What vitamin deficiency causes that? A symptom-by-symptom guide to deficiencies and the tests to ask for

Short answer

Most symptoms people google as "a deficiency" have more than one possible cause, and a vitamin or mineral is only one of them. The deficiencies that most often cause real, noticeable symptoms in adults are iron (tiredness, breathlessness, hair shedding, cold hands, cravings for ice), vitamin B12 (tiredness, numbness or tingling in hands and feet, a sore tongue, memory problems) and folate (tiredness, mouth ulcers), with vitamin D mostly affecting bones and muscles rather than causing a clear daily symptom (1, 2, 3). Magnesium, potassium, calcium and zinc deficiencies are less common in healthy people who eat a varied diet, and usually come with an underlying reason such as a medicine, a gut condition or heavy alcohol use.

The quickest honest answer to "what am I deficient in?" is a blood test, not a guess. For most people that means asking for a full blood count, ferritin, vitamin B12 (with methylmalonic acid if the result is borderline), folate, 25-hydroxy vitamin D and thyroid function (TSH) (4).

If your symptoms are real but nobody has found a reason yet, you are not imagining them and it is not your fault. Deficiencies are slow, quiet and easy to miss, and many of them sit behind ordinary complaints like "I'm just tired".

Note: this article is for general education and is not medical advice. Symptoms such as numbness, weakness, breathlessness or hair loss can have many causes; a clinician should assess them. Do not start high-dose supplements without a test, because some of them can mask another problem or cause harm.

Why deficiency symptoms are so hard to read

Deficiencies rarely announce themselves with one clear sign. They develop over months, sometimes years, and the body compensates for a long time before anything shows. Iron is a good example. Stores in the form of ferritin drop first, then the iron supply to new red blood cells falls, and only at the end does haemoglobin drop enough to be called anaemia (1). People often feel tired, short of breath on stairs or shed more hair well before that last stage.

Three things make symptoms confusing:

• They are non-specific. Fatigue, low mood, poor sleep, headaches and brain fog can come from a deficiency, but also from stress, a short night, an infection, the menstrual cycle, a thyroid problem or a medicine.

• They overlap. Iron, B12 and folate deficiency all cause tiredness and a pale look. B12 and B6 problems can both cause tingling, and too much B6 can cause it too.

• They come with a reason. A deficiency is often a sign of something else: heavy periods, a gut condition that blocks absorption, a long-term medicine, pregnancy, a restrictive diet or heavy drinking. Finding the reason matters as much as fixing the number.

That is why the table below lists other common causes next to each deficiency. If you only look for the vitamin, you may miss the thing that is actually draining it.

Symptom map: which deficiency can cause it and what else usually does

The table is a starting point for a conversation with a doctor, not a diagnosis. A single symptom is rarely enough to point to one deficiency.

Tiredness and weakness

Tiredness is the symptom most people mean when they search for a deficiency, and the one with the longest list of causes. Among deficiencies, iron is the most common, especially in women with heavy periods, during pregnancy and in people who eat little meat. Low iron can cause tiredness even before anaemia: in a meta-analysis of 18 randomised trials in people with low iron but normal haemoglobin, iron supplements reduced self-reported fatigue, though they did not improve measured fitness (5).

Vitamin B12 and folate deficiency cause a type of anaemia with large red blood cells, which brings tiredness, breathlessness, a pale or slightly yellow look and sometimes a sore tongue (3). B12 can also affect nerves and memory before the blood count changes.

Vitamin D deficiency is linked to aching bones and muscle weakness, particularly in older adults and people who get little sun (6). It is less clearly a cause of everyday tiredness, and taking vitamin D without a deficiency has not been shown to boost energy.

If tiredness has lasted more than a few weeks, the first tests are the ones above plus thyroid function. Our guide to fatigue covers the non-nutritional causes, which are more common.

