Hair Loss: The Blood Tests Worth Doing, and Why the Pattern Matters Most

Hair loss is a symptom, not a diagnosis — and it is one of the few where a primary care visit genuinely changes the outcome, because several of the causes are things a blood test finds and treatment reverses.

The pattern is what identifies the cause, far more than the amount. So the useful question is not “how much am I losing” but where from, how fast, and is the scalp normal.

And one thing worth saying immediately, because people apologise for raising it: this is a reasonable thing to come in about. Hair loss has a real effect on how people feel, and treating that as vanity is both unkind and clinically unhelpful — distress about it is a legitimate part of the presentation.

The patterns, and what each usually means

Gradual thinning at the crown and temples, over years. Pattern hair loss — the common one, strongly genetic, in men and women both. In women it usually shows as a widening part and thinning across the top rather than a receding hairline. Treatments exist and they work better early, which is the main argument for not waiting.

Sudden, diffuse shedding — handfuls in the shower, a few months after something. This is telogen effluvium, and the trigger is usually two to three months earlier: a severe illness, surgery, childbirth, rapid weight loss, a new medication, a major stressor, or a high fever. It is alarming and it is usually self-limiting — hair typically recovers over six to nine months once the cause has passed. The delay is why nobody connects it to the event, and being told the connection is often the whole of what someone needs.

One or more smooth round bald patches, with normal scalp. Alopecia areata, an autoimmune condition where the immune system attacks hair follicles. It can affect the scalp, eyebrows, beard or eyelashes. It often regrows on its own, it can recur, and there are real treatments now — including newer systemic options for extensive disease, which is a genuine change from a few years ago. It is associated with thyroid disease and other autoimmune conditions, so it prompts a wider look.

Breakage and thinning along the hairline, or where hair is pulled. Traction alopecia, from tight braids, weaves, extensions, or a habitually tight ponytail. Reversible early and permanent late, which is exactly why it is worth raising before the edges stop coming back — and it is under-discussed, particularly with Black patients, who are disproportionately affected.

Scaly, itchy, red, painful or scarring scalp, or hair loss with a changed scalp surface. This is the group that needs seeing promptly. Scarring alopecias destroy follicles permanently, and the window to prevent that is early. Fungal infection, lichen planopilaris, central centrifugal cicatricial alopecia and discoid lupus all sit here.

Broken hairs at different lengths, in an irregular patch. Sometimes hair pulling — a compulsive behaviour rather than a hair disease, and treatable when named without shame.

In a child, a scaly patch with broken hairs and sometimes swollen glands is often tinea capitis — a fungal infection that needs oral treatment, not a cream, and is contagious.

The blood tests worth doing

This is the practical reason to come in. Several reversible causes are invisible without them:

• Iron, and specifically ferritin. Iron deficiency causes hair loss before it causes anaemia, so a normal blood count does not rule it out. This is the single most commonly missed reversible cause, especially in women who menstruate
• Thyroid function. Both over- and underactive thyroid cause hair loss, and both are easily treated
• Vitamin D, which is worth checking in Arizona for the reason our other pages give — deficiency is common here precisely because people sensibly avoid the sun
• Zinc, in the right context
• A full blood count
• Screening for autoimmune conditions if the pattern suggests alopecia areata
• Hormonal tests where there are other features — irregular periods, acne, unwanted hair growth — which may point at polycystic ovary syndrome
• Coeliac screening, in the right context, since it causes hair loss through malabsorption

And a medication review, which costs nothing. Quite a few common medicines cause hair loss: some blood pressure medicines, some antidepressants, blood thinners, retinoids, certain acne treatments, some anticonvulsants, high-dose vitamin A, and hormonal contraceptive changes. Bring your list, including anything over the counter.

What to do meanwhile

Be gentle with it. Less heat, less tension, avoid tight styles and chemical relaxers while things are unsettled, and a wide-tooth comb on wet hair.

Eat enough, and enough protein. Rapid weight loss and very restrictive diets are a common trigger and one of the more fixable.

Do not take high-dose supplements on spec. Too much vitamin A and too much selenium both cause hair loss, so the well-meant hair-and-nails supplement can be the problem. Biotin is the specific one to know about: it does essentially nothing for hair loss unless you are deficient, which is rare — and it interferes with several laboratory assays, including thyroid and cardiac troponin tests. That means it can produce a wrong result at the exact appointment where you need a right one. Stop it before blood tests and tell whoever takes them.

Treat the scalp if it is itchy or scaly, and tell us about that rather than treating it indefinitely yourself.

And give it time. Hair grows about a centimetre a month. Any treatment for hair loss takes three to six months before anything is visible, which is why people abandon effective treatments as failures.

Come and see us if

• Hair loss that is sudden, rapid, or in patches
• Any scalp that is red, scaly, itchy, painful, scarring, or has visibly changed — promptly, because this is the group where waiting costs follicles
• Hair loss in a child
• Hair loss with tiredness, weight change, heavy periods, feeling the cold, or a change in mood — that combination points at thyroid or iron
• Loss of eyebrows, eyelashes or body hair as well
• Hair loss after starting a new medicine
• Hair coming out from the hairline or where it is pulled or braided — early, while it is still reversible
• You want to do something about pattern hair loss. Treatments work better early, and “wait and see” costs you the window
• It is affecting how you feel. On its own. That is enough

Sources

24. Hair loss — MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine. https://medlineplus.gov/ency/article/003246.htm
25. Alopecia areata — MedlinePlus Medical Encyclopedia, U.S. National Library of Medicine. https://medlineplus.gov/ency/article/001450.htm

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