Why is my hair falling out? The common causes of hair loss, and the blood tests that check them

When hair suddenly comes out in handfuls, the commonest reason is telogen effluvium, a burst of shedding that starts about three to five months after a shock to the body (a high fever, surgery, childbirth, a crash diet, severe stress) and usually settles by itself. ¹² Hair that slowly thins at the part or crown is usually hereditary pattern hair loss, the most common cause of hair loss in men and in women. ¹ Smooth round bald patches point to alopecia areata, where the immune system attacks the hair roots. ⁶ The pattern is the first clue; a few blood tests (iron stores, thyroid and, in some women, testosterone) check the medical causes behind it. ¹⁷

How much hair loss is normal?

The American Academy of Dermatology puts it plainly: “It's normal to shed between 50 and 100 hairs a day.” ² Each hair grows for years, rests, then falls out and is replaced, so a steady trickle in the brush and the drain is the cycle working. The worry starts when the trickle becomes clumps, when the part looks wider in photos, or when a smooth spot appears. Those three pictures have different causes, and telling them apart is most of the diagnosis. ⁵

Shedding, thinning or patches: which one is yours?

A 2024 review in American Family Physician sorts hair loss without scarring the same way, into diffuse (all over), patterned and focal (patches), because each group has a different list of causes. ⁵ A fourth, rarer group is scarring hair loss, where inflammation destroys the hair roots for good; a clinician looking at the scalp sees that the openings the hairs grow from are gone, and that picture needs a dermatologist. ¹ Several patterns can overlap, such as shedding on top of slow hereditary thinning, which is why the hair loss can seem to “suddenly” reveal a thinning that was already there.

Why is my hair falling out all of a sudden?

Sudden shedding all over the head is telogen effluvium far more often than anything else. Normally most hairs are growing and a small share are resting; a shock to the body flips a large batch into the resting phase together, and those hairs drop out together months later. ¹ That lag fools people: the clumps in the shower in March often trace back to a flu, an operation or a hard month in December. The American Academy of Dermatology notes that after childbirth, shedding “usually peaks about four months after giving birth.” ²

Hair loss that comes on fast in patches is a different story. Alopecia areata can clear a coin-sized spot in weeks. ⁶ And hair loss with fever and a peeling or blistering rash after a new drug is not a hair problem at all: it can be Stevens-Johnson syndrome, a severe skin reaction “most often triggered by particular medications,” and that needs 911 or the emergency room now. ¹⁴

Hair loss in women: what is different

Women with hereditary hair loss usually keep their hairline and thin along the part, so the first sign is often a wider part in photos or more scalp showing under bright light. ¹ Women also meet more of the shedding triggers: childbirth, stopping or switching the pill, heavy periods that drain iron, and the hormone shifts of menopause, where hair loss “often goes away after 6 months to 2 years.” ²⁴ When thinning comes with irregular periods, acne or new hair on the face, chest or belly, clinicians look at male-type hormones such as total testosterone, because polycystic ovary syndrome (PCOS) is a common cause of that mix. ³⁷

The NHS notes that minoxidil can be used for female pattern hair loss but that “Women should not use finasteride,” a tablet used in men. ¹³ That is one reason any treatment decision belongs with a clinician who knows your plans and history.

Thinning hair: is it genetic, and can it be stopped?

Most slow thinning is hereditary. In men it is so common that about half have some by age 50. ¹ The American Academy of Dermatology says that for hereditary hair loss, “The earlier treatment is started, the better it works,” because treatments slow loss and regrow some hair rather than restore what has been gone for years. ³ Thinning also weighs on people: a 2024 review in American Family Physician notes that moderate to severe hair loss comes with more anxiety and depression. ⁵ If low mood is part of it, say so to your clinician, and if you have thoughts of harming yourself, call or text 988 now.

Minoxidil (a lotion or foam) and finasteride (a tablet for men) are drugs, not supplements, and both are used for pattern hair loss. ¹¹³ The NHS is frank about them: “No treatment is 100% effective,” and they “only work for as long as they're used.” ¹³ Finasteride can lower sex drive and cause erection problems in some men. ¹ For patches, dermatologists use steroid injections and, for severe alopecia areata, newer immune-calming tablets. ⁵ LifeFrom does not recommend any of these; they are listed so you know what to ask about.

Can low iron, vitamin D or zinc cause hair loss?

Iron is the one with the firmest place: clinicians test for iron deficiency when shedding is the problem, and the NHS lists it as a cause of temporary hair loss. ¹¹³ The usual test is ferritin, the protein that stores iron; a low ferritin means stores are running down. ⁹ See iron deficiency for how it is found and why a clinician, not a supplement aisle, decides whether you need iron.

Vitamin D is the most searched and the least settled. A 2024 meta-analysis found vitamin D deficiency in 50% of women with female pattern hair loss, 53% of people with telogen effluvium and 52% of people with alopecia areata. ¹² The authors still conclude that “Further study on vitamin D supplementation as a treatment for alopecia is recommended,” because nobody has shown that topping it up regrows hair. ¹² Zinc deficiency can cause hair loss, but it is rarely the reason in people eating a varied diet. ¹³ More on reading these results: vitamin D deficiency.

That review found 18 published cases where biotin helped hair or nails, and every one involved a person with an underlying problem such as a genetic biotin disorder. ¹⁰ Biotin has a second catch: in high doses it interferes with many blood tests, including thyroid tests. ¹¹

Is there a blood test for hair loss?

Not one test, and often none. The 2017 American Family Physician review says there are “no routine tests to evaluate hair loss”; testing makes sense when the history or the scalp suggests something underneath. ¹ When shedding is the pattern, the review lists the causes worth checking: iron deficiency, zinc deficiency, thyroid disease and kidney or liver disease. ¹ In women with signs of high male hormones, hormone tests come in. ⁷ For pattern thinning in a man with no other symptoms, blood tests usually add little.

Getting a useful test

• Tell the clinician the pattern (all over, part or crown, patches) and when it started; a phone photo every month helps.
• List every medicine and supplement with start dates, including birth control changes. ¹²
• Mention biotin, including in hair, skin and nail blends and multivitamins. The American Thyroid Association says biotin most often causes “falsely high levels of T4 and T3 and falsely low levels of TSH,” and recommends stopping it for at least 2 days before thyroid testing, a step to agree with your clinician. ¹¹
• One result is a snapshot: ferritin rises with inflammation, so a normal-looking number may still be repeated or read with other tests. ⁹ See why trends beat single readings.
• Hair takes months to respond, so a repeat check after a change is usually months, not weeks, away. ¹

If you already have results, the thyroid panel guide and the CBC explainer walk through what each number means.

Questions to ask your clinician

• Does my pattern look like shedding, hereditary thinning or patches?
• Could anything from the last three to five months, including a medicine, explain it?
• Should we check ferritin, a blood count and TSH, and if so, what ferritin level would you want to see?
• Given my periods, skin or other symptoms, should we check hormone levels?
• Is there any sign of scarring or infection that needs a dermatologist?
• If treatment is an option, how long before I would know whether it works?

This page is general education, not medical advice or a diagnosis. Hair loss has many causes that overlap, and the clinician who knows your history, examines your scalp and reads your results in context is the one to put them in order.

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