Numbness, tingling and weak legs

What deficiency causes numbness in hands while sleeping? The most common reason is simpler: pressure on a nerve from sleeping on your arm or bending your wrist, which is how carpal tunnel syndrome often shows up at night. Among deficiencies, B12 is the one to rule out, because it damages nerves gradually and symmetrically, usually starting in the feet and hands (2). Untreated, nerve damage from B12 deficiency can become permanent, which is why tingling that keeps coming back deserves a test rather than a supplement guess.

A surprising cause works the other way round: too much vitamin B6. Nerve damage is best documented at very high doses, but it has been reported below 500 mg a day, and in 2023 the European Food Safety Authority set its upper limit at 12 mg a day, far below the 100 mg US limit and below the amount in some B-complex and "magnesium with B6" products (7). If you have tingling and take several supplements, check every label for B6.

What deficiency causes weakness in the legs? Vitamin D deficiency can cause proximal muscle weakness, felt as difficulty climbing stairs or getting up from a chair (6). Low potassium causes weakness and cramps, but in practice it usually comes from medicines such as diuretics, vomiting or diarrhoea rather than diet (8). B12 deficiency can cause unsteadiness and weak legs through nerve damage. Statins, thyroid problems and long periods of inactivity are common non-nutritional causes.

Tingling lips and fingers, with muscle spasms, can be a sign of low blood calcium, which is often linked to low magnesium or vitamin D (9). The same tingling also happens during over-breathing in a panic attack, when blood chemistry shifts for a few minutes. Tingling that comes with fast breathing and fear usually settles as breathing slows; tingling that keeps coming back without that context needs a calcium check.

Hair loss

What vitamin deficiency causes hair loss? The best-supported link is with iron: low ferritin is common in women with diffuse hair shedding, and many dermatologists check it first. Zinc deficiency also causes hair loss, though it is uncommon without a gut condition, a very restricted diet or heavy alcohol use (10). Very low protein intake and crash diets can trigger shedding as well.

The single most common cause of sudden diffuse shedding is not a deficiency at all. It is telogen effluvium: a delayed shedding two to three months after an illness, a fever, surgery, childbirth, a crash diet or severe stress. It usually settles on its own within months. Thyroid problems, iron deficiency and some medicines are the main things to rule out.

Biotin is the vitamin most marketed for hair, but true biotin deficiency is rare, and a 2022 systematic review of supplements for hair loss found no trial of biotin on its own (11). High-dose biotin can also distort blood tests, including thyroid and some heart tests, so stop it a few days before blood work if you take it (12).

Skin and nails: itching, dryness, white spots and peeling fingers

What deficiency causes skin itching? Itching without a rash can be a sign of iron deficiency, and it is one of the reasons doctors check ferritin in unexplained itch. More often, though, itching comes from dry skin, eczema, allergies, or less commonly from liver, kidney or thyroid problems, which is why a blood panel is better than a single guess.

Dry skin is rarely a vitamin problem in people who eat a normal diet. Weather, hot showers, soap and age are the usual reasons. White spots on the skin are mostly not deficiencies either: pityriasis alba, fungal infections and vitiligo are common causes, and a doctor can usually tell them apart by looking. Peeling skin on the fingertips is most often irritation from water, detergents or eczema.

Nails can carry more useful clues. Spoon-shaped, thin nails are a classic sign of long-standing iron deficiency (1). Vertical ridges are mostly part of normal ageing. Darkening of the skin and nails can occur with B12 deficiency, especially in people with darker skin, and it fades with treatment.

Mouth and gums

A smooth, sore, red tongue, cracks at the corners of the mouth and recurrent mouth ulcers can all come with B12, folate or iron deficiency (3). They also come with oral thrush, dry lips and ill-fitting dentures.

What deficiency causes bleeding gums? Severe vitamin C deficiency, scurvy, does cause bleeding and swollen gums, easy bruising and slow healing (13). But it needs months of almost no fruit or vegetables, and it is rare outside very restricted diets, heavy alcohol use or severe illness. Bleeding gums are far more often gum disease, so the first stop is a dentist.

