Hair loss in women: causes, tests and what actually helps
Thinning hair in women is often linked to iron, thyroid or hormones. The causes, the blood tests that matter, and which treatments are worth it.
Reviewed by Emma Carter, Senior Health & Nutrition Editor
Finding more hair in your brush or shower drain is unsettling, and for women, it is often a sign that something correctable is going on beneath the surface. Most female hair loss is not permanent, especially when the underlying trigger is found and treated. This guide walks through the common causes, the tests that pinpoint them, and what genuinely helps.
What is behind your hair loss?
Several causes overlap, and the pattern of shedding points to the likely one.
| Likely cause | What it looks like | What to check |
|---|---|---|
| Low iron | Diffuse shedding, heavy periods, tiredness | Ferritin |
| Thyroid problem | Diffuse shedding with tiredness, weight or temperature changes | TSH and free T4 |
| Telogen effluvium | A dramatic shed a few months after stress, illness, pregnancy or stopping the pill | Usually clinical; recovers on its own |
| Female-pattern hair loss | Gradual thinning at the crown or part line | Clinical assessment; early is better |
Iron stores, measured as ferritin, are commonly checked because periods steadily deplete iron. The evidence is genuinely mixed, and a meta-analysis puts numbers on it: across 36 studies and 10,029 women, those with non-scarring hair loss had ferritin about 18.5 ng/dL lower than controls, yet iron-deficiency anemia was no more common among them (Skin Appendage Disorders, 2021). So the association is real but its clinical weight is unsettled, and the threshold that matters is debated (studies often use levels around 30 µg/L). Even so, correcting a confirmed iron deficiency is a sensible, low-risk step. An underactive thyroid slows the hair-growth cycle, and because iron is needed to make thyroid hormone, low iron and low thyroid often travel together, which is one reason some women keep shedding despite thyroid medication. Read Hashimoto's and consider a TSH test.
The tests worth asking for
Because several causes overlap, testing prevents guesswork. A useful panel:
- Ferritin, your iron stores
- Complete blood count
- Thyroid function: TSH and free T4 (and thyroid antibodies if Hashimoto's is suspected)
- Vitamin D and zinc
What actually helps
- Correct low iron, with diet and, if advised, a supplement, retesting ferritin after a few months. Our iron guide covers gentle, well-absorbed options.
- Treat thyroid problems medically. Supplements do not replace thyroid treatment.
- Be realistic about supplements. Biotin only helps a genuine deficiency; other nutrients help when a blood test shows a gap.
- Mind the timing. Iron can block thyroid-medication absorption, so take them several hours apart.
A useful rule: supplement to correct a confirmed deficiency, not as a blanket fix. More is not better, and some nutrients cause harm in excess.
When to see a doctor
See your doctor or a dermatologist if shedding is sudden or heavy, if you notice bald patches, or if thinning is progressive. Early assessment widens your treatment options and catches conditions like thyroid disease or iron deficiency that are simple to treat.
Frequently asked questions
What is the most common cause of hair loss in women?
Several causes are common: female-pattern hair loss, stress- or illness-related shedding (telogen effluvium), thyroid problems and hormonal changes. Low iron stores (ferritin) are also frequently checked and can contribute, though the evidence linking them to hair loss is mixed, so iron is usually only part of the picture.
What ferritin level is linked to hair loss?
Some studies link diffuse shedding with low ferritin, often using thresholds around 30 µg/L, but the evidence is mixed and the ideal cut-off is debated. Many clinicians still aim to restore ferritin toward about 50 ng/mL when treating hair loss. Your doctor should interpret your level alongside symptoms and other tests.
Do biotin and hair supplements work?
Biotin only helps in the rare case of a true biotin deficiency. Most hair supplements work best when they correct a confirmed shortfall, iron, zinc or vitamin D, rather than as a blanket fix. Test before you supplement.
Will my hair grow back?
When shedding is driven by a correctable cause like low iron, a thyroid problem or a stressful trigger, hair usually regrows over several months once the cause is treated. Pattern hair loss is more gradual and may need specific treatments.
References
- Iron deficiency and nonscarring alopecia in women: systematic review and meta-analysis · Skin Appendage Disorders, 2021
- Hair loss · NHS
- The hormonal background of hair loss in non-scarring alopecias · PMC / NIH