measured in two different forms; 1,25-dihydroxy vitamin D (1,25-OH) and 25-hydroxy vitamin D (25-OH). 25-OH vitamin D is generally inactive, circulating in the blood until it is converted to the active 1,25-OH vitamin D when required.

25-OH vitamin D is typically the preferred measure of vitamin D in the blood as it is present for longer and in higher concentrations than 1,25-OH vitamin D. Measuring 1,25-OH vitamin D is useful for monitoring conversion between 25-OH vitamin D and 1,25-OH vitamin D by the kidneys. In the UK, the sun is only powerful enough to stimulate vitamin D production in the skin between around April and September.

Due to the fact that this is the primary source of vitamin D in the body, it is common for vitamin D deficiency to occur in many parts of the UK during the winter months without supplements. Vitamin D deficiency can result in weakness in bones and muscles and has been associated with asthma in children and cognitive impairment in adults. Excess vitamin D through supplementation can lead to hypercalcemia.

Why take the test?

This test measures the levels of 25-OH vitamin D in the blood. It is typically used to investigate vitamin D deficiency or excessive supplementation. Vitamin D deficiency can be the result of insufficient intake and production or problems absorbing the vitamin.