Causes of Hair Loss: An In-Depth Look

When hair starts falling out, the first useful question is not which product to buy. It is why the hair is falling out, because the cause decides what will actually help. Treat stress-related shedding like hereditary loss and you will be disappointed; do it the other way round and you lose valuable time.

This guide walks through the eight most common causes of hair loss, how each one typically shows itself, and how to work out which one fits your situation.

On this page:

• Hereditary hair loss (androgenetic alopecia)
• Hormonal shifts
• Stress and illness (telogen effluvium)
• Nutritional deficiencies
• Scalp conditions
• Medication as a trigger
• Traction alopecia
• Ageing, and what it changes on its own
• How to tell which cause fits you
• What to do next
• Frequently asked questions

Hereditary hair loss (androgenetic alopecia)

By a wide margin, the most common cause of hair loss in both men and women is hereditary: follicles that are genetically sensitive to the hormone DHT.

DHT, a by-product of testosterone, gradually shrinks sensitive follicles in a process called miniaturisation, which our guide to dutasteride as a DHT blocker approaches from the hormonal side rather than the follicle's. Each cycle produces a finer, shorter hair, until the follicle stops producing visible hair altogether. In men this follows a recognisable route along the temples and crown; in women it usually shows as diffuse thinning and a widening parting while the hairline stays put.

Two things are worth knowing. It is progressive: left alone, it continues. And it is the form of hair loss with the best-proven treatment options, minoxidil first among them, which makes recognising it early genuinely valuable. Our guide to the early signs of thinning shows what to look for, and if the change you have noticed is at the temples specifically, our page on whether minoxidil works on a receding hairline is honest about where the evidence is thinner.

Hormonal shifts

Hormones run the hair growth cycle, so any sizeable shift can show up on your head a few months later.

The classic triggers are pregnancy and the months after giving birth, menopause, thyroid disorders and PCOS, and our page on female pattern hair loss covers how hereditary thinning and hormonal shedding overlap in women. Post-partum shedding looks dramatic but usually recovers on its own within months. Thyroid-related loss improves when the underlying condition is treated, which is exactly why diffuse shedding plus symptoms like fatigue, weight change or feeling cold belongs at the doctor's rather than in the supplement aisle.

Stress and illness (telogen effluvium)

A serious stressor can push a large share of your hair into its resting phase at once, and that hair falls out roughly two to three months later.

The delay is what confuses people: the shedding arrives when life has often already calmed down. Typical triggers include high fever, serious illness, surgery, rapid weight loss, childbirth and periods of intense psychological strain. The reassuring part is that telogen effluvium is usually temporary; once the trigger has passed, density tends to rebuild over several months. Chronic, unrelenting stress can keep the cycle disturbed, though, which is one more reason to treat recovery seriously.

Nutritional deficiencies

Hair is one of the first places a shortage shows, because your body ranks it below every vital organ.

The usual suspects are iron (especially in women), vitamin D, zinc and simply too little protein, often after crash diets or heavily restricted eating. If you suspect a deficiency, a blood test through your doctor beats guessing: supplementing blindly can mask the real problem, and more of a nutrient you are not short of does nothing for your hair.

Scalp conditions

An inflamed scalp is a poor place to grow hair, and several common conditions cause exactly that.

Stubborn dandruff (seborrhoeic dermatitis), fungal overgrowth and psoriasis all irritate the skin around the follicle, and the scratching that comes with them does its own damage. The good news: these are treatable. Medicated shampoos with ketoconazole target the fungus behind most dandruff, something our plain-English guide to ketoconazole covers in detail. Treating the scalp first also makes any regrowth treatment more comfortable to stick with.

Medication as a trigger

Some medicines disturb the hair cycle as a side effect, and the list is longer than most people expect.

Chemotherapy is the extreme example, but certain antidepressants, blood pressure medicines, acne treatments such as isotretinoin, and starting or stopping hormonal contraception can all trigger shedding in some people. Two rules apply: this type of loss usually reverses after the medication ends, and you should never stop a prescribed medicine on your own. If you suspect a link, discuss it with the prescribing doctor; often an alternative exists.

