Why Is My Hair Falling Out? The Blood Tests That May Reveal the Cause

Hair loss is not a diagnosis in itself. It is a visible sign that can result from genetics, hormonal changes, nutritional deficiencies, illness, medication, inflammation, physical tension on the hair or disruption to the normal hair-growth cycle.

That is why there is no single blood test that can explain every case of hair loss.

Blood tests are most useful when shedding is widespread, begins suddenly, follows an illness or major physical change, or appears alongside symptoms such as fatigue, heavy periods, weight changes, feeling unusually cold, irregular menstrual cycles or digestive problems. They can help identify or rule out contributing factors such as iron deficiency, anaemia and thyroid dysfunction.

However, many common forms of hair loss—including pattern hair loss, traction alopecia and some autoimmune or inflammatory scalp conditions—are diagnosed primarily through your history and a careful examination of your scalp. A normal blood panel does not necessarily mean that nothing is wrong.

The most useful starting point is therefore not simply asking, “Which blood tests should I order?” It is understanding how your hair is being lost, when the change began and what else was happening in your body at the time.

The most common blood tests for hair loss

Depending on your symptoms, medical history and the pattern of hair loss, a clinician may consider some of the following tests:

This does not mean everyone experiencing hair loss needs every test on the list. Testing should be guided by the type of hair loss and the wider clinical picture. Ordering a large panel without a clear reason can produce borderline or incidental findings that create confusion without identifying the true cause.

Why is my hair falling out? Start with the pattern, not the blood test

Before deciding which blood tests may help, it is important to distinguish between several different problems that people commonly describe as “hair falling out.”

You may be experiencing:

• Increased shedding from the root
• Gradual thinning as individual hairs become finer
• Hair breakage along the shaft
• Smooth, defined bald patches
• Loss caused by repeated pulling or tension
• Inflammatory or scarring damage to the follicles

These processes can look similar from a distance, but they do not have the same causes or require the same investigations.

Diffuse shedding across the scalp

Diffuse shedding means that more hair than usual appears to be coming away from all over the scalp rather than from one clearly defined area. You may notice more hair in the shower, on your pillow, in your brush or when running your hands through it.

One common cause is telogen effluvium, a temporary disruption of the hair-growth cycle. A larger-than-usual number of follicles move into a resting phase and eventually release their hairs.

The trigger often occurs several weeks or months before the shedding becomes obvious. This delay means people do not always connect the hair loss with the original event.

Possible triggers include:

• A high fever or significant infection
• Surgery or hospitalisation
• Childbirth
• Rapid weight loss or a highly restrictive diet
• Severe emotional or physical stress
• Starting, stopping or changing certain medicines
• Iron deficiency
• Thyroid dysfunction
• A major change in hormone levels

According to the British Association of Dermatologists’ guidance on telogen effluvium, blood tests may be used to exclude contributing conditions such as iron deficiency and thyroid disease.

Telogen effluvium often improves once the trigger has passed or the underlying problem has been addressed. Regrowth is slow, however, because hair follicles follow a cycle measured in months rather than days. Even after the cause is corrected, it may take time for shedding to settle and for visible density to return.

Gradual thinning around the parting, crown or temples

Gradual thinning is more suggestive of pattern hair loss, also known as androgenetic alopecia.

In women, the parting may appear wider and more scalp may become visible across the top of the head. In men, the change may begin with a receding hairline, thinning at the temples or loss around the crown. These patterns vary, and some people experience a combination of patterned thinning and increased shedding.

Pattern hair loss is usually diagnosed from:

• The location and shape of the thinning
• The gradual nature of the change
• Family history
• Differences in the thickness of individual hairs
• Examination of the scalp, sometimes using magnification

Blood tests do not diagnose inherited pattern hair loss directly. They may still be useful when the presentation is unusual, shedding is pronounced or another condition could be contributing.

For example, NICE guidance on investigating female pattern hair loss advises considering thyroid function, full blood count, ferritin and vitamin D in selected circumstances, particularly when telogen effluvium or another underlying cause is suspected.

Sudden round or oval bald patches

Smooth, well-defined patches can indicate alopecia areata, an autoimmune condition in which the immune system attacks hair follicles. It may affect the scalp, beard, eyebrows, eyelashes or other areas of the body.

Alopecia areata is usually recognised through the appearance of the affected area and an examination of the scalp and nails. Blood tests do not confirm the diagnosis, although a clinician may investigate thyroid or other autoimmune conditions when symptoms or medical history provide a reason to do so.

