Lab Reference Library / Vitamin D (25-OH)
Longevity & Aging
Vitamin D (25-OH)
25(OH)DReference range, optimal functional medicine levels, and why vitamin D deficiency is one of the most prevalent and consequential nutrient deficiencies in modern medicine, affecting immune function, cardiovascular health, cancer risk, bone density, and mood.
Most SearchedLongevity Marker
Standard Sufficient30+ ng/mL
FM Optimal60 to 80 ng/mL
Fasting RequiredNo
Unitsng/mL
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Category: Longevity & Aging | Also known as: 25-Hydroxyvitamin D, 25(OH)D, Calcidiol | Sample: Serum (fasting not required)
1. What This Test Measures
The vitamin D test measures 25-hydroxyvitamin D (25-OH D), the main circulating storage form of vitamin D in the bloodstream and the most accurate indicator of overall vitamin D status. This is the correct test to order when assessing vitamin D sufficiency. It should not be confused with 1,25-dihydroxyvitamin D (calcitriol), the active hormonal form, which is tightly regulated by the kidneys and does not accurately reflect overall vitamin D stores.
Vitamin D is technically a pro-hormone rather than a vitamin. The body produces it in the skin through ultraviolet B (UVB) radiation from sunlight converting 7-dehydrocholesterol to vitamin D3 (cholecalciferol). Vitamin D3 then travels to the liver where it is hydroxylated to 25-OH D, the form measured by this test, and subsequently to the kidneys and peripheral tissues where it is converted to the active 1,25-dihydroxyvitamin D.
Vitamin D receptors (VDRs) are found in virtually every cell type in the body: immune cells, cardiac muscle, neurons, pancreatic beta cells, intestinal epithelium, and reproductive tissues. This explains why vitamin D deficiency affects so many organ systems simultaneously and why optimizing vitamin D has such broad clinical implications.
2. Why This Test Matters
Vitamin D deficiency is one of the most prevalent nutritional deficiencies in the developed world, affecting an estimated 40% of American adults and over 1 billion people globally. Its clinical consequences extend far beyond bone health:
- Immune function: Vitamin D is arguably the most important nutrient for immune regulation. It activates innate immune responses against bacterial and viral pathogens while simultaneously modulating adaptive immunity to prevent autoimmune overactivation. Low vitamin D is consistently associated with increased risk of respiratory infections, influenza, and autoimmune disease development.
- Cancer risk reduction: Epidemiological data and randomized controlled trials demonstrate that higher vitamin D levels are associated with reduced risk of colorectal, breast, prostate, and pancreatic cancers. The VITAL trial showed that vitamin D3 supplementation significantly reduced cancer mortality.
- Cardiovascular health: Vitamin D receptors are present in cardiac muscle and blood vessel walls. Deficiency is associated with increased hypertension, heart failure, arterial stiffness, and cardiovascular event risk. Vitamin D modulates the renin-angiotensin-aldosterone system (RAAS), a primary regulator of blood pressure.
- Metabolic health and insulin sensitivity: Vitamin D enhances insulin receptor expression and secretion. Deficiency impairs beta-cell function and is independently associated with insulin resistance and type 2 diabetes risk.
- Bone health: The classical role; vitamin D is required for intestinal calcium absorption. Deficiency causes rickets in children and osteomalacia and osteoporosis in adults.
- Mood and neurological health: Vitamin D receptors are densely expressed in the brain. Deficiency is strongly associated with depression, seasonal affective disorder (SAD), cognitive decline, and multiple sclerosis risk.
- Muscle function: Vitamin D is required for muscle protein synthesis and neuromuscular function. Deficiency causes proximal muscle weakness, pain, and increased fall risk.
- Thyroid function: Vitamin D modulates immune tolerance in thyroid tissue. Low vitamin D is consistently associated with Hashimoto's thyroiditis and Graves' disease activity.
