In short
- The most common causes visible in blood: iron deficiency, thyroid, zinc or B12 deficiency, protein deficiency and excess androgens in women.
- In diffuse shedding (telogen effluvium) the usual target is ferritin above 30 µg/L, and with hair loss above 70 µg/L. That is a higher cut-off than for anaemia alone.
- Shedding after childbirth, illness, surgery, rapid weight loss or severe stress appears 2–3 months after the event and resolves on its own.
- Androgenetic alopecia (patterned thinning) does not show in blood — a dermatologist makes the diagnosis by examining the scalp.
- If the hair loss comes with rashes, itching, scarring or swelling of the scalp, the right place is a dermatologist, not a blood test.
Hair loss is one of the most common reasons people pay for a blood test themselves. It is also one of the most common places where too much gets tested. This article tells you which tests have proven value and which targets are used. It also tells you when a trip to the dermatologist is more useful than a panel of tests.
Which tests make sense
| Test | What it looks for | Worth knowing |
|---|---|---|
| Ferritin + hs-CRP | Iron stores. | The most common correctable cause in women. With inflammation, ferritin is falsely high — hence CRP. |
| TSH (if abnormal, fT4, anti-TPO) | Thyroid. | Both a slow and an overactive thyroid thin the hair; with treatment, growth returns within a few months. |
| Complete blood count (CBC) | Anaemia, inflammation. | MCV points to iron (low) or to B12 and folate (high). |
| Zinc | Zinc deficiency. | Serum zinc falls with inflammation — read it with CRP; sample fasting in the morning. |
| Vitamin D | Deficiency. | The link is weaker than for iron, but in Slovenia deficiency is common in winter and simple to correct. |
| Vitamin B12, folate | Deficiency. | More common on a plant-based diet, after age 60, on metformin or acid blockers. |
| Androgens in women (testosterone, SHBG, DHEA-S, 17-OHP) | Excess male hormones. | Only with signs: acne, excess hair growth, irregular cycles. Without these signs we do not measure them. |
| Albumin or total protein | Low protein intake. | Common on strict diets and after bariatric surgery. |
Which targets are used
- Ferritin. Deficiency is below 15–30 µg/L. With hair loss the practical target is above 70 µg/L; with restless legs syndrome it is above 75 µg/L. With inflammation a higher threshold applies (WHO 2020).
- TSH. Within the reference range (roughly 0.4–4.0 mIU/L, higher in older people); if abnormal, fT4 and anti-TPO follow.
- Vitamin D. Sufficient is above 50 nmol/L (EFSA).
- Zinc and B12. Within the laboratory's reference range; for B12 in the grey zone, homocysteine helps.
When blood will not give an answer
The two most common forms of hair loss do not show in blood:
- Androgenetic alopecia. Gradual, patterned thinning (temples and crown in men, a widening parting in women). It is an inherited sensitivity of the hair follicles, not a deficiency. Blood tests are normal.
- Telogen effluvium after an event. After childbirth, illness, surgery, rapid weight loss or severe stress, hair sheds 2–3 months later. It grows back on its own within six months. Testing makes sense only if it lasts longer or if the event involved blood loss.
When to see a dermatologist, not the lab: hair loss in patches, scarring, redness and scaling of the scalp, itching or pain. Also sudden loss of eyebrows or eyelashes. These are signs that need to be looked at, not measured.
Four types of hair loss and what blood tells you in each
| Type | How it shows | What blood tells you |
|---|---|---|
| Telogen effluvium | Even thinning over the whole head, 2–3 months after a trigger (childbirth, illness, surgery, rapid weight loss, severe stress). | A lot: ferritin, TSH, blood count, protein. This is where testing makes the most sense. |
| Androgenetic alopecia | Gradual patterned thinning: temples and crown in men, a widening parting in women. | Little. Results are usually normal; it is an inherited sensitivity of the hair follicles. |
| Alopecia areata | Sharply defined bald patches, often appearing within a few weeks. | Indirectly: it is an autoimmune condition, so TSH and anti-TPO make sense. A dermatologist makes the diagnosis. |
| Traction alopecia | Thinning along the hairline from tight hairstyles, extensions or headwear. | Nothing. The fix is changing the habit while the follicles are not yet scarred. |
The same person can have two forms at once. For example, inherited thinning can be made temporarily worse by telogen effluvium after an illness. Blood explains only the second.
What you can do while you wait
- Fix the cause you measured. In iron deficiency, iron replacement as directed by a doctor is the only step proven to work. A “hair” supplement without a deficiency is not.
- Enough protein. Hair is made of protein; on strict diets and after bariatric surgery, intake is often too low.
- Do not delay the dermatologist with patterned thinning. In androgenetic alopecia, treatment keeps the hair you still have but does not bring back what is lost.
- Handle hair more gently. Less heat styling, fewer tight hairstyles, no harsh brushes on wet hair. It does not remove the cause, but it reduces breakage.
- Stop biotin 48 hours before the blood test. Otherwise thyroid results will be wrong exactly when you need them most.
How long until it improves
Hair grows slowly, so even after the cause is fixed there is no immediate effect. With iron replacement, shedding usually settles within 3–6 months. Visible new growth appears within 6–9 months. A follow-up report makes sense after three months. Before that, ferritin has not risen enough to tell you anything.
Frequently asked questions
Which test makes the most sense in hair loss?
Ferritin together with CRP. Iron deficiency is the most common correctable cause in women, and CRP prevents a falsely reassuring result.
How much ferritin is enough?
To correct a deficiency the cut-off is 30 µg/L, but with hair loss the practical target is above 70 µg/L. With inflammation the values are falsely higher.
Should I measure hormones?
Only with signs of androgen excess — acne, excess hair growth, irregular cycles. Otherwise a hormone panel does not change what you do about hair loss.
My hair has been falling out three months after giving birth. Is that normal?
Yes, that is telogen effluvium and it usually settles on its own within six months. It makes sense to check ferritin, because blood loss at childbirth is common.
How do I know whether it is inherited thinning or telogen effluvium?
By the pattern and the timeline. Telogen effluvium is even over the whole head and appears 2–3 months after a trigger. Inherited thinning progresses slowly in a typical pattern. Blood explains the first, not the second.
Do hair supplements help?
They help if the cause is a deficiency that the supplement corrects. Otherwise not. So measure first. And stop biotin supplements 48 hours before the blood test, because they interfere with the results.
Sources: WHO 2020 — ferritin and CRP · BSG 2021 and NICE NG20 — iron deficiency, coeliac disease · EFSA NDA — vitamin D · ESPEN 2022 — zinc as a negative acute-phase reactant · International PCOS guideline 2023 — androgens in women · IRLSSG/AASM 2018 — ferritin above 75 µg/L in restless legs.
