Vitamin D (25-OH)

25-hydroxyvitamin D (25-OH-D, or calcidiol) is the main circulating form of vitamin D, and the standard blood test for your vitamin D status. Your liver makes it by adding a hydroxyl group to vitamin D3 (cholecalciferol) or D2 (ergocalciferol). It reflects both what you eat and what your skin makes from UVB sunlight. Its half-life of about 2 to 3 weeks makes it a reliable index of longer-term status, unlike the short-lived active hormone 1,25-(OH)₂D. Deficiency (commonly under 50 nmol/L, or 20 ng/mL) is tied to several problems. These include poor bone mineralization, falls, and more respiratory infections. In observational data, it also goes with higher death rates. But randomized supplement trials have shown mixed results on hard outcomes. The 2024 Endocrine Society guideline (Demay et al., JCEM) actually stepped away from fixed deficiency thresholds for healthy adults. It found too little trial evidence to link specific cutoffs to outcomes. Instead, it recommended supplements for select groups (under 18, over 75, pregnancy, high-risk prediabetes), rather than universal screening. Still, many longevity-minded practitioners aim for 75 to 100 nmol/L despite the guideline shift.

Last reviewed: June 22, 2026

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