Vitamin D (25-OH Vitamin D)

Vitamin D blood tests measure 25-hydroxy vitamin D (25-OH D), the main circulating form of vitamin D and the best indicator of overall vitamin D status. It reflects both sun exposure and dietary/supplement intake. Australian pathology labs report 25-OH vitamin D in nmol/L. Despite the name, vitamin D acts more like a hormone — it regulates calcium absorption, bone metabolism, immune function and roughly 200 genes throughout the body.

Vitamin D deficiency is common in Australia despite the climate — affecting roughly 1 in 4 adults — driven by sun-protective behaviour, indoor lifestyles, dark skin, southern latitudes (Tasmania, Victoria), winter, age, obesity, and conditions affecting absorption (coeliac disease, gastric bypass, inflammatory bowel disease). Low vitamin D contributes to bone loss (osteoporosis, fractures), muscle weakness, low mood, frequent infections and possibly chronic disease risk. Medicare in Australia funds vitamin D testing only for specific indications (suspected deficiency, dark skin, veiled women, malabsorption, osteoporosis, etc.) — routine screening of asymptomatic adults is not covered.

High vitamin D levels (above 150 nmol/L) are almost always caused by excessive supplementation. True dietary or sun-driven toxicity is rare because the body limits skin synthesis. Symptoms of vitamin D toxicity include hypercalcaemia (high blood calcium), kidney stones, nausea, vomiting and confusion. Levels above 250 nmol/L should prompt review of all vitamin-containing supplements (multivitamins, fish oil, fortified foods all add up). Some patients on prescribed high-dose vitamin D for osteoporosis or malabsorption legitimately sit in the upper 100–200 nmol/L range under specialist supervision. Granulomatous diseases (sarcoidosis, tuberculosis, some lymphomas) can rarely cause endogenous vitamin D activation and elevated 1,25-dihydroxy vitamin D, but 25-OH levels are usually unremarkable.

Low vitamin D affects roughly 1 in 4 Australian adults and is more common in winter, in southern states, and in people with darker skin, who veil for cultural reasons, who work indoors, who avoid sun exposure for skin-cancer prevention, who are obese (body fat sequesters vitamin D), or who have malabsorption (coeliac disease, Crohn's, gastric bypass, pancreatic insufficiency). Older Australians have reduced skin synthesis. Strict vegans and vegetarians have lower intake from food. Treatment in Australia typically uses cholecalciferol (vitamin D3) 1,000-2,000 IU daily for mild deficiency, 4,000-5,000 IU daily or weekly bolus dosing for moderate deficiency, with retest at 3 months. Levels respond slowly — give 8-12 weeks before retesting.

Annually for at-risk groups (dark skin, veiled, obese, malabsorption, osteoporosis, on anticonvulsants or glucocorticoids). At baseline and 3 months after starting supplementation, then annually once stable. People taking high-dose prescribed vitamin D should retest every 6 months. Asymptomatic, low-risk adults without specific indications are not routinely screened (and Medicare will not fund the test).

Often interpreted alongside calcium, phosphate, parathyroid hormone (PTH) and alkaline phosphatase to assess bone metabolism. Magnesium status also affects vitamin D activation.