The Deficiencies That Show Up in Your Hair First

Your body triages. Faced with a shortage of anything, it funds the heart, the brain and the immune system first, and hair sits near the bottom of that list — it isn’t keeping you alive, and the follicle is one of the most metabolically demanding structures in the body for something so optional.

Which is why hair is often the first place a deficiency becomes visible. Months before you’d notice anything else, the follicle has already started economising.

The important consequence: shedding can be a symptom rather than a condition, and the fix is a blood test rather than a shampoo.

Iron, which is the big one

Iron deficiency is the most common nutritional deficiency in the world and it disproportionately affects menstruating women. It’s also the one most likely to show up as diffuse hair shedding.

The subtlety that catches people out: you can have normal hemoglobin and still be iron deficient. Hemoglobin is the last thing to drop, because the body drains its stores first. The measure that matters for hair is ferritin, which reflects stored iron, and a ferritin that’s technically “within range” can still be too low for hair.

Many labs set the bottom of the reference range around 15, and much of the dermatology literature discussing hair suggests a considerably higher figure is preferable. That’s a discussion to have with your doctor, and the practical point is to ask for ferritin specifically rather than accepting “your iron is fine” based on a full blood count.

Heavy periods, pregnancy, vegetarian and vegan diets, endurance training and coeliac disease all push it down.

Do not supplement iron on a guess

This is the one hard rule here.

Iron overload is genuinely dangerous. The body has no efficient way to excrete excess iron, and it deposits in the liver, heart and pancreas. Haemochromatosis — a hereditary condition causing over-absorption — is more common than people assume and is made considerably worse by supplementation.

So: test, then treat, then retest. Never the middle step alone. And if you are prescribed iron, expect it to take three to six months to rebuild stores and another few months after that before hair responds, because the follicle has to complete a cycle.

Thyroid

Both an underactive and an overactive thyroid cause hair loss, which surprises people who only associate it with the underactive kind.

Thyroid hormone regulates how quickly cells divide, and follicle cells divide faster than almost anything else in the body. Push that rate in either direction and the hair cycle destabilises.

Hypothyroidism tends to produce diffuse thinning with dry, coarse, brittle hair, sometimes with the outer third of the eyebrows thinning — a useful sign. Hyperthyroidism produces finer, softer hair and a faster shed.

Thyroid disease is far more common in women, it clusters around pregnancy and midlife, and postpartum thyroiditis in particular gets missed constantly because its symptoms are indistinguishable from having a new baby. If you’re shedding and also exhausted, cold, gaining or losing weight unexpectedly, or low in mood, ask for thyroid function to be checked.

Vitamin D

Vitamin D receptors sit in the hair follicle, and the vitamin appears to be involved in cycling the follicle back into growth. Low levels are associated with several types of hair loss, including alopecia areata and female pattern loss.

The evidence for supplementation regrowing hair is weaker than the association suggests — low vitamin D might be a marker rather than a cause in some of these studies. It’s still worth testing, because deficiency is extremely common at higher latitudes and in anyone who works indoors, and because correcting it is cheap and useful for reasons beyond hair.

Dose is worth asking about rather than guessing. Vitamin D is fat-soluble and it is possible to take too much.

B12, zinc and protein

B12 deficiency is common in vegans and vegetarians, in anyone on long-term metformin or acid-reducing medication, and in people over about sixty. It causes a diffuse shed and often shows up with fatigue and pins and needles.

Zinc is required for follicle function and for the enzymes involved in protein synthesis. Deficiency is less common than iron but turns up in restrictive diets, after bariatric surgery, and in inflammatory bowel conditions. It’s another one where excess causes its own problems — too much zinc interferes with copper absorption — so testing rather than guessing applies here too.

Protein deserves a mention because it’s the one people under-eat without realising. Hair is roughly ninety percent keratin, keratin is built from amino acids, and a body short of protein deprioritises hair immediately. Crash dieting produces a shed two to three months later with reliable timing.

What biotin is doing

Almost nothing, for almost everyone.

True biotin deficiency is rare, and in that rare case supplementation works well. In everyone else, the evidence that extra biotin improves hair is very thin, and it dominates the supplement aisle mostly because it’s cheap and the name sounds relevant.

It also has a real downside that gets no attention: high-dose biotin interferes with a range of laboratory immunoassays, including thyroid tests and — importantly — troponin, the test used to diagnose a heart attack. It can produce falsely reassuring results. Tell any doctor ordering blood work that you take it, and expect to be asked to stop for a few days beforehand.

One more that gets missed: medication. Several common drugs cause hair shedding as a known effect, including some antidepressants, beta blockers, anticoagulants, retinoids taken orally, and hormonal contraception when started or stopped. If your shedding began within a few months of a prescription change, that’s the first thing to raise — and not something to solve by stopping the medication on your own.

What to reach for

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Questions we get

Which tests should I ask for? Ferritin, a full thyroid panel rather than TSH alone, vitamin D, B12 and a full blood count is a reasonable starting request. Bring the specific words, since “check my iron” often returns hemoglobin only.

How long until hair responds? Three to six months to rebuild stores, then another cycle before the follicle produces visibly better hair. Nine to twelve months to a fair verdict.

Can I just take a multivitamin? It’s unlikely to contain enough of any single thing to correct a real deficiency, and it may contain enough biotin to skew your tests. Talk to your doctor.

Do hair supplements work? They work if they’re correcting something you’re short of. That’s the whole mechanism, and it’s why testing first is both cheaper and more likely to work than buying the bottle first.

Is my diet the problem? It can be, particularly with restriction, rapid weight loss or a recent switch to a plant-based diet. It’s also frequently blood loss from heavy periods, which is a medical issue rather than a dietary one. More across our hair archive.

Where this leaves you

Hair is the early warning system for a handful of deficiencies that are common, cheap to test for and straightforward to correct. That’s good news, because it means a proportion of the shedding people spend years treating cosmetically has a blood test underneath it.

Ask for ferritin by name, get thyroid checked properly, and don’t take iron without a result in front of you. Then be patient — the follicle can’t respond faster than a cycle.