Blood tests for hair loss: what to ask for

When hair starts to fall out, it is natural to want "tests for everything". But they are not always necessary, and a huge panel without criteria can bring more confusion than answers. Tests are useful for finding causes that can be corrected, such as iron deficiency or a thyroid problem, and for understanding whether there are hormonal changes behind the hair loss.

In most cases, a short set is enough: full blood count, ferritin and TSH (thyroid function), with vitamin D, vitamin B12, folic acid or zinc depending on the clinical history. Hormone tests are reserved for women with signs of androgen excess, such as irregular cycles, acne or excess hair. And there are tests for specific situations, requested only when the picture justifies it.

In this guide we explain when they make sense, what each test shows, how to interpret the results and how to prepare.

When they make sense, and when they do not

They make sense when

• the shedding is diffuse and has clearly increased for more than 2 to 3 months;
• there was an event that may have depleted reserves: childbirth, surgery, illness, restrictive diet, rapid weight loss, bariatric surgery;
• there are general symptoms: tiredness, pallor, palpitations, unusual sensitivity to cold or heat, changes in weight;
• there are heavy or irregular periods;
• there are signs of androgen excess in women: acne, facial or body hair, irregular cycles;
• the diet is vegetarian or vegan, or there are conditions affecting absorption;
• you are about to start a treatment and deficiencies need to be ruled out.

Often not necessary

• Typical male pattern baldness, with receding temples and crown, in a healthy man: the diagnosis is clinical and tests rarely change the decision.
• Female androgenetic alopecia with no other signs: routine tests are not usually needed for the diagnosis; ferritin, a full blood count and TSH are considered depending on the history, especially if an associated effluvium is suspected (GPnotebook).
• Effluvium with an obvious, recent cause, such as childbirth 3 months ago with no other symptoms, may simply be monitored.

The cause of hair loss is diagnosed mainly from the clinical history and examination of the scalp, including trichoscopy. Blood tests complete the picture; they do not replace it.

The basic tests: what each one shows

Full blood count

It shows whether there is anemia (low hemoglobin) and gives clues about the cause: small red blood cells suggest iron deficiency; large ones suggest a lack of vitamin B12 or folic acid. A normal blood count does not rule out iron deficiency: iron stores run out before hemoglobin drops.

Ferritin: the most useful test

Ferritin reflects the body's iron stores. It is the test with the most evidence in hair loss, especially in women.

• A meta-analysis of 36 studies found significantly lower ferritin in women with non-scarring hair loss, even without anemia (Treister-Goltzman et al., 2022).
• In one study, ferritin of 30 ng/mL or less was strongly associated with telogen effluvium (Zhang et al., 2023).

How to interpret it:

Some laboratories consider any value above 10 to 15 ng/mL "normal". That is why a value within the reference range can still be low for someone with hair loss. It has not yet been proven that replacing iron increases hair density in every case, but correcting a deficiency is always beneficial for health.

CRP (C-reactive protein)

It measures inflammation. It is requested together with ferritin because, if there is inflammation or infection, ferritin rises and can mask iron deficiency. In some cases the doctor also requests transferrin saturation.

TSH: the thyroid

Hypothyroidism and hyperthyroidism can cause diffuse hair loss. TSH is the screening test. If it is abnormal, it is followed up with free T4 and sometimes antibodies. It is especially important after childbirth, at menopause and when there is tiredness, weight changes, intolerance to cold or heat, or palpitations.

Vitamin D (25-OH vitamin D)

Low values are common in people with hair loss, including effluvium and alopecia areata. It has not been shown that supplementing without a deficiency makes hair grow, but correcting a deficiency has general benefits.

Vitamin B12 and folic acid

They are needed for cell division, including in the follicle. B12 should be checked in vegetarians and vegans, after bariatric surgery, and in people who take metformin or stomach medication for a long time. After bariatric surgery, low folic acid was associated with hair loss (Global Bariatric Research Collaborative, 2021).

Zinc

Deficiency is uncommon in people who eat a varied diet, but more frequent after bariatric surgery, on very restrictive diets and in some bowel diseases. In those cases, testing makes sense.

Hormone tests in women

When to request them

Male hormones (androgens) are involved in androgenetic alopecia, but most women with this type of hair loss have normal values. Hormone tests are requested when there are signs of androgen excess (GPnotebook):

• irregular menstrual cycles or absence of periods;
• excess hair on the face or body;
• persistent or severe acne in adulthood;
• very oily scalp;
• suspected polycystic ovary syndrome;
• hair loss that progresses rapidly together with other signs of virilization (deeper voice, increased muscle mass), which requires prompt assessment.