The British Association of Dermatologists’ information on alopecia areata explains that the condition often causes sudden, patchy hair loss and may sometimes be accompanied by nail changes or sensations such as tingling, itching or burning.

Hair loss around the edges, temples or areas under tension

Hair loss around the hairline may be caused by traction alopecia. This develops when repeated pulling places excessive stress on the follicles.

Styles and practices that may contribute include:

• Tight braids
• Cornrows
• Tightly secured wigs or extensions
• Repeatedly pulled-back ponytails or buns
• Heavy added hair
• Chemical processing combined with tension
• Frequent use of high heat

Blood tests will not diagnose traction alopecia. The history and distribution of the loss are usually more informative. Early changes may be reversible when tension is reduced, but prolonged traction can damage follicles permanently.

The American Academy of Dermatology’s advice on traction-related hair loss recommends changing hairstyles that repeatedly pull on the scalp and seeking assessment before the damage becomes advanced.

Hair breakage rather than shedding

Sometimes hair is not being released from the follicle at all. Instead, the shaft is snapping.

Breakage may produce shorter hairs of uneven length, rough ends, dryness or areas that seem unable to grow beyond a certain point. It can be associated with bleaching, relaxing, heat styling, friction, overmanipulation, tight styling or structural hair-shaft disorders.

A simple way to begin distinguishing breakage from shedding is to inspect the hair that comes away. A naturally shed hair may have a small pale club at one end, whereas a broken strand may be shorter and lack the expected root end.

Blood tests are unlikely to explain damage caused primarily by hair care or chemical processing. However, severe nutritional restriction and certain medical conditions can affect both the quality and growth of hair, so the wider context still matters.

1. Full blood count: looking for anaemia and broader clues

A full blood count, often abbreviated to FBC or CBC, measures several components of the blood. These include haemoglobin, red blood cells, white blood cells, platelets and measurements that describe the size and characteristics of red blood cells.

It can help identify anaemia, but it does not establish the cause by itself.

Iron deficiency is one cause of anaemia, but low haemoglobin can also result from vitamin deficiencies, chronic inflammation, inherited blood conditions, bleeding and other medical problems.

A full blood count may be particularly relevant when hair loss appears alongside:

• Persistent tiredness
• Breathlessness
• Palpitations
• Pale skin
• Headaches
• Heavy menstrual bleeding
• Pregnancy or recent childbirth
• A restricted diet
• Recent surgery or blood loss

It is also possible to have depleted iron stores before haemoglobin falls enough to meet the definition of anaemia. That is why a normal full blood count does not always rule out iron deficiency, and ferritin is often considered separately.

2. Ferritin and iron studies: checking the body’s iron stores

Ferritin is a protein that stores iron. A ferritin blood test is commonly used to assess how much stored iron is available in the body.

Iron supports many essential processes, including the production of haemoglobin, which allows red blood cells to carry oxygen. Low iron may be associated with fatigue, reduced exercise tolerance, headaches, brittle nails and hair loss.

Possible reasons for iron deficiency include:

• Heavy menstrual bleeding
• Pregnancy
• Bleeding from the digestive tract
• A diet that does not provide enough absorbable iron
• Reduced absorption caused by a digestive condition
• Frequent blood donation
• Increased physical demand
• Recent surgery or blood loss

The NHS overview of iron-deficiency anaemia identifies blood loss and pregnancy among the common causes and stresses that the cause of iron deficiency should be investigated rather than simply assumed.

Ferritin requires careful interpretation. A clearly low result supports iron deficiency, but ferritin can rise during infection, inflammation, liver disease and some chronic conditions. A result that falls within the laboratory range may therefore need to be considered alongside symptoms, the full blood count and, in selected cases, additional iron measurements such as transferrin saturation.

There is also no single ferritin level that explains every case of hair loss. A low result may represent an important contributing factor, but it should not automatically be treated as proof that iron deficiency is the only cause.

Should you take iron for hair loss?

Iron should not be taken simply because hair is shedding.

Too much iron can be harmful, and supplementation may obscure the need to identify ongoing blood loss or another underlying cause. Treatment is most appropriate when deficiency has been demonstrated and the dose, duration and follow-up plan are suitable for the individual.

The objective is not merely to improve a number on a laboratory report. It is to understand why the iron level is low and whether the deficiency is clinically relevant.

3. Thyroid function tests: assessing an important metabolic cause

The thyroid is a gland in the neck that produces hormones involved in metabolism, temperature regulation, energy, digestion and many other body functions.

Both underactive and overactive thyroid conditions can affect the hair-growth cycle.

