What is Vitamin D?
Vitamin D is a fat-soluble vitamin that acts like a hormone. The blood test measures 25-hydroxyvitamin D, the best indicator of your overall vitamin D status. It is made in the skin from sunlight and obtained from a few foods and supplements. Deficiency is very common, especially in higher latitudes and darker skin tones.
Why it matters
Vitamin D is essential for absorbing calcium and maintaining bone density, and it supports immune function and muscle strength. Deficiency is linked to bone loss, falls, and fatigue.
What it measures
Serum 25-hydroxyvitamin D, the storage form that best reflects total body vitamin D status.
Reference & optimal ranges
Reference ranges vary by lab, assay, age, and sex. The ranges below reflect commonly published adult intervals and are for education only always interpret results with the range printed on your own lab report and a clinician.
High vitamin D is almost always from over-supplementation and can cause dangerous high calcium levels (toxicity is rare below 100 ng/mL).
Common symptoms
- Nausea, vomiting
- Excessive thirst and urination
- Confusion
- Kidney stones (from high calcium)
Potential causes
- Excessive supplement doses
- Rare granulomatous diseases (e.g., sarcoidosis)
Low vitamin D is very common and can weaken bones and muscles and impair immune function.
Common symptoms
- Fatigue
- Bone or muscle aches
- Frequent infections
- Low mood
- Muscle weakness
Potential causes
- Limited sun exposure
- Darker skin, higher latitude, winter
- Malabsorption (celiac, IBD)
- Obesity (sequestration in fat)
- Older age
How to improve your Vitamin D
Lifestyle
Sensible sun exposure
Short, regular sun exposure supports vitamin D synthesis, balanced against skin cancer risk.
Nutrition
Vitamin D-rich foods
Fatty fish (salmon, sardines), egg yolks, and fortified foods provide dietary vitamin D.
Supplement if deficient
Vitamin D3 supplementation effectively raises low levels; dosing should be guided by your result and a clinician.
Exercise
Outdoor activity
Combines movement with sun exposure.
Sleep
General health
No direct sleep link, but overall wellness supports immunity.
Frequently asked questions
Scientific references
- Vitamin D Fact Sheet for Health Professionals NIH Office of Dietary Supplements (2024)
- Vitamin D Test MedlinePlus, U.S. National Library of Medicine
Educational information, not medical advice. This page is for general education and does not replace diagnosis or treatment by a licensed clinician. Do not start, stop, or change any medication, supplement, or treatment based on this content. Reference and optimal ranges vary between laboratories interpret your results with the range on your own report and a qualified professional.