What is requested

Depending on the results, the doctor may request other tests, such as 17-OH-progesterone, FSH, estradiol or cortisol, to rule out adrenal diseases, hormone-producing tumors or Cushing's syndrome.

When to take the sample

• In the morning, between 8 and 9 a.m.
• Between day 2 and day 5 of the cycle, when there are cycles.
• Off hormonal contraception: the pill raises SHBG and can "normalize" the results. Ideally, take a break of at least 2 months, decided with the doctor (GPnotebook).

And in men?

In typical male pattern baldness, hormone tests are not necessary: men with baldness usually have normal testosterone. The cause is the sensitivity of the follicles to androgens, not an excess. They are requested only if there are other symptoms.

Tests for specific situations

These tests are not part of the routine. They are requested when the history or examination justifies them.

• Syphilis serology (VDRL or RPR and TPHA). Secondary syphilis can cause hair loss, usually in small patches with a "moth-eaten" look or diffuse thinning. It is uncommon, affecting 3% to 7% of people with secondary syphilis, but it is important not to miss it, because hair grows back with treatment and the disease must be treated (Vafaie et al., 2005). It is requested when the patches look atypical or there are other signs.
• Antinuclear antibodies (ANA). When lupus or another autoimmune disease is suspected: hair loss with joint pain, sun-sensitive patches on the face, mouth ulcers, marked tiredness.
• Liver and kidney function. In chronic diseases or before some oral treatments.
• Blood glucose or glycated hemoglobin. When diabetes or metabolic syndrome is suspected, which is common in women with polycystic ovary syndrome.
• Scalp biopsy. It is not a blood test, but it is the examination that confirms some diagnoses, especially scarring alopecias.

How to interpret the results

"Normal" is not always optimal

The laboratory's reference values indicate the range in which most of the population falls, not necessarily the ideal value for hair. The clearest example is ferritin: a value of 20 ng/mL may appear as normal on the report and still be low for a woman with hair loss. The interpretation should be made by the doctor, taking into account the symptoms and the history.

Common pitfalls

• Biotin. Biotin supplements, very common in products "for hair", interfere with several tests done by immunoassay. They can give falsely high T4 and T3 and falsely low TSH, mimicking hyperthyroidism, and even distort troponin, which is used to diagnose heart attacks. Stop biotin at least 1 day before the tests, or longer if the dose is high, and tell the doctor (Fan et al., 2019).
• Inflammation and infections. A cold, an infection or an inflammatory disease raise ferritin and can mask iron deficiency. That is why CRP is requested at the same time.
• Recent iron supplements. They can temporarily raise the values. If you are taking iron, follow the doctor's instructions on the interval before the tests.
• Hormonal contraception. It changes SHBG and, with it, the interpretation of male hormones.
• Phase of the menstrual cycle. Hormones vary throughout the cycle; that is why the sample is requested in the first days.
• Vitamin D and the season. Values are lower at the end of winter and higher at the end of summer.

An abnormal result does not always explain the hair loss

A slightly low vitamin D, for example, is very common and may not be the main cause of the hair loss. The doctor integrates the results with the history, the examination and trichoscopy before drawing conclusions.

How to prepare for the tests

• Fasting: check with the laboratory. For most of these tests, fasting for 8 to 12 hours is not mandatory, but it may be requested if other tests are done at the same time, such as blood glucose or cholesterol.
• Time: early in the morning, especially for hormones (between 8 and 9 a.m.).
• Biotin: stop at least 1 day before, or longer if the dose is high.
• Iron: if you are taking supplements, ask the doctor whether you should pause them beforehand.
• Menstrual cycle: for hormones, sample between day 2 and day 5 of the cycle, if you have cycles.
• Hormonal contraception: do not stop it on your own. If the doctor needs hormone tests without the influence of the pill, they will agree with you on the best way to do it, including an alternative contraceptive method.
• Recent illness: if you had a fever or an infection in the last few weeks, let them know; it may be better to postpone.
• Bring the list of medicines and supplements you take, and previous test results for comparison.

Summary: which tests for each situation

This table is a general guide. The tests should be chosen by the doctor, after the consultation.

When to seek a medical assessment

Ideally, have the tests after a consultation, not before. That way only those that make sense in your case are requested, and the results are interpreted together with the history and the examination of the scalp.

Book an appointment if:

At Luxmed, the Hair Medicine consultation starts with the clinical history and trichoscopy and, when indicated, requests the right tests for your case. If you already have recent test results, bring them.

This article is for information only and does not replace a medical consultation.

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