An underactive thyroid may cause symptoms such as:

• Fatigue
• Feeling unusually cold
• Weight gain
• Constipation
• Dry skin or hair
• Difficulty concentrating
• Low mood
• Heavy or irregular periods

The NHS guidance on underactive thyroid lists dry hair and hair loss among the possible symptoms and explains that diagnosis usually involves testing thyroid-stimulating hormone and thyroxine.

The initial investigation generally includes thyroid-stimulating hormone, or TSH. Free T4 may be measured at the same time or added when the TSH result is outside the expected range. The exact approach depends on the laboratory, symptoms, medication use, pregnancy status and previous thyroid history.

Thyroid-related hair loss is often diffuse rather than confined to one small patch. However, hair loss alone is not enough to diagnose a thyroid condition. Thyroid results must be interpreted alongside symptoms and the laboratory’s own reference ranges.

4. Vitamin D: potentially relevant, but not an explanation for every case

Vitamin D has roles in bone health, immune function and several biological processes involving the skin and hair follicle. Low levels have been observed in some people with different forms of hair loss, but an association does not prove that vitamin D deficiency caused the shedding.

Testing may be reasonable when there are additional reasons to suspect deficiency, such as:

• Limited exposure to sunlight
• Darker skin in a low-sunlight environment
• Clothing that covers most of the skin
• Malabsorption
• Bone or muscle symptoms
• Pregnancy
• A diet with few vitamin D sources
• Medical conditions or medicines that affect vitamin D metabolism

A low result should be corrected appropriately, but it should not prevent investigation of other causes. Conversely, a normal vitamin D result does not exclude pattern hair loss, telogen effluvium, alopecia areata or a scalp disorder.

High-dose vitamin D should not be taken indefinitely without a clinical reason. As with iron, more is not automatically better.

5. Vitamin B12 and folate: useful when the history points towards deficiency

Vitamin B12 and folate are needed for normal blood-cell production. Deficiency may lead to anaemia and symptoms such as fatigue, weakness, a sore tongue and, in the case of B12 deficiency, neurological changes.

Testing may be considered when a person has:

• A vegan or highly restricted diet
• A history of stomach or intestinal surgery
• Coeliac disease or another malabsorption condition
• Long-term use of medicines that can affect B12 absorption
• Unexplained anaemia
• Numbness, tingling or balance problems
• Persistent digestive symptoms
• Reduced appetite or unintentional weight loss

B12 and folate are not universal first-line hair-loss tests. They are more useful when the medical history or other blood results create a reasonable suspicion of deficiency.

The NICE guideline on vitamin B12 deficiency emphasises assessing symptoms, dietary intake, medicines and possible causes rather than interpreting the result in isolation.

6. Zinc and other micronutrients: targeted testing is better than guesswork

Zinc supports tissue growth, immune function and wound healing. Severe or prolonged deficiency may affect the skin and hair, but routine zinc testing is not necessary for everyone experiencing hair loss.

It may be considered when there is a history of:

• Severe dietary restriction
• Malabsorption
• Chronic diarrhoea
• Intestinal disease
• Bariatric or other digestive surgery
• Poor wound healing
• Other signs of nutritional deficiency

Commercial hair supplements often contain zinc, selenium, vitamin A, vitamin E, biotin and several other nutrients. Taking these without confirming a need is not risk-free. Excessive intake of some nutrients can contribute to hair loss or cause other adverse effects.

The American Academy of Dermatology’s guidance on hair-loss diagnosis and treatment advises taking iron, zinc or biotin supplements only when testing shows that a deficiency is present.

A blood test should not be treated as a shopping list for supplements. The objective is to identify meaningful deficiencies, understand why they developed and correct them safely.

7. Hormone tests: useful when there are signs of androgen excess

Hormonal testing is not required for every person with pattern hair loss.

It becomes more relevant when thinning is accompanied by symptoms that suggest increased androgen activity or another endocrine disorder. These may include:

• Irregular or absent periods
• Difficulty becoming pregnant
• New or worsening acne
• Increased facial or body hair
• Rapidly progressing scalp hair loss
• A deepening voice
• Other signs of virilisation

Depending on the presentation, a clinician may consider tests such as total testosterone, sex hormone-binding globulin, a calculated free androgen index, DHEAS or prolactin. The exact selection and timing should be based on the symptoms rather than ordered as a generic “female hormone panel.”

Polycystic ovary syndrome is one possible explanation for a combination of irregular cycles, acne, increased facial or body hair and scalp thinning. However, it cannot be diagnosed from one hormone result, and not everyone with pattern hair loss has a hormonal disorder.