Traction alopecia

Hairstyles that pull, worn day after day, can physically damage follicles, and this is one form of hair loss that is entirely preventable.

Tight braids, high ponytails, heavy extensions and tightly wound buns put constant tension on the hairline and temples. Early on it is reversible: loosen the styles and hair recovers. Keep the tension up for years and the follicles scar, at which point the loss becomes permanent. A receding edge or small broken hairs around the hairline are the warning signs to take seriously.

Ageing, and what it changes on its own

Even without any of the above, hair changes with age: growth slows, strands become finer, and overall density drifts downward.

This is normal biology rather than a condition. You cannot stop the clock, but gentle handling, a healthy scalp and, where hereditary thinning is also in play, a proven treatment can keep the difference between ageing gracefully and thinning fast surprisingly large.

How to tell which cause fits you

Pattern, speed and accompanying symptoms separate the causes better than any single test. Read the table by what you can actually see in the mirror, not by what you suspect.

What you are seeing	Most likely cause	Where it goes next
Gradual thinning at the temples, crown or parting	Hereditary hair loss	The one form with well-proven treatment; start early
Sudden, diffuse shedding two to three months after illness, childbirth or a rough period	Telogen effluvium	Usually recovers on its own; our page on diffuse thinning versus pattern loss tells them apart
Patchy, circular bald spots	Alopecia areata	A doctor, not a product
Itching, flaking or soreness before the shedding	A scalp condition	Treat the scalp first, then judge the hair
A tight hairstyle worn daily, plus a receding edge	Traction alopecia	Loosen the style now; scarred follicles do not come back

More than one cause can be active at once; a stressful spring can unmask hereditary thinning that was creeping along quietly anyway.

What to do next

Match the response to the cause, and give whatever you choose enough months to be judged fairly.

• Hereditary loss: minoxidil is the most widely used over-the-counter option, with many users seeing results within 3 to 6 months of daily use. Kirkland Signature 5% is the value pick for men; women usually start with a 2% product such as Foligain's 2% lotion. Sensitive scalp? Start with a foam.
• Suspected medical causes: thyroid symptoms, patchy loss, or shedding with fatigue mean a doctor first, treatment second.
• Scalp trouble: treat the dandruff or irritation before judging what your hair is doing.
• Everything else: eat properly, sleep properly, loosen the ponytail, and track progress with monthly photos. Our page on hair loss myths versus facts clears out the advice that wastes those months, and our customer reviews show how often the unglamorous route is the one that works.

Frequently asked questions

Can hair loss have more than one cause at the same time?

Yes, and it is common. A stressful period or a deficiency can accelerate shedding on top of slow hereditary thinning. That is why sudden change deserves a proper look even if hair loss runs in your family: fixing the temporary cause often reveals how much of the loss was reversible.

Does DHT cause hair loss in women too?

It plays a role, though female pattern loss is less strictly DHT-driven than the male version and hormone shifts around menopause matter more. The visible result differs as well: diffuse thinning and a wider parting rather than a receding hairline.

Is stress-related hair loss permanent?

Usually not. Telogen effluvium tends to recover over several months once the trigger has passed. If shedding continues beyond six months, or you can see the pattern of hereditary loss emerging underneath, it is worth reassessing rather than waiting longer.

Should I get blood tests for hair loss?

If the loss is sudden, diffuse or comes with other symptoms, yes: iron, vitamin D and thyroid values are the usual starting points, arranged through your doctor. For slow, classic pattern thinning with a family history, testing rarely changes the plan.

When should I see a doctor rather than try a product?

See a doctor first for patchy or circular bald spots, sudden heavy shedding, hair loss with scalp pain or visible inflammation, or loss accompanied by symptoms such as exhaustion or weight change. Products are for the causes that do not need a diagnosis.