Vitamin D, 25-OH

Routine annual

also 25(oh)d, calcidiol

What it is25-hydroxyvitamin D is the storage form of vitamin D circulating in blood. It's produced in skin from sunlight, absorbed from a few foods and supplements, and converted in the liver from inactive vitamin D.

DoesServes as the substrate the kidneys convert to active calcitriol; also acts directly on immune cells, neurons, and other tissues.

GoodAdequate 25-OH vitamin D supports bone density, immune balance, mood, cardiovascular health, and may reduce cancer risk.

BadLow 25-OH vitamin D drives osteoporosis, depressed mood, immune dysregulation, autoimmunity, and may increase risk of cardiovascular disease and several cancers.

Standard

30–100 ng/mL sufficient.

Optimal

60–80 ng/mL (higher is good, ceiling ~100)

Modifiers

Latitude, season, skin pigmentation, BMI, age, sunscreen use; obesity lowers serum vitamin D for the same dose.

How to optimize

Lifestyle: Daily sun exposure on bare skin when UV index is adequate (10–30 minutes midday for lighter skin, more for darker).
Diet: Fatty fish (salmon, sardines, mackerel), egg yolks, mushrooms grown under UV light; vitamin D-fortified foods are weak sources.
Supplements: Vitamin D3 (cholecalciferol) 4,000–8,000 IU/day to reach target, with vitamin K2 MK-7 (180–360µg/day) to direct calcium to bones; magnesium glycinate supports activation.
Peptides: —
Drugs: Prescription D3 for documented deficiency.

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