Cramps, twitching and shaky hands

Night leg cramps, eyelid twitching and muscle flickers are frequently blamed on magnesium. In most adults they have other causes: exercise, long standing, dehydration, caffeine, stress, short sleep and some medicines. A Cochrane review found that magnesium is unlikely to prevent ordinary night cramps in older adults (14). Real magnesium, calcium or potassium deficiency does cause cramps and spasms, usually with a clear reason such as diuretics, long-term acid-reducing medicines, diarrhoea or heavy drinking (15).

Shaky hands are more often caffeine, anxiety, low blood sugar, an overactive thyroid or a benign tremor than a vitamin problem.

Cold hands and feet, dark circles and cravings

What deficiency causes cold feet? Iron-deficiency anaemia can make you feel cold, and so can an underactive thyroid. Raynaud's phenomenon, where fingers turn white or blue in the cold, is the other common cause and is not a deficiency.

What deficiency causes dark circles? Iron deficiency can make skin look paler and circles more visible, but most dark circles are genetics, thin skin, allergies, rubbing and short sleep.

Cravings are mostly not a sign of a missing nutrient, with one well-documented exception: a strong urge to chew ice, called pagophagia. The NHS lists wanting to eat non-food items such as ice among the symptoms of iron deficiency anaemia (17), so it is a good reason to check ferritin. Salt cravings can follow sweating and fluid loss, and very occasionally signal an adrenal problem.

Mood, anxiety, brain fog and weight

Low B12 and low folate can affect mood and memory, and low vitamin D is more common in people with depression and anxiety (6). But "linked with" does not mean "caused by". People who feel low often go out less and eat differently, which can lower vitamin D on its own. Correcting a real deficiency is worthwhile; expecting a supplement to fix anxiety is usually disappointing.

Can vitamin D deficiency cause weight gain? People with obesity often have lower vitamin D levels, partly because vitamin D is stored in fat tissue. Trials of vitamin D supplements have not shown meaningful weight loss. Iron and B12 deficiency do not cause weight gain directly, although tiredness can reduce activity. An underactive thyroid is a more likely medical reason for unexplained weight gain.

Who is most likely to be deficient

Some situations raise the risk enough that testing makes sense even before symptoms:

• Heavy periods, pregnancy and recent childbirth: iron and folate (1).

• Vegan or mostly plant-based diet: B12 (there is no reliable plant source), sometimes iron, iodine and zinc (2).

• Over 60: B12, because stomach acid and absorption decline with age; vitamin D.

• Long-term metformin or acid-reducing medicines (PPIs, H2 blockers): B12, and with PPIs also magnesium (4, 15).

• Coeliac disease, Crohn's disease, weight-loss surgery: iron, B12, folate, vitamin D, zinc.

• Heavy alcohol use: thiamine (B1), folate, magnesium, zinc.

• Little sun, darker skin, covering clothing, long winters: vitamin D (6).

• Diuretics or frequent vomiting and diarrhoea: potassium and magnesium.

Which blood tests to ask for

For most adults with unexplained tiredness, hair shedding or tingling, a sensible first panel is:

1. Full blood count (CBC): shows anaemia and the size of red blood cells, which hints at iron (small cells) or B12/folate (large cells).

2. Ferritin: the best single test for iron stores. A ferritin in the low end of "normal" can still mean low stores in someone with symptoms, so ask what your number means for you.

3. Vitamin B12, plus methylmalonic acid (MMA) if B12 is borderline. Levels below about 180 pg/mL indicate deficiency, and 180–350 pg/mL is a grey zone where MMA helps (4).

4. Folate.

5. 25-hydroxy vitamin D, especially in winter or with little sun exposure (16).