Hormonal results are also affected by age, menstrual timing, pregnancy, menopause, contraception and other medicines. Context is essential.

When additional blood tests may be appropriate

Sometimes the history points beyond iron, thyroid function or common nutritional concerns.

A clinician may consider additional investigations when hair loss occurs alongside digestive symptoms, joint pain, rashes, kidney problems, liver symptoms, recurrent illness or other systemic changes.

Examples include:

Coeliac disease screening

Coeliac disease can reduce the absorption of iron, folate and other nutrients. Screening may be considered when iron deficiency is unexplained or accompanied by bloating, diarrhoea, weight loss, abdominal discomfort, a family history of coeliac disease or another autoimmune condition.

Testing should generally be performed while the person is still eating gluten, because removing gluten beforehand can affect the result.

Liver and kidney function

Chronic liver or kidney conditions can affect nutrition, metabolism and the interpretation of certain blood markers. These tests are not specific to hair loss, but they may be appropriate when other symptoms or existing conditions make them relevant.

Autoimmune investigations

Autoimmune blood tests are not routine screening tools for ordinary shedding. They may be considered when there are signs of a wider autoimmune or connective-tissue condition, such as persistent joint symptoms, unusual rashes, mouth ulcers or marked sensitivity to sunlight.

Broad autoimmune panels ordered without supporting symptoms can produce results that are difficult to interpret and may not explain the hair loss.

What blood tests cannot tell you

Blood testing can uncover important contributors, but it has clear limitations.

A blood test cannot reliably determine:

• Whether tight hairstyles have damaged the follicles
• Whether hair shafts are breaking from chemical or heat damage
• Whether gradual thinning follows a genetic pattern
• Whether a smooth bald patch is alopecia areata
• Whether inflammation is beginning to scar the scalp
• Whether a fungal infection is affecting the hair
• Whether two different forms of hair loss are occurring together

These questions often require visual examination, magnification of the scalp, a hair-pull test, fungal testing or, occasionally, a scalp biopsy.

This is why the American Academy of Dermatology recommends professional assessment before assuming that a supplement, shampoo or blood result provides the full explanation.

Why a “normal” blood result may not settle the question

Laboratory reference ranges describe the results seen in a defined comparison population. They do not guarantee that every person within the range is free from symptoms or disease.

A normal result may still require context because:

• Reference ranges vary between laboratories
• Pregnancy and age can affect expected values
• Recent infection or inflammation can alter some markers
• Medicines and supplements may interfere with testing
• A result may have changed substantially while remaining inside the stated range
• Several borderline findings may matter more together than separately
• The cause of hair loss may not be detectable through blood testing

A result flagged as high or low is not automatically a diagnosis either. Mild abnormalities may be temporary, unrelated or require repeat testing.

Where possible, results should be reviewed alongside symptoms, medications, diet, menstrual history, recent illness and the visible pattern of hair loss. A pharmacist-led results review can help place individual markers in context and identify questions that may need follow-up with a doctor or dermatologist.

How to prepare for a useful hair-loss assessment

The quality of the investigation depends partly on the information available before testing.

Create a brief timeline that includes:

1. When you first noticed the change. Record whether the onset was sudden or gradual.
2. What the hair loss looks like. Note whether it is diffuse, patchy, concentrated around the parting or hairline, or associated with breakage.
3. Events in the preceding months. Include infections, fever, surgery, childbirth, major stress, rapid weight loss or changes to medication.
4. Other symptoms. Fatigue, heavy periods, temperature sensitivity, digestive problems, acne and menstrual irregularity may affect which tests are relevant.
5. Dietary patterns. Mention vegan or vegetarian diets, restrictive eating, low appetite and recent changes in weight.
6. Hair practices. Include braids, extensions, wigs, adhesives, relaxers, bleaching and frequent heat styling.
7. Medicines and supplements. Provide the full names and doses where possible.

High-dose biotin is especially important to disclose because it can interfere with certain laboratory tests. Do not stop prescribed medication unless advised to do so, but tell the testing provider what you take before your sample is analysed.

Photographs can also be useful. Take images in consistent lighting and from the same angles every few weeks. Day-to-day checking is often misleading because meaningful changes in density occur gradually.

