Vitamin D3 (25-OH)

Measurement of 25-hydroxyvitamin D (calcidiol), the primary circulating storage form of vitamin D. This marker reflects total vitamin D status from both dietary intake and skin synthesis, making it the gold standard for assessing deficiency. Vitamin D plays critical roles in calcium metabolism, bone health, immune function, and mood regulation.

Vitamin D deficiency is endemic in central Europe, with studies showing that 50 to 60 percent of the German population falls below the optimal threshold of 30 ng/mL, particularly during the winter months when UVB radiation is insufficient for cutaneous synthesis at Munich's latitude. The 25-hydroxyvitamin D test captures total body stores by measuring the liver's conversion product of both dietary and sun-derived vitamin D. Beyond its well-established role in calcium absorption and bone mineralisation, vitamin D functions as a secosteroid hormone with receptors in virtually every tissue. Adequate levels support innate and adaptive immune responses, modulate inflammatory pathways, contribute to insulin sensitivity, and influence serotonin production. Deficiency has been associated with increased susceptibility to respiratory infections, autoimmune conditions, cardiovascular risk, and depressive symptoms. SYNLAB München Zentrum measures 25-OH vitamin D using liquid chromatography–tandem mass spectrometry or competitive immunoassay, depending on clinical context. No fasting is required. Results are typically available within one to three working days. For patients found to be deficient, supplementation dosing should be guided by a physician based on the severity of the deficit and individual factors such as body weight and absorption capacity.