6. TSH: thyroid problems mimic many deficiency symptoms.

7. Electrolytes, calcium and magnesium if you have cramps, spasms, tingling or take diuretics. Blood magnesium can look normal even when body stores are low, so it is read together with symptoms and medicines.

Zinc, copper, vitamin A and others are checked only when there is a specific reason.

Why "just take a multivitamin" is not the same as finding the cause

It is tempting to skip the test and buy something that covers everything. That approach has three problems. First, it can hide the cause: iron tablets may lift your energy while heavy periods or a bleeding stomach ulcer go unexplored. Second, it can mask a problem: high-dose folic acid can correct the anaemia of B12 deficiency while nerve damage continues (2). Third, more is not always safer: excess B6 harms nerves, extra iron in people who do not need it can build up, and fat-soluble vitamins A and D accumulate.

A test also gives you a baseline. If you then take a supplement, you can see whether the number actually moved, rather than relying on how you feel in a given week.

Living with a suspected deficiency: the questions people ask next

How long does it take to fix a deficiency?

It depends on the nutrient and the cause. With iron tablets, haemoglobin usually starts to rise within a few weeks, but refilling ferritin stores often takes about three months. B12 symptoms can improve within days to weeks of treatment, while nerve symptoms may take months and do not always fully recover. Vitamin D levels rise over about two to three months of regular intake. Ask when to re-test, so you can see whether treatment is working rather than guessing.

Can I just eat better instead of taking supplements?

For mild shortfalls, often yes: meat, fish, eggs and dairy for B12 and iron; leafy greens and legumes for folate; oily fish and eggs for vitamin D. But diet alone usually cannot correct a true deficiency quickly, and it cannot overcome an absorption problem such as pernicious anaemia, coeliac disease or long-term metformin use. Use food to maintain levels and treatment to correct them, with your doctor deciding which you need.

Should I stop my supplements before a blood test?

Tell your doctor everything you take. Biotin in particular can distort several blood tests, so labs often recommend stopping it for a few days beforehand. Iron tablets taken that morning can affect iron results. Do not stop prescribed medicines without asking.

Can a deficiency cause anxiety or heart palpitations?

Iron-deficiency anaemia can cause a faster heartbeat and palpitations, because the heart works harder to deliver oxygen (17). Low B12 and low vitamin D are associated with low mood and anxiety, but the link is not proven to be causal. Palpitations with chest pain, fainting or breathlessness at rest need a 911 call, whatever the suspected cause. Our guide to heart palpitations explains common triggers.

Is my ferritin "normal" if it is in the reference range?

Not necessarily for you. Reference ranges are wide, and many clinicians treat a low-normal ferritin as relevant when someone has symptoms such as tiredness, hair shedding or restless legs. Inflammation can also push ferritin up and hide low iron. Ask your doctor to interpret your number in the context of your symptoms rather than only the range.

Can I have a deficiency if I eat a healthy diet?

Yes. Absorption, not intake, is often the issue: pernicious anaemia blocks B12 absorption, coeliac disease reduces iron and folate uptake, and some medicines interfere with B12 and magnesium. Blood loss from heavy periods can outpace even an excellent diet. A healthy diet lowers the odds of a deficiency; it does not rule one out.

Before you blame yourself

Many people with a deficiency spend months hearing, and telling themselves, that they are just stressed, lazy or getting older. Iron runs low slowly; B12 can take years to drop. By the time symptoms are obvious, you may have adjusted to feeling worse without noticing. That is not a lack of willpower. It is a body running on less than it needs.

The useful step is not to push harder, and not to buy every supplement in the aisle, but to get one round of tests, fix what is found, and watch what changes over the following weeks. If the tests are normal, that is information too: it points the search towards sleep, stress, thyroid, hormones or another cause.

When to see a doctor

See a doctor soon if you have tiredness or breathlessness that has lasted more than a few weeks, hair loss in patches or with scalp changes, tingling or numbness that keeps coming back or is spreading, unexplained weight loss, or very heavy periods.

