Why Hair Falls: A Surgeon’s Guide to the Causes

Last reviewed 8 July 2026

As a hair transplant surgeon, the single most useful thing I do in a first consultation is not measure grafts. It is answering one question honestly: why is your hair actually falling out? Get that wrong and every treatment that follows is aimed at the wrong target. Get it right and the plan almost writes itself.

This is a plain-English map of the real causes, how a doctor tells them apart, and what each one means for you. It is education, not a diagnosis — your scalp needs to be examined for that.

The two big patterns behind most hair loss

1. Androgenetic alopecia — the genetic, progressive one

This is pattern hair loss: the receding hairline and thinning crown in men, the widening part in women. It is the most common cause of hair loss worldwide, and it is largely inherited — variation in the androgen receptor gene accounts for a substantial share of the risk in men.^[1] The mechanism is follicular miniaturisation: under the influence of dihydrotestosterone (DHT), genetically susceptible follicles shrink with each growth cycle, producing progressively finer, shorter, lighter hairs until they stop producing visible hair at all.

Two features matter clinically. First, it is patterned — it follows predictable zones (the Norwood scale in men, the Ludwig scale in women), while the back and sides are usually spared. That spared zone is exactly why hair transplantation works: those follicles are genetically resistant to DHT and keep growing even when moved. Second, it is progressive. It does not stop on its own, which is why planning has to assume future loss.

2. Telogen effluvium — the diffuse, temporary one

If your hair is coming out all over rather than in a pattern — handfuls in the shower, a thinner ponytail, hair on the pillow — telogen effluvium is often the culprit. Here, a shock to the body pushes an abnormal number of follicles out of their growth phase and into shedding at once.^[2] The classic triggers: a high fever or illness, major surgery, significant weight loss or crash dieting, childbirth, severe stress, thyroid disturbance, or starting/stopping certain medications.

The signature clue is timing. Because of the hair cycle, the shed usually appears 2–4 months after the trigger, which is why people so often can’t connect them. The good news: pure telogen effluvium is usually self-limiting and reverses once the trigger is corrected. The important part: a hair transplant does nothing for it — operating on a telogen shed is a mistake.

The factors that make everything worse: iron, vitamin D, thyroid

These rarely cause dramatic baldness by themselves, but they are common, checkable, and often the reason a treatment isn’t working as well as it should. They matter most in women and in anyone with a diffuse shed.

Iron stores (ferritin)

Ferritin reflects your body’s iron reserves. A meta-analysis of 36 studies covering over 10,000 people found that women with non-scarring hair loss had significantly lower serum ferritin than women without it.^[3] Low iron won’t always be the whole story, but in a menstruating woman with diffuse shedding it is one of the first things worth checking.

Vitamin D

Vitamin D receptors sit in the hair follicle and appear to play a role in cycling. A 2024 meta-analysis found patients with non-scarring alopecia had significantly lower vitamin D levels and over three times the odds of deficiency compared with healthy controls.^[4] Common in India despite the sunshine, and easily missed.

Thyroid

Thyroid hormone is required for normal follicle cycling, so both an underactive and an overactive thyroid can trigger diffuse shedding.^[5] A simple blood test rules it in or out.

One honest caveat: these are associations. A low lab value flags a contributing factor worth correcting — it does not guarantee that fixing it alone will regrow your hair. That’s why a number on a report is read alongside a clinical exam, never instead of one.

How a doctor actually tells them apart

A proper hair-loss work-up is not complicated, but it is specific:

• History — the pattern of loss, its speed, family history, recent illnesses, diet, medications, and (for women) menstrual and hormonal history.
• Scalp and pull test — is the loss patterned or diffuse? How much comes away on a gentle pull?
• Trichoscopy — magnified examination of the scalp to see miniaturisation, follicle density and the ratio of thick to thin hairs.
• Targeted bloods — ferritin, vitamin D, thyroid function, and others where the history suggests them.

Only after that does treatment planning begin — and for many patients the right first step is medical management, not surgery.

What this means for your next step

If your hair loss is diffuse and recent, the priority is finding and correcting the trigger — surgery is not the answer. If it is patterned, progressive, and your donor area is strong, then medical therapy, and in the right cases a transplant, become the conversation. Non-surgical treatments like PRP, GFC and exosome therapy sit in between — supportive tools that can help maintain and thicken hair in suitable patients, which is why they deserve their own honest, evidence-based look rather than a sales pitch.

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