3. Standard Lab Reference Range
| 25-OH Vitamin D Level | Conventional Classification |
|---|---|
| Below 20 ng/mL | Deficient; bone disease risk, severe immune impairment |
| 20 to 29 ng/mL | Insufficient; below sufficiency threshold |
| 30 to 59 ng/mL | Sufficient (conventional); adequate for bone health |
| 60 to 100 ng/mL | Optimal (functional medicine target) |
| Above 150 ng/mL | Potentially toxic; hypercalcemia risk |
The conventional sufficiency threshold of 30 ng/mL was established based on the prevention of bone disease, not optimal immune function, cancer risk reduction, or cardiovascular protection. Research on populations with regular sun exposure and hunter-gatherer communities consistently shows vitamin D levels of 40 to 80 ng/mL, supporting the higher functional medicine target.
4. Optimal Functional Medicine Range
| 25-OH Vitamin D Level | Functional Interpretation |
|---|---|
| 60 to 80 ng/mL | Optimal: strong immune, cardiovascular, and cancer-protective range |
| 40 to 59 ng/mL | Adequate: above conventional sufficiency; improve if possible |
| 30 to 39 ng/mL | Insufficient by functional medicine standards; supplementation indicated |
| 20 to 29 ng/mL | Deficient; immediate supplementation required; loading protocol may be appropriate |
| Below 20 ng/mL | Severely deficient; loading dose protocol under physician supervision |
The upper tolerable range extends to approximately 100 to 150 ng/mL before toxicity risk. The 60 to 80 ng/mL functional medicine optimal range provides a wide safety margin while delivering the clinical benefits documented in research literature.
5. Symptoms Associated With Vitamin D Deficiency
Low Vitamin D (below 30 ng/mL)
- Persistent fatigue and low energy
- Bone pain and muscle aches
- Proximal muscle weakness
- Depression and seasonal mood changes
- Frequent respiratory infections, colds, flu
- Impaired wound healing
- Hair loss
- Back pain (vertebral bone density loss)
- Brain fog and cognitive difficulty
- Poor sleep quality
- Increased autoimmune disease activity
Vitamin D Toxicity (above 150 ng/mL)
- Toxicity is rare; almost never occurs from sun exposure alone
- Typically requires sustained supplemental doses above 10,000 IU per day for months
- Symptoms of hypercalcemia: nausea, vomiting, weakness
- Increased urination and thirst
- Kidney stones
- Confusion and fatigue
- The functional medicine target of 60 to 80 ng/mL has a wide safety margin; toxicity generally begins above 150 ng/mL
6. What Causes Low Vitamin D
- Insufficient sun exposure: the primary cause; indoor lifestyle, northern latitudes (above 35 degrees N), winter months, and office-based work dramatically reduce UVB exposure
- Sunscreen use: SPF 30 reduces vitamin D synthesis by approximately 95%; necessary for skin cancer prevention but must be paired with vitamin D supplementation
- Darker skin pigmentation: melanin reduces UVB absorption; individuals with darker skin tones require 3 to 5 times more sun exposure to produce equivalent vitamin D
- Obesity: vitamin D is fat-soluble and sequesters in adipose tissue, reducing circulating levels; obese individuals typically require higher supplemental doses
- Age: skin synthesis efficiency declines by approximately 75% from age 20 to 70; kidney conversion to active form also declines with age
- Malabsorption: celiac disease, Crohn's disease, gastric bypass surgery, and cholestatic liver disease reduce fat-soluble vitamin absorption including vitamin D
- Medications: corticosteroids accelerate vitamin D catabolism; anticonvulsants (phenytoin, carbamazepine) and rifampin induce liver enzymes that break down vitamin D
- Chronic kidney disease: impairs conversion of 25-OH D to the active 1,25-dihydroxyvitamin D form
- Low dietary intake: very few foods contain meaningful vitamin D naturally; fatty fish, egg yolks, and fortified foods provide modest amounts
- VDR gene polymorphisms: genetic variants affecting the vitamin D receptor can reduce biological response to vitamin D, requiring higher levels for equivalent physiological effect
7. How to Improve This Marker
Important note on Vitamin K2: When supplementing vitamin D3 at doses above 2,000 IU daily, co-supplementation with Vitamin K2 (MK-7 form, 90 to 200 mcg per day) is recommended. Vitamin D increases calcium absorption; K2 ensures that calcium is directed to bones and teeth rather than depositing in arterial walls. The combination is significantly safer than vitamin D alone at therapeutic doses.