Call 911 or go to the emergency room for chest pain, fainting, breathlessness at rest, black or tarry stools or a lot of blood in your stool, vomiting blood, sudden numbness or weakness in the face, arm or leg (especially on one side of the body), or sudden confusion or trouble speaking (18, 19, 20).

How to bring this up with your doctor

What to ask for, by name. "Could we check a full blood count, ferritin, B12, folate, vitamin D and TSH?" If you have tingling or cramps: "Could we also check electrolytes, calcium and magnesium?" If B12 is borderline: "Would a methylmalonic acid test help?"

What to bring. A short list of symptoms with dates, how your periods are if you menstruate, everything you take (including supplements and their doses), your diet pattern and any gut symptoms. A few weeks of sleep and energy notes help show that this is a pattern, not one bad week.

What to say, out loud. "I've been [tired / losing hair / getting tingling in my feet] for [N] weeks. I'd like to rule out common deficiencies and thyroid problems before we assume it's stress."

If a result is low-normal and you still feel unwell. Ask: "What would this number mean for someone with my symptoms? When should we re-test, and what else should we check?"

How long until you notice a difference? Most supplements take weeks, not days. See how long supplements take to work, and how to tell if one is working.

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How Welltory helps — and what it cannot do

The limits first. Welltory is a general wellness product, not a medical device. It cannot measure iron, B12, vitamin D or any other nutrient, and it does not diagnose deficiencies or any other condition. A blood test is the only way to know your levels.

What it can do is record the footprint that a deficiency, and its treatment, may leave on your daily state, so that "I've been tired for months" becomes a dated pattern.

Sleep and recovery. Sleep Analysis uses your Apple Watch or Oura data on iOS, and syncs sleep from Samsung Health, Fitbit or Garmin Connect through Health Connect on Android (Sleep Analysis for Android). On iOS with Apple Watch or Oura, Battery starts each morning from your resting heart rate on waking and the quality of your sleep (Today screen). Anaemia can push resting heart rate up, so a resting heart rate drifting above your usual range alongside tiredness is something worth mentioning to your doctor.

Heart rate variability. If you take morning HRV measurements, take them at the same time after waking, before coffee or exercise, in the same position (accurate HRV measurements). HRV works with Apple Watch, Samsung Galaxy Watch, compatible chest straps or your phone camera.

Tag the change you make. On iOS with Apple Watch or Oura, when Welltory marks a stress or rest episode and asks "What happened?", tap, type or talk: "started iron", "heavy period day", "B12 injection", "poor night". Over weeks, My Patterns shows which tags tend to appear with stress and which with recovery; some sections need a paid plan.

Compare before and after. Once a deficiency is found and treated, look at four to eight weeks before and after: sleep, resting heart rate, stress minutes and Battery. If you feel better and your numbers settle closer to your usual range, that is a useful sign to share at the follow-up test. If nothing changes, that is useful too.

A record for your doctor. From the web app you can export a CSV (Dashboard → choose a chart → Export) (export guide) to bring to an appointment. Whatever shows up is an association to discuss with your doctor, not proof of a deficiency or of what fixed it.

How we made it

We based this article on the US National Institutes of Health Office of Dietary Supplements fact sheets, NHS guidance on anaemia, the American Academy of Family Physicians review of vitamin B12 deficiency (2025), a meta-analysis of iron in non-anaemic iron deficiency a 2022 systematic review of supplements for hair loss, and MedlinePlus and CDC pages on urgent warning signs. Search questions were taken from what people in the United States most often ask about deficiency symptoms. For the product section, we checked every function against the Welltory Help Center on October 21, 2026.

Made with AI tools, then edited and fact-checked by the Welltory team. See our Editorial & AI policy.

Jane Smorodnikova, co-founder of Welltory, built the methodology behind how we read physiological data.

Written by Tatsiana Yashyna.

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