What's Causing My Hair Loss? A Diagnostic Guide

You’re losing more hair than usual and you want to know why. The honest answer is that there are roughly a dozen distinct causes, and which one you have changes everything that follows: what to do, what’s reversible, whether to bother with treatment.

This guide walks the same triage a dermatologist runs¹ . Most people can narrow their cause to one or two possibilities at home before they ever sit down in a clinic.

Step 1: Look at the pattern

Stand in front of a mirror with the brightest light you can find. Look at the top of your head from above (a phone selfie or two angled mirrors helps). Where is the thinning?

• Temples and crown, with the sides and back largely preserved → androgenetic alopecia (male pattern). The “M-shape” recession at the temples plus a thinning crown is its signature.
• Widening of the central part, with the frontal hairline preserved → female pattern hair loss. Often subtle for years before it becomes visible. Covered in our female pattern hair loss guide.
• Diffuse, where the whole scalp is thinner with no specific zone → most likely telogen effluvium, possibly chronic telogen effluvium, possibly diffuse alopecia areata, occasionally a thyroid or nutritional issue.
• Sharply-bordered round or oval patches with smooth scalp inside them → almost certainly alopecia areata.
• Hairline thinning where you wear a tight ponytail, braids, weave, or extensions → traction alopecia.
• Patchy with redness, scaling, itching, or pain → a scarring alopecia. This is the urgent category.

Our types of hair loss guide goes deeper on each pattern.

Step 2: Look at the speed and timing

A sudden shed that started 2–4 months ago. Almost certainly telogen effluvium. The trigger is whatever happened 2–4 months before the shedding began: a flu, surgery, childbirth, severe stress, a new medication, a crash diet. Hair shedding has a built-in delay; the cause is usually in your past, not your present. See our telogen effluvium guide.

A patch that appeared in days or weeks. Alopecia areata.

Gradual thinning over months to years. Pattern hair loss (AGA / FPHL), usually in the 20s–40s for men, and often after pregnancy, perimenopause, or menopause for women.

Slow thinning at the hairline that matches a hairstyle you’ve worn for years. Traction alopecia.

Rapid, weeks-scale loss in someone on chemotherapy. Anagen effluvium: expected, usually reversible after treatment ends.

Step 3: Look at the scalp itself

This is the step most people skip and dermatologists do first² .

Get a partner or a phone camera and look at the skin in the thinning areas. What you’re looking for:

• Smooth, clear, normal-coloured scalp → non-scarring alopecia (pattern hair loss, telogen effluvium, alopecia areata, traction). Reassuring.
• Redness, scaling, flaking → could point to seborrhoeic dermatitis, psoriasis, or, more concerning, lichen planopilaris (LPP) or other scarring alopecias.
• Smooth, shiny, slightly pale skin where hair should be → possible scarring. Each of those follicles is gone permanently.
• Itching, burning, or pain → inflammation. Worth a dermatologist’s examination.
• Pustules or scaling around individual follicles → folliculitis decalvans or similar inflammatory scarring.

If anything in this column is present, do not stop at home triage. Book a dermatologist who specialises in hair disorders, sooner rather than later.

Step 4: Inventory the triggers

If you suspect telogen effluvium (diffuse, sudden, recent), think back 2–4 months and tick anything that applies:

• Major illness, COVID-19, hospitalisation, or surgery
• Childbirth
• Significant emotional stress (bereavement, divorce, job change, prolonged sleep loss)
• Crash dieting, sudden weight loss, low protein intake, restrictive eating
• Starting a new medication, particularly hormonal contraceptives, retinoids, beta-blockers, anticonvulsants, lithium, or certain antidepressants
• Stopping a medication you’d been on long-term, especially hormonal birth control or HRT
• Known thyroid dysfunction or recent thyroid medication change
• Iron deficiency, low ferritin, or vegan/vegetarian diet without supplementation
• Heavy or prolonged menstrual bleeding (a major cause of low ferritin)
• Crash dieting, bariatric surgery, gastric issues affecting absorption

If you find a trigger, you’re likely looking at telogen effluvium and the path forward is to address the trigger and wait. Hair density usually returns to baseline within 6–9 months after the trigger is gone⁴ .

Step 5: Consider blood work

Not everyone needs blood tests. Most cases of obvious androgenetic alopecia in a man with a positive family history don’t require any. But blood work is high-value when:

• Hair loss is diffuse (whole-scalp thinning)
• Hair loss is disproportionate to your age and family history
• You’re a woman with hair loss plus other symptoms (period changes, weight changes, fatigue, cold intolerance, skin changes, acne)
• You have a chronic illness, restrictive diet, or recent significant weight change
• You’re considering pharmacological treatment, where a baseline is useful

The standard panel:

• Ferritin (iron stores, the most useful single test for hair loss)
• TSH (thyroid function), often with free T4
• Complete blood count (CBC)
• Vitamin D
• Vitamin B12
• Zinc
• For women: a hormonal panel (testosterone, DHEAS, prolactin) if PCOS or androgen excess is suspected

We cover this in detail in our blood tests for hair loss guide, including what the cutoff values mean and what to ask your GP for if they’re hesitant to order them.

Quick differential by presentation

When to see a dermatologist

• Any redness, scaling, itching, burning, or pain on the scalp
• Visible scarring or smooth shiny skin where hair used to be
• Rapid, progressive loss without an obvious trigger
• A patch that appeared in days to weeks
• Diffuse hair loss in a woman, especially with other symptoms (period changes, fatigue, weight change, skin/nail changes)
• You’ve had diffuse shedding for more than 6 months
• You want to start prescription treatment (finasteride, oral minoxidil, JAK inhibitors)
• You’ve done home triage and you’re still not sure what you have

A dermatologist will examine the scalp under magnification (trichoscopy)³ , perform a pull test, possibly take a punch biopsy if a scarring alopecia is suspected, and order targeted blood work. The biopsy is a 3–4 mm skin sample under local anaesthetic, a minor procedure that’s often the only way to definitively distinguish certain conditions.

What this article doesn’t cover

This is a triage guide, not a treatment plan. Each cause above has its own dedicated article on Regrowth Index covering treatment, prognosis, and what to expect. We’ve also left out paediatric hair loss and most rare hair-shaft disorders, which warrant specialist evaluation.

If hair loss is affecting your wellbeing, talk to a GP or dermatologist sooner rather than later, and consider reaching out to a peer community. You’re not making this up, and you’re not alone in finding it hard. Anything in this article is general education, not personal medical advice.