Fatigue, muscle weakness, or aching bones can have many explanations, so they should not automatically be attributed to low vitamin D. If thyroid concerns are also part of your health picture, our multivitamin for hypothyroidism guide can help you review supplement considerations more carefully.
Vitamin D status is evaluated through blood testing rather than symptoms alone. In the United States, recent national data also show why balance matters: low levels remain relevant, while excessive levels have become more common among some supplement users.
What does low vitamin D mean?
Vitamin D is a fat-soluble nutrient that helps the body absorb calcium and phosphorus. Those minerals support bone structure, muscle function, and other physiological processes. The body obtains vitamin D from certain foods, fortified products, supplements, and production in the skin after ultraviolet exposure.
The phrase “low vitamin D” may refer to a result below a laboratory or clinical reference range. It does not always mean that a person has severe deficiency or that symptoms are caused by vitamin D. Test interpretation depends on the measurement, the patient’s age, health conditions, medications, season, and clinical history.
The latest national report from the CDC, published in June 2026 and based on data collected through August 2021 to August 2023, found that vitamin D insufficiency, defined in that report as 25-hydroxyvitamin D below 50 nmol/L, affected 22% of US people aged 6 years and older. That was down from 28% in 2001 to 2002. The same report found that vitamin D levels above 125 nmol/L increased from less than 1% to 8% over those periods, according to the 2026 CDC report.
These figures describe population patterns, not an individual diagnosis. A person can have symptoms with a borderline result, or have no symptoms despite a low result. This is why clinical context matters.
What are the symptoms of low vitamin D?
Many people with low vitamin D have no clear symptoms. When symptoms occur, they may be mild and overlap with sleep problems, anemia, thyroid disorders, medication effects, mood disorders, and other common conditions.
Possible symptoms include:
- Fatigue or reduced energy
- Muscle aches, cramps, or weakness
- Bone pain or tenderness
- Increased difficulty climbing stairs or rising from a chair
- Changes in mood
- More frequent falls in vulnerable older adults
Severe or prolonged deficiency can interfere with bone mineralization. In adults, this may contribute to osteomalacia, a condition involving softer bones. In children, serious deficiency can cause rickets, which may affect normal bone growth.
Fatigue and low mood deserve particular caution. Research may find an association between vitamin D status and these symptoms, but an association does not prove that low vitamin D caused them. Correcting a confirmed deficiency is appropriate medical care, but vitamin D supplements should not be presented as a universal treatment for depression, chronic fatigue, or other unrelated conditions.
Race and skin pigmentation can also complicate interpretation. The 2026 CDC vitamin D factsheet reported that the lowest measured levels were found more often among non-Hispanic Black participants, while also noting that bone health outcomes do not always correspond directly with serum vitamin D concentrations across population groups. The CDC vitamin D factsheet advises interpreting these patterns carefully.
What causes low vitamin D?
Several factors can reduce vitamin D levels. A person may consume too little vitamin D, absorb it poorly, produce less through the skin, or have difficulty converting it into forms the body can use.
Limited sun exposure
People who spend little time outdoors may produce less vitamin D in the skin. However, sun exposure is not a simple treatment strategy. Ultraviolet radiation increases skin cancer risk, and vitamin D production varies with season, latitude, skin pigmentation, clothing, age, cloud cover, and sunscreen use.
Skin pigmentation and age
Higher melanin levels reduce the skin’s efficiency in producing vitamin D from ultraviolet exposure. The skin also becomes less efficient with age. Older adults, especially those who are homebound or living in long-term care, may therefore have a higher risk of low levels.
Digestive and medical conditions
Celiac disease, Crohn’s disease, cystic fibrosis, inflammatory bowel disease, pancreatic disorders, and some forms of liver disease can interfere with digestion or absorption. Bariatric surgery may also reduce the absorption of fat-soluble nutrients.
Kidney and liver conditions can affect the conversion of vitamin D into active forms. Obesity is another recognized risk factor because vitamin D may become distributed into body fat, reducing its concentration in the blood.
Medications and overlapping symptoms
Some medications can lower vitamin D levels or affect its metabolism. Examples include certain steroids, seizure medicines, cholesterol-binding drugs, and other treatments. A complete medication review is important before starting a supplement.
If you are also managing thyroid disease, symptoms such as fatigue, dry skin, muscle discomfort, or mood changes may have several possible causes. Our guide to vitamins to avoid with hypothyroidism discusses why supplement ingredients and medication timing require careful review.
How is low vitamin D tested?
The usual test is a blood measurement of serum 25-hydroxyvitamin D, also written as 25(OH)D. This marker reflects vitamin D obtained from food, supplements, and skin production over time.
Testing is not necessarily needed for every healthy adult. Clinicians may consider it when a person has bone pain, muscle weakness, osteoporosis, fractures, malabsorption, kidney disease, liver disease, limited mobility, or another risk factor.
Reference categories vary by guideline and clinical setting. A 2026 analysis of US National Health and Nutrition Examination Survey data classified levels below 30 nmol/L as being at risk of deficiency, 30 to 49 nmol/L as being at risk of inadequacy, 50 to 125 nmol/L as sufficient, and levels above 125 nmol/L as potentially concerning. The 2026 national analysis also emphasized that vitamin D status should be interpreted using the full clinical picture.
Laboratories may report results in nanomoles per liter or nanograms per milliliter. The conversion is approximately 2.5 nmol/L for each 1 ng/mL. Your clinician can explain which range applies to your situation and whether repeat testing is appropriate.
