Why Is My Hair Thinning? The Moment You Notice
It usually starts with something small. The ponytail feels thinner. The parting looks wider. There’s more hair in the drain than there used to be. You’re not sure when it started — but you’re sure something has changed.
That moment of noticing is unsettling because it comes without an explanation. You don’t know whether this is temporary, progressive, normal, or something you should be worried about. And the internet offers ten contradictory answers in ten seconds.
So here’s what I’d tell you if you were sitting in my clinic right now: the fact that your hair is thinning tells me something is happening. But it doesn’t tell me what. Thinning is not a diagnosis — it’s a signal. And the most useful thing you can do is investigate it properly, not guess at the cause.
Why Is My Hair Thinning? The Most Common Causes
In clinic, these are the causes I see most often. Many of my clients have more than one at work simultaneously — which is why identifying the full picture matters more than landing on a single explanation.
Telogen Effluvium — Stress, Illness, and Shock
The most common cause of sudden, diffuse thinning. A trigger — stress, illness, surgery, rapid weight loss, a medication change, childbirth — pushes a large number of hair follicles into the resting phase at once. The shedding typically appears two to three months after the trigger, which is why most people blame the wrong thing. Telogen effluvium is usually temporary, but identifying and addressing the trigger is what determines how quickly it resolves. Read more about GLP-1 medication and hair shedding.
Androgenetic Alopecia — Genetic and Hormonal
The most common cause of progressive thinning. In women, this typically presents as gradual widening of the parting or overall reduction in density, rather than the receding hairline pattern more common in men. It’s driven by genetic sensitivity to androgens and tends to progress without intervention. Early identification through trichoscopy makes a significant difference to the options available.
Nutritional Deficiency
Low iron stores are one of the most common nutritional contributors to hair thinning — and one of the most frequently missed. Standard blood tests may flag ferritin as “normal” when it’s not optimal for hair. Vitamin D, zinc, and B12 can also play a role. The key is not guessing which supplement to take, but testing and reading the results in context. Read more about blood tests and hair loss.
Thyroid Dysfunction
Both underactive and overactive thyroid can cause diffuse thinning. Thyroid-related hair changes are often accompanied by fatigue, weight changes, and temperature sensitivity — but not always. A full thyroid panel, not just TSH alone, gives the clearest picture.
Hormonal Changes
Perimenopause, menopause, polycystic ovary syndrome (PCOS), changes to contraception, and the postpartum period can all trigger thinning through shifts in oestrogen, progesterone, and androgen levels. Hormonal thinning often overlaps with other factors — which is why it’s rarely just “hormones.”
Medication
Certain medications can trigger shedding as a side effect — including GLP-1 weight-loss drugs, some antidepressants, blood thinners, beta-blockers, and retinoids. The mechanism is usually telogen effluvium: the medication, or the rapid physiological change it causes, pushes follicles into the resting phase. Understanding whether the medication or the underlying condition is the driver matters for management.
Scalp Conditions
A congested, inflamed, or unhealthy scalp is not the ideal environment for healthy hair growth. Seborrhoeic dermatitis, psoriasis, persistent scalp buildup, and chronic inflammation can all contribute to thinning over time — sometimes without the person realising their scalp is involved at all.
Traction and Physical Damage
Tight hairstyles, extensions, heavy braiding, and chronic tension on the hair follicles can cause traction alopecia — a form of thinning that starts at the hairline and temples. Caught early, it’s reversible. Left too long, the follicle damage can become permanent.
Autoimmune Conditions
Alopecia areata — where the immune system attacks the hair follicles — typically presents as distinct, smooth, circular patches rather than gradual thinning. It requires specific investigation and management, often alongside a dermatologist.
Why It’s Rarely Just One Thing
When clients ask me why is my hair thinning, my honest answer is almost always: it’s more than one thing.
In clinic, it’s unusual for me to identify a single cause. Much more often, I see two or three factors interacting — low ferritin alongside perimenopause, a stressful year compounded by a medication change, a scalp condition sitting underneath hormonal thinning that nobody had looked at.
That interaction is exactly why self-diagnosis is so unreliable. You might correctly identify one factor and miss two others. You might treat the obvious one and wonder why nothing improves. A friend might tell you what worked for her — but her causes weren’t yours.
The value of a trichological assessment is seeing the full picture at once. Not guessing at one piece of it.
Under the trichoscope, thinning reveals itself in ways invisible to the naked eye. I look for miniaturisation — where individual hairs are becoming progressively finer with each growth cycle. In early stages, a person might have full coverage but the strands themselves are thinner than they should be. Nobody notices until the volume drops.
I also look at follicle groupings. Healthy follicles typically produce two to three hairs from a single opening. In thinning, that number drops to one — or the follicle produces only fine, vellus-like hairs that barely contribute to coverage.
The trichoscope also distinguishes between active miniaturisation, diffuse shedding, and conditions like alopecia areata. These look completely different under magnification — but to the person experiencing them, they all feel like “my hair is thinning.”
What to Do First
If your hair is thinning, resist the urge to panic-buy. The supplement, the laser cap, the miracle shampoo — none of them will help if the underlying cause hasn’t been identified. Assessment before intervention. Always.
See your GP for baseline bloods. Request a full panel: ferritin (not just haemoglobin — they’re different), full thyroid function, vitamin D, and hormones if appropriate. These results establish whether anything medical is contributing.
