Hair shedding, decoded.

What's typically going on.

Hair grows in cycles, not continuously. Each follicle moves between an active growth phase (anagen, which can last several years), a brief transition phase (catagen, weeks), and a resting phase (telogen, several months) before the old hair sheds and a new hair begins growing. At any given moment, roughly 85–90% of the hairs on a healthy scalp are in growth, and the rest are quietly cycling through rest and shed.

The most common pattern in women 25–55 — far more common than the male-pattern thinning women fear — is telogen effluvium. A physiological stressor pushes an unusually large proportion of follicles into the telogen phase all at once. Two to four months later, those follicles release their hairs more or less simultaneously, producing the alarming shedding women notice in the shower drain and on the brush. The trigger is rarely happening on the day the shedding starts. It happened months earlier — and that lag is the single most confusing feature of the pattern.

Triggers that may shift follicles into mass telogen include: childbirth (the classic postpartum shed, peaking at around three to four months), high fever or significant illness, surgery or general anaesthesia, rapid weight loss, restrictive dieting, low iron stores, untreated thyroid disease, a course of certain medications, and major emotional or psychological events. Several of these are bloodwork-visible, which is why the workup begins with a panel rather than a topical.

The perimenopausal pattern is slightly different. As estrogen levels begin fluctuating in a woman's 40s, the relative balance of estrogen to androgens shifts, and hair density may gradually decline — particularly at the crown and along the part line. This pattern is less dramatic than telogen effluvium but more persistent, and it overlaps with the iron and thyroid patterns above. A bloodwork workup catches the modifiable contributors regardless of which pattern is dominant.

"The shedding you are noticing today is most often the echo of something that happened two to four months ago. The hair cycle hides the timing on purpose."

The biomarkers most worth knowing.

You do not need every marker on this list to start. You need enough to triangulate. The panel that catches most of the bloodwork-visible drivers of diffuse shedding in women is below — read together by a qualified healthcare provider, ideally one who has treated hair shedding before.

Worth knowing: zinc, selenium and biotin are sometimes added to hair-loss panels, and may have a role in specific cases — but they are second-line in most workups. The big four (ferritin, thyroid, vitamin D, B12) capture the high-volume causes first.

When this may be more than "just stress."

Telogen effluvium that does not resolve within six to nine months — particularly if shedding continues at the same intensity rather than tapering — warrants a wider look. So does shedding accompanied by other symptoms: fatigue, breathlessness on stairs and exercise intolerance point toward iron depletion; weight changes, cold intolerance and constipation point toward thyroid dysfunction; and new cycle changes alongside shedding in a woman in her 40s point toward perimenopause.

Postpartum shedding deserves its own paragraph. It is real, almost universal, and it usually peaks around three to four months after birth before tapering across the following six to nine months. If shedding is heavier than expected, persists beyond a year, or is accompanied by fatigue and mood changes while breastfeeding, the iron, B12 and thyroid panels are particularly worth running together — the postpartum year is the highest-risk window for all three.

Patterns that warrant earlier referral to a board-certified dermatologist, rather than waiting on bloodwork, include: patchy rather than diffuse hair loss (which may point toward alopecia areata), scarring with redness or scaling on the scalp, hair loss along a defined area like the front hairline or beard distribution, broken hairs that look like exclamation points, and pattern thinning concentrated at the crown or part line in women with a family history of female pattern hair loss. These warrant in-person scalp evaluation rather than telehealth alone.

One specific pattern worth flagging: shedding alongside acne, irregular periods, weight gain centred at the waist and excess facial or body hair growth may belong to a polycystic ovary syndrome (PCOS) picture. The relevant bloodwork shifts toward fasting insulin, HbA1c, total and free testosterone, DHEA-S and SHBG — a conversation worth having with a women's-health clinician who treats PCOS specifically.

What to ask your provider.

These are not a script. Your clinician will steer the conversation where it is most useful. They are a starting point that tends to widen the panel and produce a clearer plan than the default 15-minute visit usually yields.

Related Heme reading.

If hair shedding is the entry point, the related guides below help build a fuller bloodwork picture — particularly if fatigue, cycle changes or postpartum context are also part of the story.

• Ferritin, in plain English — the marker dermatologists ask about most often when treating hair shedding.
• Fatigue, decoded — the four biomarker patterns most often behind persistent tiredness in U.S. women.
• Heavy periods, decoded — the most-overlooked driver of low iron stores in pre-menopausal women.
• TSH and the full thyroid panel — what each marker adds when shedding is part of a broader thyroid picture.
• Vitamin D, decoded — what the marker measures and where ranges sit.
• The postpartum hub — the depletion window most U.S. providers under-investigate.

Frequently asked.