Around four in five cases of long-term hair loss are inherited pattern loss, in which follicles are genetically sensitive to the hormone DHT. The rest is divided between thyroid and hormonal disorders, iron and protein deficiency, autoimmune disease, medication, severe illness and physical damage to the follicle.
The detail that misleads most people is that in pattern loss the hormone level itself is almost always normal. What is abnormal is the follicle’s response to it.
Knowing what type of hair loss you have and knowing what caused it are two different investigations. The first reads the shape of the loss; the second asks what set it in motion, because that determines whether it can be reversed. This guide from HairX Clinics works through the causes themselves — inheritance, hormones, illness, nutrition, medication and physical injury — and explains how each one damages the growth cycle.
Why are hormone tests normal in hormonal hair loss?
This is the most common wrong turn in the whole subject, so it is worth taking first.
Someone notices their hairline moving, asks for blood tests, and everything comes back within range. Testosterone normal. Thyroid normal. The conclusion drawn is that the cause must be stress, or shampoo, or water quality — anything but hormones.
But androgenetic alopecia does not require abnormal hormone levels, and in men it essentially never involves them. The hormone driving it, dihydrotestosterone, is produced by nearly every adult male at an ordinary concentration. What varies between people is how the androgen receptors inside the follicle respond to it, and that sensitivity is inherited. A follicle on the crown of a susceptible man will miniaturise at a hormone level that leaves the follicles at the back of his own head entirely unaffected — which is precisely why the donor area at the back is permanent, and why transplantation works at all.
So a normal androgen panel does not rule out hormonal hair loss. In women the picture differs: markedly raised androgens do occur, usually with polycystic ovary syndrome, and there the numbers are worth checking because they are often abnormal and often treatable.
How much of it is inherited?
Most of it. Pattern hair loss is polygenic, meaning many genes each contribute a small amount of risk, and it comes from both sides of the family rather than only the mother’s. The old rule about looking at your maternal grandfather captures one real genetic contribution on the X chromosome, but treats a single signal as the whole story.
What inheritance sets is susceptibility and roughly when it begins. It does not fix how far it will go, and it is not a sentence, because the speed of progression responds to treatment. The practical value of family history is in timing: loss that starts in the early twenties in a family with heavy baldness tends to progress further than loss that starts at forty-five, which matters enormously for anyone weighing surgery. Our guide to the Norwood scale explains how that progression is staged.
Which illnesses and hormonal disorders cause it?
Several, and unlike pattern loss most of them are reversible once the underlying condition is treated.
| Cause | How it damages the cycle | Typical picture | Reversible? |
|---|---|---|---|
| Thyroid disease, over or under active | Alters the metabolic signalling that controls cycle length | Diffuse thinning all over, hair texture changes | Usually, once thyroid function is stable |
| Polycystic ovary syndrome | Genuinely raised androgens acting on sensitive follicles | Widening part, thinning at the crown, often with acne | Partly, with hormonal treatment |
| Iron deficiency | Depletes a mineral the follicle needs for rapid cell division | Diffuse shedding, often with fatigue | Yes, with correction |
| Severe illness, surgery, high fever, childbirth | Pushes many follicles into the resting phase at once | Sudden heavy shedding two to four months later | Yes, typically within six to nine months |
| Autoimmune disease, including alopecia areata and lupus | Immune attack on the follicle | Smooth round patches, or scarring loss | Depends on whether scarring has occurred |
| Rapid weight loss and low protein intake | Withdraws the raw material for keratin production | Diffuse shedding, brittle hair | Yes, with adequate nutrition |
| Scarring scalp conditions | Inflammation destroys the follicle and replaces it with fibrous tissue | Smooth shiny skin with no visible pores | No. Loss is permanent |
The last row is the one that changes urgency. Everything above it can usually wait a few weeks for an appointment. Scarring alopecia cannot, because each week of untreated inflammation destroys follicles that no treatment and no surgery will bring back.
Can diet and deficiency cause hair loss?
Yes, but far less often than the supplement aisle implies, and only when the deficiency is real.
Hair is one of the body’s least essential tissues, so when resources run short the follicle is among the first to be deprioritised. Low iron stores, inadequate protein and severe calorie restriction all show up in hair, usually as diffuse shedding some months after the event rather than during it. Vitamin D and zinc are plausible contributors when genuinely low.
The catch is that correcting a deficiency helps only if a deficiency existed. Supplementing a normal level does nothing for hair, and some supplements cause harm at high doses — excess vitamin A and excess selenium both cause hair loss in their own right, as does iron overload. Biotin deficiency is rare in people eating an ordinary diet, and high-dose biotin interferes with several laboratory tests, including thyroid and cardiac assays, which can produce dangerously misleading results.
Which medications are involved?
A long list, and one worth reviewing with whoever prescribed it rather than acting on alone. Chemotherapy is the widely known example, where hair loss is expected and almost always temporary. Less obviously, some blood thinners, retinoids for acne, certain antidepressants and mood stabilisers, beta blockers, some hormonal contraceptives, gout treatment and high-dose steroids can all trigger shedding.
Timing is the giveaway. Drug-related shedding typically begins two to four months after starting, and settles a similar interval after stopping. Never stop a prescribed medication because of hair. The condition being treated is almost always more serious than the shedding, and an alternative within the same class is often available.
How do you work out which cause applies to you?
Three questions narrow it down quickly.
How fast did it happen? Pattern loss is slow, measured in years, and people usually cannot name the month it started. Sudden heavy shedding that you can date to a particular few weeks points towards a triggering event two to four months earlier.
What shape is it? Loss concentrated at the temples and crown, sparing the back and sides, is pattern loss. Even thinning everywhere including the sides suggests a systemic cause. Discrete round patches suggest an immune cause. Loss following the line of a tight hairstyle is mechanical.
Is the skin normal? Healthy scalp skin with visible pores means the follicles are present, whatever else is wrong. Redness, scale, pain or smooth poreless patches change the diagnosis and the urgency.
Women should be aware that the pattern differs from the male version and is frequently mistaken for general shedding, as covered in our guide to hair loss and transplantation in women. Once the cause is established, the options divide cleanly between what is reversible and what needs long-term management, which our overview of hair loss treatment sets out in full.
Can stress alone cause hair loss?
Severe physical or emotional stress can push follicles into the resting phase, producing heavy shedding two to four months later. It usually recovers. Everyday work stress is far more often blamed than responsible, and it does not cause pattern baldness.
My blood tests were all normal. Does that rule out a hormonal cause?
No. Male pattern loss is driven by follicle sensitivity to normal hormone levels, so the panel is expected to be normal. In women, abnormal androgens do occur and are worth checking.
Does hair loss come from my mother’s side?
Partly. One important gene sits on the X chromosome, which comes from the mother, but many other genes contribute and both parents matter. A bald father raises risk regardless.
Can hard water or shampoo cause hair loss?
Neither causes loss from the follicle. Hard water and harsh detergents can roughen the shaft and increase breakage, which looks similar in the sink but is a different problem entirely.
Will hair grow back once the cause is treated?
If the follicle survives, usually yes, over six to twelve months. If the cause was scarring, or if pattern loss has run to the point where follicles no longer produce hair, it will not, and surgery becomes the only way to add density.