01 Fatigue and tiredness
Persistent, unexplained fatigue is one of the most common signs. In one 2014 study, 89% of people presenting with chronic fatigue had 25(OH)D below 30 ng/mL.
Vitamin D Deficiency: Symptoms and Warning Signs
Roughly 1 billion people worldwide have low serum 25-hydroxyvitamin D [25(OH)D]. Symptoms are usually subtle and non-specific, which is why deficiency often goes undiagnosed for years. Below are the nine most common signs, with the evidence behind each one — and a note on when to ask your doctor for a blood test.
02 Bone pain and back pain
Vitamin D is required for calcium absorption and bone mineralisation. Sub-clinical deficiency often manifests as diffuse aching, especially in the lower back, hips, and shins (osteomalacia-related pain).
03 Muscle weakness and cramps
Vitamin D receptors are present in muscle. Deficiency causes proximal muscle weakness, difficulty rising from a chair, and increased fall risk in older adults.
04 Frequent infections
Vitamin D modulates innate and adaptive immunity. Deficient individuals have more upper respiratory infections; meta-analyses show supplementation reduces acute respiratory infection risk by ~12%.
05 Slow wound healing
Adequate 25(OH)D is needed for the production of cathelicidin and defensins that support tissue repair; deficient patients heal surgical and diabetic wounds more slowly.
06 Hair loss
Severe deficiency is associated with alopecia areata and generalised hair thinning. Vitamin D receptor knockout mice develop alopecia — the pathway is well established.
07 Mood changes and depression
Observational studies consistently link low 25(OH)D with depressive symptoms and seasonal affective disorder, though randomised supplementation trials show mixed results.
08 Bone loss (osteopenia / osteoporosis)
Chronic deficiency leads to secondary hyperparathyroidism, bone resorption, and reduced BMD on DXA. This is often silent until a fracture.
09 Impaired glucose control
Deficiency is associated with insulin resistance; some trials show modest improvements in HbA1c with supplementation in deficient individuals.
When should I ask for a 25(OH)D blood test?
A serum 25(OH)D test is reasonable if you have any of these risk factors:
- Live above ~40° latitude (most of the US, all of Canada, northern Europe)
- Fitzpatrick skin phototype IV, V, or VI
- Age 70+ (skin 7-DHC declines with age)
- BMI > 30 (adipose sequestration lowers serum 25(OH)D)
- Chronic kidney disease, malabsorption (Crohn's, celiac), or bariatric surgery
- On glucocorticoids or anticonvulsants (accelerate 25(OH)D catabolism)
- Persistent bone pain, unexplained fatigue, or frequent infections
Once you have a result, you can interpret it with our blood test interpreter.
What to do if you're deficient
If your 25(OH)D is below 20 ng/mL (50 nmol/L), most guidelines recommend a loading dose of 50,000 IU cholecalciferol weekly for 8 weeks, followed by 1,500–2,000 IU/day maintenance. Below 12 ng/mL (30 nmol/L), see a physician for urgent repletion and additional workup (calcium, phosphorus, PTH, kidney function). For insufficiency (20–30 ng/mL), 1,000–2,000 IU/day cholecalciferol with re-testing at 3 months is a common approach.
Use our reserves calculator to model how quickly your levels will rise on a given dose, or the main calculator to see how much sun exposure you'd need to reach the same target from UVB alone.