Vitamin D Deficiency: Symptoms, Testing, and What to Do

Vitamin D is often called the "sunshine vitamin" because your skin produces it when exposed to UV light. But despite living in a sun-rich country, over 40% of American adults have insufficient vitamin D levels — and most have no idea.

Vitamin D deficiency is one of the most common and most correctable nutritional deficiencies in the world. It is also one of the most consequential: low vitamin D is linked to bone loss, immune dysfunction, depression, cardiovascular disease, and increased cancer risk.

The good news is that testing is simple, and treatment is straightforward.

Vitamin D is technically a hormone, not a vitamin. It regulates hundreds of genes and plays a role in nearly every system in the body:

• Bone health — vitamin D is essential for calcium absorption. Without enough vitamin D, your body cannot absorb calcium effectively, leading to bone loss and increased fracture risk.
• Immune function — vitamin D activates immune cells and helps regulate the inflammatory response. Low levels are associated with increased susceptibility to infections and autoimmune conditions.
• Muscle function — vitamin D receptors are present in muscle tissue. Deficiency is associated with muscle weakness and increased fall risk in older adults.
• Mood and mental health — vitamin D receptors are found throughout the brain. Low levels are associated with depression, seasonal affective disorder, and cognitive decline.
• Cardiovascular health — vitamin D influences blood pressure regulation and inflammation. Low levels are associated with higher cardiovascular risk.
• Cancer prevention — vitamin D plays a role in cell growth regulation. Low levels are associated with higher rates of colorectal, breast, and prostate cancer.

Vitamin D deficiency is often called a "silent" deficiency because many people have no obvious symptoms, especially in the early stages. When symptoms do appear, they are often vague and easy to attribute to other causes:

• Fatigue and low energy — one of the most common complaints; often dismissed as stress or poor sleep
• Bone pain or achiness — especially in the back, hips, and legs
• Muscle weakness — difficulty climbing stairs, getting up from a chair, or carrying groceries
• Frequent illness — getting sick more often than usual, or taking longer to recover
• Depression or low mood — especially in winter months (seasonal affective disorder is strongly linked to low vitamin D)
• Hair loss — severe deficiency can contribute to hair thinning
• Slow wound healing — vitamin D plays a role in the healing process
• Bone loss — detected on a DEXA scan; often the first clinical sign in older adults

Because these symptoms overlap with so many other conditions, the only reliable way to know if vitamin D is the cause is to test your blood level.

Vitamin D status is measured with a blood test called 25-hydroxyvitamin D (also written as 25(OH)D or Vitamin D, 25-OH). This is the storage form of vitamin D in your body and the best indicator of your overall vitamin D status.

The test is simple — a single blood draw, no fasting required.

Note: Reference ranges vary slightly between labs and medical organizations. The Endocrine Society defines deficiency as below 20 ng/mL; many integrative medicine practitioners aim for 40–60 ng/mL.

Almost everyone is at some risk, but certain groups are at significantly higher risk:

• People with limited sun exposure — office workers, people who live in northern latitudes, those who cover their skin for cultural or religious reasons
• People with darker skin — melanin reduces the skin's ability to produce vitamin D from sunlight
• Older adults — the skin's ability to produce vitamin D decreases with age; older adults also spend less time outdoors
• Obese individuals — vitamin D is fat-soluble and gets sequestered in fat tissue, reducing circulating levels
• People with malabsorption conditions — Crohn's disease, celiac disease, and bariatric surgery can impair vitamin D absorption
• People who avoid dairy — dairy is one of the few dietary sources of vitamin D
• Breastfed infants — breast milk is low in vitamin D; supplementation is recommended

Vitamin D3 (cholecalciferol) supplements are inexpensive, widely available, and highly effective. Dosing depends on how deficient you are:

• Mild insufficiency (20–29 ng/mL): 1,000–2,000 IU/day
• Deficiency (below 20 ng/mL): 2,000–4,000 IU/day, or a short-term high-dose protocol under physician guidance
• Severe deficiency: 50,000 IU/week (prescription dose) for 8–12 weeks, then maintenance dosing

Vitamin D3 is better absorbed than D2 and is the preferred form for supplementation. Taking it with a meal that contains fat improves absorption.

10–30 minutes of midday sun exposure on the arms and legs several times per week can meaningfully raise vitamin D levels — but this varies significantly by latitude, season, skin tone, and age. Sun exposure is not a reliable sole strategy for most people.

Few foods are naturally high in vitamin D:

• Fatty fish (salmon, mackerel, sardines) — 300–600 IU per serving
• Egg yolks — 40 IU each
• Fortified milk and orange juice — 100 IU per cup
• Fortified cereals — varies

Diet alone is rarely sufficient to correct a deficiency.

If you start supplementing, retest your 25(OH)D level after 3 months to confirm your levels have normalized. Vitamin D toxicity is rare but possible with very high doses — retesting ensures you are in the optimal range without overshooting.

You do not need a physician's order to check your vitamin D level. At Reliable Exams, a Vitamin D (25-OH) test can be ordered online and drawn at any LabCorp or Quest Diagnostics location near you. No fasting required. Results in 1–2 business days.

If you want to check vitamin D alongside other key wellness markers, explore our Female Wellness Panel or Male Wellness Panel.