Vitamin D25-hydroxyvitamin D · 25(OH)D

Vitamin D is a fat-soluble nutrient that behaves more like a hormone in the body. Most of it is made in the skin on exposure to sunlight, with a smaller amount from food; the blood test measures 25-hydroxyvitamin D, the stored form.

It matters because vitamin D is important for bone health, immune function and muscle, and deficiency is genuinely common — especially in winter, at higher latitudes and with darker skin or little sun exposure. It is one of the more worthwhile deficiencies to find and correct.

In practice, a low level is usually corrected with supplementation and sensible sun exposure; because it is stored in fat, it is checked periodically rather than day to day.