Sun Exposure
- 10 to 30 minutes of midday sun (10am to 2pm) with face, arms, and legs exposed; most efficient time for UVB-driven vitamin D synthesis
- Frequency needed varies dramatically by latitude, season, and skin tone
- Meaningful sun synthesis is not possible above approximately 35 degrees N latitude from October through March regardless of exposure time
- Sun exposure cannot cause vitamin D toxicity; the body self-regulates synthesis
- Glass blocks UVB; sitting by a window does not produce vitamin D
Diet
- Wild-caught fatty fish: salmon (447 to 600 IU per 3oz), mackerel, sardines, herring; the richest natural sources
- Cod liver oil: historically the primary supplemental source; approximately 400 IU per teaspoon
- Egg yolks from pasture-raised hens; higher vitamin D than conventional eggs
- Mushrooms exposed to UV light can produce significant vitamin D2
- Fortified foods (milk, orange juice, cereals) provide modest amounts; D3 fortification preferred over D2
Supplementation
- Vitamin D3 (cholecalciferol) strongly preferred over D2; raises 25-OH D levels approximately 2 to 3 times more effectively and sustains levels longer
- Dosing by level: 20 to 30 ng/mL requires 4,000 to 6,000 IU per day; below 20 ng/mL requires 6,000 to 10,000 IU per day loading under physician supervision
- Maintenance: 2,000 to 4,000 IU per day for most adults once optimal levels are achieved
- Take with fat: vitamin D is fat-soluble; taking with a fat-containing meal improves absorption by 50% or more
- Pair with Vitamin K2 (MK-7): directs calcium to bones rather than arteries; important at higher doses
- Magnesium: required cofactor for vitamin D activation; correct deficiency alongside vitamin D supplementation
- Retest at 3 to 4 months to confirm adequate response and adjust dose
8. Related Lab Tests
Vitamin D is most informative when interpreted alongside these related markers:
RBC MagnesiumRequired Cofactor for Vitamin D
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Vitamin K2Synergistic Calcium Director
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TPO AntibodiesVitamin D Deficiency and Hashimoto's
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hs-CRPAnti-inflammatory Codeficiency
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FerritinCommonly Co-deficient in Women
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Fasting InsulinVitamin D Supports Insulin Sensitivity
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9. When Testing Is Recommended
- Annual testing for all adults; vitamin D deficiency is endemic and largely asymptomatic until significant
- Any individual living above 35 degrees N latitude, especially October through March
- Individuals with limited sun exposure: indoor workers, night shift workers, those who cover skin for cultural or medical reasons
- Darker skin tones; significantly higher deficiency risk at all latitudes
- Obesity; adipose tissue sequesters vitamin D, reducing circulating levels
- Autoimmune disease: Hashimoto's, multiple sclerosis, rheumatoid arthritis, IBD; vitamin D plays a critical immune-regulatory role
- Depression, seasonal mood changes, or chronic fatigue
- Osteoporosis or low bone density; vitamin D is foundational to bone health management
- Malabsorption conditions: celiac disease, Crohn's, gastric bypass
- Monitoring response to supplementation; retest at 3 to 4 months after initiating or changing vitamin D protocol
10. Clinical Perspective
Clinical Perspective
Vitamin D is the nutrient deficiency I correct in nearly every patient I see at The Lamkin Clinic. The average new patient presents with a level around 28 to 35 ng/mL, told they are "sufficient" by their previous provider. But 30 ng/mL is the level that prevents rickets, not the level that optimizes immune defense, supports cancer prevention, or protects cardiac and neurological health. When I explain that a hunter-gatherer living outdoors year-round typically has levels between 50 and 80 ng/mL, and that our indoor lifestyle has fundamentally cut us off from what our biology expects, patients understand immediately why supplementation is so important. We target 60 to 80 ng/mL. We always pair high-dose D3 with K2 and confirm adequate magnesium status first. Retesting at four months is essential because the dose-response varies significantly between individuals. Some patients need 5,000 IU to reach 65 ng/mL; others need 8,000. You simply cannot know without testing.
