By Ciobanu Sebastian — Founder & Head of Product, Folliq · Published 5 September 2026 He is not a doctor, dermatologist or trichologist, and does not present himself as one — he reads the primary studies and links them throughout.
Key Takeaways
- Ferritin is the test that finds something most often. In a review of 2,851 women with shedding, it was low in 46.5% of those tested.
- Folate is the test that almost never finds anything — under 1%. Knowing what to skip is as useful as knowing what to ask for.
- If you take a hair supplement, biotin can distort your results. Stop it 48 to 72 hours before the blood draw.
- "Normal" isn't one number. The ferritin threshold used in hair research is genuinely disputed, so ask for the value, not the verdict.
- These tests rule things in or out. They don't diagnose why your hair is shedding, and no result should be read without your doctor.
Every article about hair loss tells you to get bloodwork. Almost none of them tell you which bloodwork.
So you book the appointment, the ten minutes go quickly, you leave with a general panel, and a week later someone tells you everything looks normal. You still don't know anything.
This is the list you need beforehand. Which tests actually find something, what each one is looking for, and the one preparation step most people get wrong.
Which Tests Actually Find Something
A retrospective review of 2,851 women with telogen effluvium — the diffuse shedding kind — recorded how often each test came back abnormal. That gives you something a generic list can't: a ranking.
| What was tested | How often it was abnormal |
|---|---|
| Ferritin | 1,123 of 2,413 — 46.5% |
| Iron | 685 — 29.5% |
| Haemoglobin | 317 — 11.1% |
| Vitamin B12 | 150 — 5.8% |
| Folate | 8 — 0.6% |
Two things jump out.
Ferritin found something in nearly half of the women tested. It's the single highest-yield test on the list, and it's the one most likely to be left off a general panel.
Folate found something in fewer than one in a hundred. It's on almost every "vitamins for hair" list you'll read. In this cohort it was nearly useless.
The authors' own conclusion names the priorities: vitamin B12, ferritin and T3 are "particularly important in the 18–45 age group."
What Each Test Is Actually Looking For
Ferritin measures stored iron, not circulating iron. That's why it can be low while a standard iron test looks acceptable — your reserves are drawn down before the level in your blood drops. It's the reason ferritin is the test worth naming explicitly.
Serum iron and a full blood count fill in the rest of the picture. Haemoglobin below range points to anaemia, which is a different conversation than depleted stores with normal haemoglobin.
Thyroid function matters because both an underactive and an overactive thyroid affect the hair cycle. A TSH alone is the usual starting point; the study above flags T3 as worth including for younger women.
Vitamin B12 was abnormal in about one in seventeen women here. Worth including, especially if you eat little or no meat.
Vitamin D is commonly ordered alongside these. Note it for later — it's one of the tests biotin can distort.
Nothing on this list explains why hair sheds on its own. Each one rules a contributor in or out, which narrows what your doctor is looking at.
Your Hair Supplement Can Distort Your Hair Bloodwork
This is the part almost nobody mentions, and it applies to a large share of the people reading this.
Biotin is the headline ingredient in nearly every "hair, skin and nails" supplement. It also interferes with the immunoassays used to run many of these tests — and the direction of the error depends on how each test is built.
- Competitive assays read falsely high. That includes T3, T4 and 25-OH vitamin D.
- Sandwich assays read falsely low. That includes TSH.
Put those together and the pattern is a raised free T4 and T3 with a suppressed TSH. That looks like an overactive thyroid. One case series describes it as "pseudohyperthyroidism" and notes it can appear indistinguishable from Graves' disease.
So the supplement you take for your hair can make the tests you take for your hair report a thyroid problem you don't have. It skews your vitamin D at the same time.
How much matters. The same source states that "the ingestion of 20 mg or more of biotin may lead to a clinically relevant thyroid assay interference," and notes some manufacturers flag the potential above 5 mg a day. Check the label: hair supplements are often dosed in micrograms, and 10,000 mcg is 10 mg.
What to do. The guidance is to discontinue biotin "for at least 48–72 hours before thyroid assaying." Tell the person taking your blood that you've been taking it, even if you stopped. That one sentence can save you a repeat test, or a wrong conclusion.
"Normal" Isn't One Number
Here's the thing that catches people out. Results come back, you're told they're normal, and the conversation ends.
But a reference range answers a specific question, and it isn't always the question you're asking. The ferritin threshold used in hair research is openly disputed. Some studies use a cutoff around 15 ng/ml. Work on women with chronic shedding has used a level closer to 40. There's no settled figure the field agrees on.
Which means "normal" may mean not anaemic rather than nothing here worth discussing about your hair.
The practical move is simple: ask for the actual numbers, not just the verdict. Request a copy of the results. Then, if a value is called fine but hair research often uses a different threshold, you have something specific to ask about. That beats a vague feeling of being brushed off.
To be clear about the boundary: those study cutoffs are research thresholds, not personal targets. What your number means for you is a judgement your doctor makes with the rest of your picture. The point isn't to self-diagnose from a range you found online — it's to have the number in hand so the conversation can happen at all.
What to Actually Say at the Appointment
Ten-minute appointments reward specificity. Something close to this covers it:
"I've had increased shedding for about [X] months. I'd like to rule out the treatable causes — could we check ferritin, iron, a full blood count, thyroid function and B12? I've also been taking a biotin supplement, so I've stopped it for the last three days."
Three details make that work. You've given a duration. You've named the tests rather than asking an open question. And you've pre-empted the interference problem without being asked.
If you've already had bloodwork and were told it was normal, bring the printout rather than the memory of the conversation.
What These Tests Can't Tell You
Bloodwork rules contributors in or out. It doesn't identify the pattern.
It can't tell you whether this is temporary shedding or the early stage of something longer-term. That distinction comes from examining the scalp, and it's the subject of what normal shedding actually looks like. It can't tell you if hormonal change is driving it, which is harder to read in your forties and fifties. And it can't tell you whether the trigger was something that happened months ago, as with postpartum shedding.
Normal results are not a dead end. They're a genuinely useful outcome — they mean the treatable contributors have been excluded, and the conversation can move on to what's actually happening at the scalp.
Nothing here is medical advice, and it can't assess your situation. Use it to prepare for the appointment, not to replace it.
Frequently Asked Questions
Should I stop my hair supplement before a blood test?
If it contains biotin, yes — the guidance in the literature is to stop 48 to 72 hours beforehand. Also tell whoever takes your blood that you've been taking it. Check the label first: hair supplements are often dosed in micrograms, so 5,000 mcg is 5 mg.
My results came back normal but my hair is still shedding. What now?
That's a common and informative outcome. It means the treatable contributors have been ruled out. That shifts the question from "is something wrong internally?" to "what's happening at the scalp, and on what timeline?" Ask for the actual values before moving on.
Is ferritin the same as an iron test?
No. Ferritin measures stored iron, while a serum iron test measures what's circulating right now. Stores are depleted first, so ferritin can be low while iron looks acceptable. That's exactly why it's worth naming separately when you ask.
The Bottom Line
The single most useful thing you can do before your appointment is stop guessing what to ask for.
Name ferritin first — it found something in nearly half the women in the largest study here. Add iron, a full blood count, thyroid function and B12. Skip the panels that promise to test everything; folate came back abnormal in less than one percent.
Pause the biotin three days ahead, and say you were taking it.
Then ask for the numbers themselves, not just the verdict.
Once you know what the bloodwork rules out, the next question is what to do at the scalp. That's what our guide to building a scalp care routine that actually works is for.