Vitamin D results —
a clear answer
on what's actually enough.
Your Vitamin D came back low and your doctor mentioned "deficiency." But is 18 ng/mL actually serious? Does it explain your fatigue? We answer both questions.
Dr. Emma Walsh, MD, MPH
Preventive Medicine, Thyroid Disorders ·
The essentials — before you read the full guide below.
What's actually measured
25-hydroxyvitamin D (25-OH-D) is the storage form in your blood — the best indicator of your overall vitamin D status. It reflects both sun exposure and dietary intake.
Optimal vs deficient
Below 20 ng/mL is deficient. 20–29 is insufficient. 30–100 ng/mL is generally considered sufficient. Above 100 ng/mL risks toxicity.
D2 vs D3 matter
Vitamin D3 (from sun and supplements) raises 25-OH levels more effectively than D2. This distinction matters when your doctor recommends supplementation.
Symptoms are non-specific
Deficiency causes bone pain, muscle weakness, and fatigue — but these symptoms are common to many conditions. Only the blood test confirms status.
Reference Ranges
What does your number
actually mean?
Use the interactive slider below, or read the range cards for a full clinical breakdown.
Vitamin D (25-OH) Reference Ranges
ng/mLEnter your result
Drag to see what your Vitamin D (25-OH) means
The Science
Why does sunlight affect a blood test?
Vitamin D is unique — it's both a vitamin and a hormone. When UVB rays hit your skin, a cholesterol precursor converts to Vitamin D3, which then travels to the liver and kidneys for activation into the form that regulates calcium absorption and immune function.
90% from sun, 10% from diet
In sun-exposed regions, most Vitamin D comes from UVB radiation. Diet (oily fish, fortified foods) contributes little. Indoor lifestyles and SPF use significantly reduce production.
Two-step activation
D3 from skin first goes to the liver (→ 25-OH-D, what's measured) then kidneys (→ active hormone). Kidney disease can cause deficiency even with normal 25-OH levels.
World's most common deficiency
Over 1 billion people worldwide are Vitamin D deficient — making it the most common nutritional deficiency in developed countries, despite being entirely preventable.
When to Test
Signs your doctor will
order this test
These are the most common reasons a Vitamin D (25-OH) test is requested — from symptoms to routine screening.
Bone pain and tenderness
Vitamin D is essential for calcium absorption. Deficiency causes softening of bones — presenting as diffuse bone pain, especially in the spine and pelvis.
Classic signMuscle weakness
Vitamin D receptors in muscle tissue affect function. Deficiency causes proximal muscle weakness — difficulty climbing stairs or rising from chairs.
Common signFatigue and low mood
Low Vitamin D is associated with chronic fatigue and depression. Worth testing if fatigue persists without another explanation.
Non-specific signFrequent infections
Vitamin D plays a key role in innate immunity. Deficient individuals have higher rates of respiratory infections including influenza.
Immune effectDark skin or limited sun
Melanin reduces UVB conversion efficiency. People with darker skin in northern latitudes are at high risk of deficiency.
Risk factorAge over 65
Skin thins with age, reducing Vitamin D synthesis. Older adults spend less time outdoors — deficiency is very common.
Age-related riskTesting Schedule
How often should
you get tested?
Frequency depends on your current health status and your doctor's guidance.
General population
Routine Vitamin D testing not recommended for asymptomatic healthy adults. Test when deficiency is clinically suspected.
High-risk groups
Dark skin, limited sun, elderly, malabsorption (Crohn's, coeliac), obesity, or CKD. Baseline + annual monitoring.
During supplementation
If you start high-dose D3, retest in 3–4 months to confirm levels have reached the sufficient range.
On maintenance dose
Once sufficient levels are confirmed, annual testing to ensure maintenance dosing keeps levels stable.
If Your Result Is Abnormal
How to correct Vitamin D deficiency
Vitamin D deficiency is one of the easiest nutritional deficiencies to correct — with the right approach.
Sun exposure
10–20 minutes of midday sun on arms and legs (without SPF) 3–4 times per week can maintain adequate levels in fair-skinned adults.
+10–20 ng/mL (seasonal)D3 supplementation
Vitamin D3 (cholecalciferol) is more effective than D2 at raising 25-OH levels. Typical therapeutic dose: 2000–4000 IU/day.
Most effective methodDietary sources
Oily fish (salmon, sardines), egg yolks, and fortified dairy/plant milks contain Vitamin D but not enough to correct existing deficiency alone.
Supplementary benefitTake with fat & K2
Vitamin D is fat-soluble — take with the largest meal for optimal absorption. Vitamin K2 taken alongside D3 directs calcium to bones.
Optimise absorption