Fatigue that does not lift after a full night of sleep, a dull ache in the bones and legs, muscles that feel heavier than they should – these are the complaints that most often lead people to look up vitamin D. In this guide, Dr. Ergenekon Karagöz, Specialist in Internal Medicine, explains what vitamin D deficiency is, how it usually shows itself, how it is diagnosed and what treatment involves.
Rather than a collection of anonymous stories, the sections below describe the picture as it is seen in practice: what people report, how complaints progress, when they improve and when they need attention.
What is vitamin D deficiency?
Vitamin D is unusual among vitamins because most of it does not come from food. It is produced in the skin under ultraviolet B light from the sun, then converted in the liver and kidneys into its active form. Its central task is to keep calcium and phosphate in balance so that bone can mineralise properly; it also supports muscle function and immune regulation.
Deficiency means that store has dropped below what these tasks require. Calcium is then absorbed less efficiently from the gut, and the parathyroid glands raise their hormone output to keep blood calcium steady. That compensation works by drawing calcium out of the skeleton, which is why long-standing deficiency eventually affects bone strength and not only energy.
Symptoms of vitamin D deficiency
Mild deficiency is frequently silent. Many people have low levels for a long time without noticing anything specific, which is why it is often found incidentally on a blood test requested for another reason.
When complaints do appear, they are usually some combination of the following:
- Persistent tiredness that is not explained by workload or sleep and does not improve much with rest.
- Diffuse bone and muscle pain, most often in the lower back, hips, thighs and rib cage, and usually on both sides rather than in one spot.
- Muscle weakness, noticed as difficulty climbing stairs, rising from a low chair or lifting the arms above the head.
- Low mood, irritability and difficulty concentrating.
- Increased hair shedding and a sense of catching infections more easily.
- In infants and children, delayed walking, bowing of the legs and bone deformity (rickets).
The pain is typically described as deep and internal rather than sharp, present through most of the day rather than triggered by a particular movement. Morning stiffness and heavy, aching legs are common descriptions.
Why symptoms alone are not enough
Every one of these complaints occurs in other conditions too. Thyroid disease, anaemia, vitamin B12 deficiency, depression, sleep disorders and chronic pain syndromes all produce fatigue and diffuse aching. Recognising yourself in this list points towards being assessed, not towards a conclusion. Our articles on thyroid disorders and vitamin B12 deficiency cover two of the conditions most often confused with it.
What causes vitamin D deficiency?
In most cases the cause is not enough effective sun exposure, often combined with something that reduces production or absorption:
- Spending most daylight hours indoors, or living where winter sunlight is weak for months.
- Clothing that covers most of the skin, and consistent high-factor sunscreen use.
- Darker skin tone, since more melanin means longer exposure is needed.
- Older age, as the skin produces vitamin D less efficiently.
- Obesity, because vitamin D is fat soluble and held in adipose tissue.
- Malabsorption: coeliac disease, inflammatory bowel disease or previous bariatric surgery.
- Chronic kidney or liver disease, which interferes with conversion to the active form.
- Some long-term medicines, including certain anti-epileptic drugs and corticosteroids.
- Exclusively breastfed infants not given the supplement advised for their age.
Food plays a smaller part than most people expect. Oily fish, egg yolk, liver and fortified dairy products contribute, but an ordinary diet rarely covers the daily requirement alone.
How is vitamin D deficiency diagnosed?
Diagnosis is made with a blood test measuring serum 25-hydroxyvitamin D, shown on most laboratory reports as 25(OH)D. This is the storage form and the one that reflects your actual status.
Reference ranges differ between guidelines and between laboratories, so a number on a report does not interpret itself. Your doctor reads it together with your symptoms and, where relevant, with calcium, phosphate, alkaline phosphatase and parathyroid hormone. A value that needs treatment in one person may need only follow-up in another.
Testing is not needed for everyone; it is generally appropriate with suggestive symptoms, a clear risk factor, a bone health concern or a condition affecting absorption.
How is vitamin D deficiency treated?
Treatment means replacing what is missing and, where possible, correcting why it went missing. Replacement is prescribed by a physician; the amount and schedule depend on the measured level, age and weight, kidney function, the underlying cause and other medication. No dose is given here, because a plan that suits one person can be unsuitable for another.
A follow-up blood test after an agreed interval is part of treatment, not an optional extra. It is the only objective way to confirm the level has risen and to decide what comes next: a maintenance amount, continued treatment, or a search for an absorption problem.
Where malabsorption or a chronic illness lies behind the deficiency, treating that condition matters as much as replacement; otherwise levels fall again once supplementation stops.
More is not better
Vitamin D is fat soluble, so it accumulates. High amounts taken for long periods without supervision can push blood calcium too high, causing nausea, unusual thirst, frequent urination, constipation, confusion and, in time, kidney stones. High-dose regimens copied from another person are a common route to this problem.
Sunlight and diet
Short, regular exposure of the face and forearms in the warmer months supports natural production, balanced against skin cancer risk; burning is never the goal and does not increase production. Sunlight through window glass does not work, because ultraviolet B is filtered out.
How long does it take to feel better?
Blood levels respond faster than symptoms. With appropriate replacement the laboratory value usually rises within weeks, while aching, bone pain and fatigue ease more gradually, often over several weeks to a few months. Recovery is genuinely variable: initial severity, the underlying cause, body weight and how consistently treatment is taken all matter.
If the level normalises but symptoms do not, that is useful information rather than a failed treatment: vitamin D was probably not the whole explanation, and another cause deserves attention. Our detailed guide to vitamin D deficiency covers the wider picture.
When should you see a doctor?
- Fatigue or diffuse aching lasting weeks without an obvious explanation.
- Muscle weakness affecting stairs, standing up or carrying things.
- Bone pain concentrated in one point, pain that wakes you at night, unexplained weight loss or fever; these need assessment for other causes.
- Numbness or tingling around the mouth or in the hands, or muscle cramps and spasms, which may point to low calcium and should be assessed promptly.
- In children: delayed walking, bowed legs, swelling at the wrists or ribs, or poor growth.
- Nausea, marked thirst, frequent urination or confusion while taking a supplement; stop it and seek advice.
Frequently asked questions
Can I start a supplement without a blood test?
Routine preventive amounts appropriate to age exist, but when there are symptoms the sensible order is test first, then treat. Starting alone can mask the problem without addressing why it happened, and leaves no baseline to compare with later.
Does vitamin D deficiency cause hair loss?
Increased shedding is often reported with low levels and frequently settles as they are corrected. It is not the only cause of hair loss, so shedding that continues after treatment should be looked at separately.
Is sunlight enough in summer?
It depends on how much skin is exposed, at what hours, for how long, and on skin tone. A sunny climate is no guarantee; deficiency is regularly found in people who spend the day indoors in any season.
Can the deficiency come back?
Yes. If the circumstances that caused it have not changed, levels commonly fall again after treatment ends, which is why a maintenance plan and periodic re-testing are often advised.
Information note: this article is for general information and does not replace a medical consultation. Vitamin D deficiency is diagnosed with a blood test and treated individually; do not start, change or stop a supplement without consulting your own physician.