Brian Lamkin, DO | Founder, The Lamkin Clinic | Edmond, Oklahoma
11. Frequently Asked Questions
What is the optimal vitamin D level?
In functional medicine, the optimal 25-OH vitamin D level is 60 to 80 ng/mL. The conventional sufficiency threshold of 30 ng/mL prevents bone disease but does not reflect the level associated with optimal immune function, cancer risk reduction, cardiovascular protection, or mood regulation. Research on populations with regular sun exposure shows typical levels of 40 to 80 ng/mL, supporting the higher functional medicine target.
What does low vitamin D cause?
Low vitamin D impairs immune function and increases infection susceptibility, contributes to depression and seasonal affective disorder, causes muscle weakness and bone pain, reduces insulin sensitivity, increases cardiovascular disease risk, and is associated with higher rates of autoimmune disease. Because vitamin D receptors are present in virtually every tissue, deficiency creates a systemic problem rather than simply a bone health issue.
How much vitamin D should I take?
Dosing should be guided by your blood level. For levels below 30 ng/mL, loading protocols of 5,000 to 10,000 IU daily under physician supervision are appropriate. For maintenance once optimal levels are reached, 2,000 to 4,000 IU daily is typical. Always use vitamin D3 (cholecalciferol) over D2, take with a fat-containing meal, and pair with Vitamin K2. Retest at 3 to 4 months to confirm response; individual dose requirements vary widely.
What vitamin D level is considered deficient?
Conventionally, below 20 ng/mL is deficient and 20 to 29 ng/mL is insufficient. In functional medicine, levels below 40 ng/mL are considered inadequate for optimal health, and below 30 ng/mL requires active treatment. Approximately 40% of American adults are below 20 ng/mL, with much higher rates at northern latitudes.
Can vitamin D levels be too high?
Yes, but toxicity is rare and almost exclusively occurs from excessive supplementation, not from sun exposure, which self-regulates. Toxicity generally does not occur below 150 ng/mL. The functional medicine target of 60 to 80 ng/mL provides a wide safety margin. Symptoms of toxicity (hypercalcemia) include nausea, weakness, increased urination, and kidney stones. Doses above 5,000 IU daily should be monitored with periodic testing.
What is the difference between vitamin D2 and D3?
Vitamin D3 (cholecalciferol) is the form produced in human skin and found in animal-derived foods. Vitamin D2 (ergocalciferol) is derived from plant sources and fungi. D3 raises 25-OH D levels approximately 2 to 3 times more effectively than D2 and sustains those levels for longer. For supplementation, vitamin D3 is strongly preferred.
Content authored and clinically reviewed by Brian Lamkin, DO, founder of The Lamkin Clinic in Edmond, Oklahoma. Brian Lamkin, DO has 25+ years of experience in functional and regenerative medicine. This page reflects current functional medicine practice standards and is updated as new clinical evidence becomes available.
30 ng/mL prevents rickets. 60 to 80 protects everything else.
Most patients are deficient and don't know it. Schedule a consultation to test your vitamin D level and build a targeted protocol with D3, K2, and magnesium.
Schedule a ConsultationMedical Disclaimer: This content is provided for educational purposes only and is not intended as a substitute for professional medical advice, diagnosis, or treatment. Lab interpretation should always be performed in clinical context by a qualified healthcare provider. Reference ranges and optimal targets may vary based on individual patient history, clinical presentation, and laboratory methodology. Schedule a consultation to discuss your specific results with Dr. Lamkin.