If bloods come back “normal” but the thinning continues — that’s not the end of the investigation. It’s the beginning of the trichological one. The gap between “medically normal” and “optimal for your hair” is real, and it’s exactly where a trichologist operates.
Look at your scalp, not just your hair. Is it itchy? Flaky? Oily? Tight? Sore? These are signals most people overlook because they’re focused on the hair. But the scalp is where the hair grows from — and its condition matters.
Track the timeline. When did you first notice the change? What else was happening in your life at that time — stress, illness, medication, weight loss, pregnancy, a major life event? The timeline is often the most revealing piece of information in an assessment.
When Thinning Needs Professional Assessment
Not all thinning requires clinical intervention. Some resolves on its own once the trigger passes. But certain signals tell me a closer look is needed:
The thinning has been going on for more than three months and isn’t improving.
You can see your scalp through your hair in places where you couldn’t before.
Your GP says bloods are normal but the problem continues. This is more common than you’d think.
The pattern is uneven — thinning more on one side, or in patches rather than all over.
Your scalp is involved — itching, flaking, redness, or tenderness alongside the thinning.
You’ve already tried things that didn’t work — and you’re not sure what to try next.
A trichological assessment doesn’t mean something is seriously wrong. It means you’re investigating properly instead of guessing. Under the trichoscope, I can see whether the thinning is active or stable, what type it is, and what’s most likely driving it — usually within the first appointment.
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Frequently Asked Questions — Hair Thinning
Not exactly. “Thinning” usually describes a gradual reduction in hair density or strand thickness — you still have hair, but less of it or finer strands. “Hair loss” can mean the same thing, but clinically it often refers to more significant or permanent changes like androgenetic alopecia. Shedding and breakage are different again. The distinction matters because the cause and the approach are different for each.
It depends entirely on the cause. Telogen effluvium — triggered by stress, illness, or medication — is usually temporary, and hair typically recovers once the trigger is addressed. Androgenetic alopecia is progressive and won’t reverse on its own, but early intervention can slow or stabilise it significantly. The sooner the cause is identified, the more options are available.
The American Academy of Dermatology puts normal shedding at 50 to 100 hairs per day. But counting hairs isn’t particularly useful. What matters more is whether the pattern has changed — more in the drain than usual, a thinner ponytail, visible scalp where there wasn’t before. A change in your normal is the signal, not a number.
Start with your GP for baseline blood tests — ferritin, thyroid function, vitamin D. This rules out or identifies medical causes and it’s important. If bloods come back normal but the thinning continues, that’s where a trichologist adds the next layer of investigation: trichoscopy, pattern analysis, and reading your results alongside what the hair and scalp are actually showing.
Yes — significant stress can trigger telogen effluvium, where a large number of follicles shift into the resting phase at once. The shedding usually appears two to three months after the stressful period, which is why people rarely connect the two. Stress can also worsen existing conditions or unmask thinning that was developing quietly underneath.
Not necessarily — and this is one of the most common fears I hear. Many causes of thinning are temporary or manageable. Even progressive conditions like androgenetic alopecia don’t inevitably lead to baldness, especially with early identification. The anxiety of not knowing is often worse than the clinical reality, which is exactly why assessment matters.
Ask your GP for ferritin (iron stores — different from haemoglobin), full thyroid function (TSH, T3, T4), vitamin D, and a full blood count. Depending on your history, hormone levels may also be relevant. Bring the results to your trichology appointment — I read them alongside what the hair and scalp are showing, which often reveals more than the numbers alone. Read more about blood tests for hair loss.
Yes. Hair requires adequate protein, iron, zinc, B vitamins, and vitamin D to grow well. Crash diets, restrictive eating, and rapid weight loss are among the most common nutritional triggers for thinning that I see in clinic. But supplementing blindly isn’t the answer — testing identifies what’s actually low, so you address the right deficiency rather than guessing.
Key Takeaways
- Hair thinning is a signal, not a diagnosis — identifying the cause matters more than treating the symptom.
- Most thinning involves more than one contributing factor, which is why self-diagnosis and single-product solutions rarely work.
- Start with your GP for baseline blood tests, then consider a trichological assessment if the thinning continues despite normal results.
- Under a trichoscope, a trichologist can distinguish between different types of thinning in minutes — the uncertainty is usually worse than the answer.
This content is for informational purposes only and does not constitute medical advice. Please consult a qualified trichologist or GP for personalised guidance.
Hair Health Essentials was founded by Clare Devereux, one of Ireland and the UK’s leading IAT-certified clinical trichologists. We combine trichoscopy, blood biomarker analysis, and personalised treatment protocols with naturally formulated, COSMOS-certified products — because your hair deserves both the science and the care. Harley Street, London. Clinics also in Dublin.
This article is for information purposes and doesn’t replace medical advice. Please discuss any medication concerns with your prescribing doctor.
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IAT-Certified Clinical Trichologist
Clare Devereux
Clare is the founding clinical trichologist at Hair Health Essentials, practising in Dublin and London. With over a decade of clinical trichology experience underpinned by a lifetime in professional hair and scalp care, she specialises in personalised diagnostics — from trichoscopy and blood biomarker analysis to genetic testing — to identify what’s really happening with your hair and scalp.
Clinics: Blackrock, Dublin · Eden One, Dublin · Harley Street, London
Hair Health Essentials provides specialist trichological guidance. This content is for informational purposes and does not constitute medical advice. If you have concerns about your health, please consult your GP or medical practitioner.
With naturally formulated, COSMOS-certified products
— because your hair deserves both the science and the care.