25-hydroxyvitamin D is the hepatic-storage form of vitamin D. It's the right blood test for vitamin D status — not 1,25-OH-D (the active form), which is tightly regulated.
Mechanism — Vitamin D acts more like a hormone than a vitamin: binds vitamin D receptors across nearly every tissue. Modulates calcium, immune function (including LL-37 production), insulin sensitivity, and cellular proliferation. Skin synthesis from UVB is the natural source.
Lab range — 30-100 ng/mL is the standard range, but 'optimal' for most populations is 50-70 ng/mL. Levels under 30 are insufficient; under 20 deficient.
Caveats — Always supplement with K2 (MK-7) to direct calcium into bone rather than soft tissue. Most northern-latitude adults need 4,000-5,000 IU/day to maintain optimal levels. Check, supplement, recheck — don't guess the dose.