Vitamin D is a fat-soluble vitamin that the body produces when skin is exposed to ultraviolet B (UVB) radiation from sunlight. It is also obtained in smaller amounts from dietary sources including fatty fish, eggs, and fortified foods. Vitamin D supports calcium absorption, bone mineralisation, immune function, and muscle function.
Despite Australia's sunny climate, vitamin D deficiency is one of the most common nutritional deficiencies in the country. This happens because many people spend most of their day indoors, use sunscreen (which blocks UVB), wear covering clothing, or live in southern states where winter sun intensity is insufficient for adequate vitamin D production. People with darker skin require longer sun exposure to produce equivalent amounts. Older adults produce less vitamin D from sun exposure than younger people, and those who are obese may have lower circulating levels because vitamin D is sequestered in body fat.
This test measures 25-hydroxyvitamin D (25-OH D), the standard marker for assessing vitamin D status. It reflects the total amount of vitamin D circulating in your blood from both sun exposure and dietary or supplemental sources. It is distinct from the Active Vitamin D (1,25 Dihydroxy) test, which measures calcitriol, the hormonally active form produced by the kidneys and used for different clinical purposes.
Vitamin D deficiency does not always cause obvious symptoms. When symptoms do occur, they can include fatigue, bone pain or tenderness, muscle weakness, difficulty concentrating, and low mood. Prolonged severe deficiency can affect bone density, increasing the risk of osteoporosis and fractures.
Levels are interpreted as deficient (below 30 nmol/L), insufficient (30 to 49 nmol/L), adequate (50 to 150 nmol/L), and potentially excessive (above 150 nmol/L). Seasonal variation is normal, with levels typically lowest at the end of winter and highest at the end of summer. Testing at different times of year can help you and your practitioner understand how your levels fluctuate and whether supplementation needs adjusting.
To measure circulating 25-OH vitamin D and determine whether levels are deficient, insufficient, adequate, or potentially excessive, informing decisions about supplementation and sun exposure.
