The only way to confirm low vitamin D is a blood test, but your body often drops hints before you ever see a lab result. Fatigue, bone pain, muscle weakness, and mood changes are the most common signs in adults. Because these symptoms overlap with dozens of other conditions, recognizing which combination of clues points toward vitamin D deficiency can help you decide whether testing is worthwhile.
Physical Symptoms to Watch For
Low vitamin D doesn’t always announce itself clearly. Many people with mild deficiency feel nothing at all. When symptoms do appear, they tend to build gradually, which makes them easy to dismiss as normal aging or stress. The classic cluster includes:
- Persistent fatigue that doesn’t improve with more sleep
- Bone pain, especially in the lower back, hips, or legs
- Muscle weakness, aches, or cramps that seem disproportionate to your activity level
- Mood changes, particularly low mood or depression
In children, low vitamin D can show up as weak or sore muscles. Severe, prolonged deficiency in kids may lead to rickets, where bones grow incorrectly and bend or bow. Adults don’t get rickets, but the equivalent condition, called osteomalacia, causes soft, weakened bones that fracture more easily and can produce deep, aching bone pain.
Less Obvious Signs
Beyond the well-known symptoms, a few subtler changes may point to low vitamin D. Hair thinning is one. Research over the past decade has linked low levels to several types of hair loss, including female pattern hair loss and a condition called telogen effluvium, where hair sheds more than usual after a stressor. In at least one documented case, frontal hair thinning caused by vitamin D deficiency reversed after supplementation.
Vitamin D also plays a role in immune regulation. If you find yourself catching colds or other infections more frequently than usual, low vitamin D could be a contributing factor. Slow wound healing is another signal some people notice, though it’s harder to pin down without other context.
Mood, Cognition, and Seasonal Patterns
Vitamin D receptors are found throughout the brain, and low levels have been associated with both depression and poorer cognitive performance in older adults. The seasonal connection is worth noting: your skin produces vitamin D from sunlight, so levels naturally dip during fall and winter months. Seasonal changes in vitamin D have led researchers to investigate whether supplementation helps people with seasonal mood disorders, though the evidence there is mixed. Some small trials showed improvement, while others found no significant benefit.
If you notice that your mood reliably drops in darker months, or you experience a foggy, sluggish feeling that lifts in summer, low vitamin D is at least worth considering alongside other explanations.
Who’s Most Likely to Be Low
Certain factors make deficiency far more probable. Darker skin contains more melanin, which reduces the skin’s ability to produce vitamin D from sunlight. People over 65 synthesize vitamin D less efficiently. Living at higher latitudes (roughly north of Atlanta in the U.S., or north of Rome in Europe) means months of weak winter sunlight that can’t trigger meaningful vitamin D production. Spending most of your time indoors, consistently wearing sunscreen, or covering most of your skin for religious or cultural reasons all limit sun exposure.
People with higher body weight are also at greater risk because vitamin D is fat-soluble and gets sequestered in fat tissue, making less of it available in the bloodstream. Conditions that affect fat absorption, like Crohn’s disease or celiac disease, can reduce the amount of vitamin D you absorb from food and supplements.
The Blood Test That Confirms It
The standard test is called 25-hydroxyvitamin D. It’s a simple blood draw, and it’s the most accurate reflection of your body’s vitamin D stores. Here’s how to read the results, measured in nanograms per milliliter (ng/mL):
- Below 10 ng/mL: severe deficiency
- 10 to 19 ng/mL: mild to moderate deficiency
- 20 to 50 ng/mL: optimal range
- 51 to 80 ng/mL: risk of excess calcium in urine begins to rise
- Above 80 ng/mL: toxicity is possible
At-home finger-prick test kits are now widely available. The samples go through the same laboratory processing and validation as blood drawn in a clinic, so they can be a reasonable option if getting to a doctor’s office is a barrier.
Should You Get Tested?
This is where guidelines may surprise you. The Endocrine Society’s 2024 clinical practice guideline suggests against routine vitamin D screening for healthy adults of any age, including people with darker skin and people with obesity. The reasoning: clinical trials haven’t established a specific blood level that reliably predicts better health outcomes for the general population, so testing healthy people and chasing a number hasn’t been shown to help.
That said, “routine screening” is different from “testing someone with symptoms.” If you’re experiencing the fatigue, bone pain, muscle weakness, or mood changes described above, especially combined with risk factors like limited sun exposure or a malabsorption condition, a 25-hydroxyvitamin D test is a reasonable step. The guideline is aimed at discouraging blanket testing in people who feel fine, not at discouraging a targeted workup when something feels off.
What Happens if You Don’t Address It
Mild deficiency is common and usually easy to correct. Left untreated for a long time, though, it can progress to serious bone problems. Severe, chronic vitamin D deficiency leads to osteomalacia in adults, where bones soften and weaken, raising the risk of fractures and skeletal deformities. It also contributes to osteoporosis by impairing the body’s ability to absorb calcium properly. In children, prolonged deficiency causes rickets, which distorts bone growth.
Daily Intake Recommendations
The recommended dietary allowance set by the National Institutes of Health is 600 IU (15 mcg) per day for most people ages 1 through 70, including during pregnancy and breastfeeding. Adults over 70 need 800 IU (20 mcg) per day. Infants up to 12 months have an adequate intake set at 400 IU (10 mcg) daily.
These numbers represent the baseline for maintaining bone health and normal calcium metabolism in healthy people. If a blood test shows you’re already deficient, your provider will typically recommend a higher dose for a set period to bring levels back up, then a maintenance dose to keep them there. The Endocrine Society does recommend empiric supplementation (meaning without testing first) for adults 75 and older, because of a potential reduction in mortality risk, and during pregnancy, where supplementation may lower the risk of preeclampsia, preterm birth, and neonatal complications.
Good food sources include fatty fish like salmon and mackerel, fortified milk and orange juice, egg yolks, and fortified cereals. For most people, though, food alone doesn’t provide enough, which is why sunlight exposure and supplements matter. About 10 to 30 minutes of midday sun on your arms and legs a few times per week can make a significant difference, depending on your skin tone, latitude, and the time of year.