Which foods contain vitamin D?
Food can contribute to vitamin D intake, although relatively few foods naturally contain substantial amounts. Common sources include fatty fish such as salmon, trout, tuna, sardines, and mackerel. Egg yolks, beef liver, and some mushrooms also contain vitamin D in varying amounts.
Fortified foods may provide a more consistent dietary source. Depending on the product, these can include cow’s milk, plant-based milk, breakfast cereals, yogurt, and orange juice. Nutrition labels can help you compare serving sizes and vitamin D amounts.
Food choices are useful for prevention and general nutritional adequacy, but they may not correct a confirmed deficiency on their own. The amount needed depends on the person’s blood level, age, body size, absorption, pregnancy or breastfeeding status, kidney function, and other medical factors.
Sun exposure should not be used to justify unprotected ultraviolet exposure. Dermatology guidance generally prioritizes skin protection, because the amount of vitamin D produced by sunlight is difficult to predict and excessive ultraviolet exposure increases skin cancer risk.
How is low vitamin D treated safely?
Treatment may include vitamin D2 or vitamin D3, dietary changes, management of an absorption problem, or a combination of these approaches. The dose and duration should be selected by a healthcare professional when deficiency is confirmed.
Higher doses may be used for a limited period under medical supervision. They are not automatically appropriate for routine self-treatment. Taking more vitamin D does not necessarily produce better results, and excess intake can raise blood calcium levels.
Possible warning signs of excessive vitamin D intake include nausea, constipation, unusual thirst, frequent urination, weakness, confusion, and poor appetite. These symptoms can have other causes, but they warrant medical advice if they occur after starting high-dose supplementation.
Review every supplement with your clinician or pharmacist. This is especially important if you take medications, have kidney stones, kidney disease, liver disease, sarcoidosis, hypercalcemia, or a condition affecting calcium metabolism.
People managing thyroid disease may also need to consider the timing of mineral-containing products with levothyroxine. Our information on nutritional supplements for thyroid health explains why medication timing and ingredient review should be individualized rather than assumed.
What does the latest US evidence suggest?
Recent US data support a balanced interpretation. Vitamin D insufficiency has declined across the national time series, but low levels remain present in a meaningful portion of the population. At the same time, excess levels increased between the 2001 to 2002 and August 2021 to August 2023 survey periods.
A 2026 study using national survey data also found that the proportion of people taking two or more vitamin D-containing supplements who had levels considered potentially concerning rose from 4.7% to 30.6% across the study period. This finding does not mean that supplements are unsafe when medically indicated. It does show why combining several products can unintentionally increase total intake, as reported in the peer-reviewed analysis.
Global findings also vary widely. A pooled analysis published in 2023 included more than 7.9 million participants from 81 countries and found that prevalence estimates changed substantially according to the threshold used, latitude, season, sex, assay, and study design. The global pooled study illustrates why one universal percentage cannot describe every population.
The practical lesson is straightforward: test when there is a clinical reason, address the underlying cause, use food and supplements thoughtfully, and avoid treating a laboratory number in isolation.
What to remember about low vitamin D
Low vitamin D can be silent, nonspecific, or clinically important depending on its severity and the person’s health history. A 25-hydroxyvitamin D blood test can help clarify status, but the result should be interpreted alongside symptoms, medications, absorption issues, bone health, and other laboratory findings. Food, fortified products, and supplements can all contribute to vitamin D intake, while excessive supplementation can create avoidable risks. Discuss testing and dosing with a qualified healthcare professional instead of relying on symptoms or high-dose self-treatment.
Take action with Core Axis
If you are evaluating nutritional support alongside care for Hashimoto’s or autoimmune thyroiditis, a transparent daily routine may make supplement review easier. The goal is not to replace vitamin D testing, prescribed treatment, or individualized medical advice.
Our thyroid support supplement is formulated as a three-capsule daily protocol with ten disclosed active compounds, including vitamin D3, selenium, zinc, magnesium, and selected vitamin forms. We publish ingredient, dose, and testing information so you can review the formula with your healthcare professional. The product also includes our Hashimoto’s Protocol Guide for structured evaluation.
Frequently Asked Questions
Can low vitamin D cause fatigue?
It can contribute to fatigue in some people, particularly when deficiency is significant. Fatigue is nonspecific, so anemia, thyroid disease, sleep disorders, medication effects, and other conditions should also be considered.
What is the best test for vitamin D deficiency?
The commonly used test is serum 25-hydroxyvitamin D, also called 25(OH)D. Your clinician can determine whether testing is appropriate and explain the result using the laboratory range and your health history.
Can sunlight correct low vitamin D?
Sunlight can stimulate vitamin D production in the skin, but the amount varies widely and unprotected ultraviolet exposure increases skin cancer risk. Food, fortified products, or supplements may be safer and more predictable options when recommended by a healthcare professional.
Should you take vitamin D without a blood test?
Some people may use routine amounts found in standard supplements, but high-dose treatment should not be started without professional guidance. Testing is especially important when symptoms, risk factors, chronic illness, medication use, or a history of deficiency is present.
Can a thyroid support supplement replace vitamin D treatment?
No. Our thyroid support supplement is intended as nutritional support for people evaluating a Hashimoto’s-focused routine, not as a replacement for vitamin D treatment, thyroid medication, laboratory testing, or medical care.