When hair loss needs prompt clinical assessment

Arrange an assessment rather than relying only on a private blood panel when hair loss is:

• Sudden and rapidly progressing
• Confined to smooth, well-defined patches
• Associated with scalp pain, burning or intense itching
• Accompanied by redness, scaling, crusting, pus or bleeding
• Causing areas of shiny or scar-like skin
• Affecting the eyebrows or eyelashes
• Occurring in a child
• Accompanied by unexplained weight loss or systemic illness
• Associated with marked menstrual or hormonal changes
• Continuing for several months without improvement
• Causing significant emotional distress

Pain, burning, inflammation and visible scarring can indicate a condition capable of permanently damaging follicles. The British Association of Dermatologists’ information on lichen planopilaris, for example, explains that inflammatory destruction of the follicles can result in permanent hair loss.

Early assessment matters because treatment may be more effective before extensive follicle damage occurs.

Common mistakes when investigating hair loss

Assuming every case is caused by low iron

Iron deficiency is important, but it is only one possible contributor. Pattern hair loss, traction, autoimmune disease and scalp inflammation can occur with completely normal iron results.

Ordering every available test

More testing does not always produce more clarity. Large panels increase the likelihood of incidental borderline results that may have no relationship to the hair loss.

Treating a supplement as a diagnosis

Feeling better after taking a supplement does not prove which condition was present. Conversely, failing to improve does not necessarily mean the original result was irrelevant. Hair regrowth takes time, and more than one process may be involved.

Ignoring the scalp

A blood panel cannot replace examining the skin, follicle openings, scale, inflammation, breakage pattern and distribution of thinning.

Expecting immediate regrowth

Correcting a deficiency does not produce visible density within days. The follicles must re-enter their growth phase, and new hairs must grow long enough to become noticeable.

Continuing tight or damaging styles during investigation

Even when a nutritional or hormonal issue is present, repeated tension and chemical damage may continue to worsen the overall problem.

Frequently asked questions about blood tests for hair loss

What is the most important blood test for hair loss?

There is no single most important test for everyone. Full blood count, ferritin and thyroid function are commonly considered when shedding is diffuse or when symptoms suggest anaemia, iron deficiency or thyroid disease. The best selection depends on the pattern of loss and the person’s wider health history.

Can low ferritin cause hair loss without anaemia?

Iron stores can become depleted before haemoglobin falls into the anaemic range. A person may therefore have low ferritin while their full blood count remains relatively normal. Whether that deficiency explains the hair loss still requires clinical interpretation.

Can a thyroid problem cause hair loss?

Yes. Both underactive and overactive thyroid conditions can disrupt the hair-growth cycle. Thyroid-related shedding is often diffuse and may appear alongside changes in energy, weight, temperature tolerance, digestion or menstrual cycles.

Should vitamin D be tested for hair loss?

Vitamin D testing may be helpful in selected cases, especially when there are clear risk factors for deficiency. It is not a universal explanation for hair loss, and correcting a low result does not guarantee regrowth when another condition is responsible.

Do I need a hormone test for thinning hair?

Not necessarily. Hormonal testing is most useful when thinning occurs with irregular periods, acne, increased facial or body hair, fertility concerns or rapid progression. Pattern hair loss can occur even when routine hormone results are normal.

Can stress cause hair to fall out?

A major physical or emotional stressor can trigger telogen effluvium. The shedding usually begins after a delay, so the connection may not be immediately obvious. Stress should not automatically be assumed to be the cause until other relevant factors have been considered.

Will biotin help my hair grow back?

Biotin is unlikely to correct hair loss unless a genuine deficiency is present. Deficiency is uncommon, while high-dose biotin supplements can interfere with certain laboratory tests. Supplements should not be used as a substitute for identifying the type and cause of hair loss.

What happens if all my blood tests are normal?

Normal results make some underlying problems less likely, but they do not rule out pattern hair loss, alopecia areata, traction alopecia, breakage or inflammatory scalp disease. A scalp assessment may be the most important next step.

The practical next step

Blood tests can be valuable when they answer a focused clinical question. They may reveal iron deficiency, anaemia, thyroid dysfunction or another issue contributing to shedding. They are less useful when ordered without considering the pattern, timing and wider symptoms.

The most effective approach combines three elements:

1. A clear account of when and how the hair loss began
2. Appropriate blood tests selected for the individual situation
3. Professional interpretation of both the results and the scalp changes

For people experiencing widespread shedding or symptoms that may suggest an underlying deficiency, the Hair Loss Clarity Panel provides a structured starting point for investigating relevant markers. Testing should still be followed by appropriate interpretation, particularly when results are abnormal, symptoms are significant or the visible pattern suggests a condition that cannot be diagnosed through blood work alone.

Hair loss is rarely explained by one number in isolation. The aim is not simply to collect more results, but to understand which findings matter, what they may mean together and what should happen next.