What Actually Affects Hair Health, Inside and Out
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No topical product changes what happens at the follicle. If your hair is thinning, shedding more than usual, or has lost its condition, the explanation is usually somewhere other than your bathroom shelf.
Here's what genuinely affects hair from the inside — nutrition, hormones, thyroid, stress and medication — and which of those are worth testing for.
Hair is sensitive to iron deficiency, low protein intake, thyroid problems, rapid weight loss, hormonal changes and significant stress. Most of those are identifiable with simple tests, and several are reversible. Supplements help when you're genuinely deficient and rarely otherwise — and some, taken in excess, can cause hair loss themselves.
Why hair is an early indicator
Hair follicles are among the most metabolically active structures in the body, dividing rapidly and continuously. That makes them demanding — and expendable.
When your body is under strain, whether from illness, deficiency or stress, it prioritises essential functions. Hair growth is not one of them, so follicles can shift from their growing phase into a resting phase, and a few months later you notice increased shedding.
That delay is worth understanding: hair often responds to something that happened two to four months earlier. If you're shedding now, the trigger may well have been a bout of illness, a crash diet or a stressful period in the spring.
Iron: the most common culprit in women
Iron deficiency is one of the most frequent nutritional causes of hair shedding, and it's particularly common in women who menstruate.
The important nuance is that hair can be affected before full anaemia develops. Your haemoglobin can look normal while your iron stores — measured as ferritin — are low. Many doctors will check ferritin specifically when hair shedding is the complaint.
Who's more likely to be affected: women with heavy periods, anyone pregnant or recently postpartum, people eating little or no meat without planning for iron, those with coeliac disease or other absorption issues, and frequent blood donors.
Dietary sources: red meat, poultry and fish provide the more readily absorbed form. Lentils, beans, tofu, fortified cereals and dark leafy greens provide the plant form, which absorbs better alongside vitamin C — a squeeze of lemon or some peppers with your lentils genuinely helps.
Don't self-prescribe iron. Excess iron is harmful and accumulates. Get tested first.
Protein, and why crash diets show up in your hair
Hair is made almost entirely of keratin, a protein. Severely restricting protein, or losing weight very rapidly, can push follicles into their resting phase.
This is a common and under-recognised cause of shedding after an aggressive diet, and it typically appears two to three months after the restriction rather than during it, which is why people often don't connect the two.
Including a protein source at each meal — eggs, fish, poultry, dairy, beans, lentils, tofu — covers this for most people without any special effort.
Thyroid: easy to test, easy to miss
Both underactive and overactive thyroid conditions can cause hair changes, and thyroid disorders are substantially more common in women.
Hair changes rarely appear alone. An underactive thyroid often comes with fatigue, weight gain, feeling cold and dry skin; an overactive one with weight loss, palpitations, anxiety and heat intolerance.
It's a straightforward blood test, and treating the thyroid typically improves the hair, though it can take some months.
Hormonal changes across life
Postpartum shedding is extremely common and alarming when you don't know to expect it. During pregnancy, raised oestrogen keeps more hairs in their growing phase, so you shed less than usual. After birth, those hairs enter the resting phase together and shed at once, typically peaking around three to four months postpartum. It usually resolves on its own within six to twelve months.
Perimenopause and menopause bring falling oestrogen, and many women notice hair becoming finer, drier or less dense, sometimes alongside changes to texture. Our guide to the stages covers what else changes in this period.
PCOS and androgen-related changes can cause thinning in a pattern similar to male pattern loss, often with other symptoms. Worth raising with a doctor, as there are specific treatments.
Stopping or starting hormonal contraception can trigger temporary shedding in some people.
Stress and illness
Significant physical or emotional stress can push a large number of follicles into their resting phase at once — a pattern called telogen effluvium.
Common triggers include major illness or high fever, surgery, bereavement, severe emotional stress and rapid weight loss. The shedding appears two to three months later, which is why it so often feels like it came from nowhere.
The reassuring part is that it's usually temporary and self-resolving once the trigger has passed. The less reassuring part is the delay, which means recovery takes months rather than weeks.
Supplements: when they help and when they don't
| Nutrient | Helps hair when | Honest summary |
|---|---|---|
| Iron | You're genuinely deficient | Test first — excess is harmful |
| Vitamin D | You're deficient | Common deficiency; worth testing |
| Protein | Your intake is genuinely low | Food first |
| Zinc | You're deficient | Uncommon in general populations |
| Biotin | You have a true deficiency | Genuine deficiency is rare |
| Vitamin A | — | Excess can cause hair loss |
| Selenium | — | Excess can cause hair loss |
Two points worth emphasising. Biotin is the headline ingredient in most hair supplements, yet true biotin deficiency is rare in people eating a varied diet, and supplementing when you aren't deficient is unlikely to do much. High-dose biotin can also interfere with certain blood tests, including thyroid and cardiac markers — tell your doctor if you take it.
And excess vitamin A and selenium can cause hair loss themselves, which is a real risk when stacking multiple supplements that each contain them.
What to ask your doctor about
- Full blood count — to check for anaemia
- Ferritin — your iron stores, often the more useful number for hair
- Thyroid function
- Vitamin D
- Hormonal assessment if you have other symptoms suggesting PCOS or menopause
- A medication review — several common medicines list hair loss as a side effect
These are inexpensive, routine tests, and they turn guesswork into something you can act on.
When to see a doctor
- Sudden, rapid or patchy hair loss
- A widening parting or visible thinning at the crown
- Hair loss with scalp pain, redness, scaling or scarring
- Shedding alongside fatigue, weight change, or changes to your periods
- Hair loss after starting a new medication
- Shedding that continues beyond six months
Some scarring conditions cause permanent loss if untreated, so early assessment matters more than it might seem.
Habits that genuinely help
Alongside addressing anything internal, the external basics still count: gentle detangling, less heat, protecting hair overnight, regular trims and reducing friction.
Those protect the length you already have — covered in our guide to breakage — and they work alongside, rather than instead of, whatever is happening internally.
Where a hair oil fits
Bellawell's Hair Care with Argan is a conditioning oil for the strand: smoothing the cuticle, softening dry ends and reducing the friction that causes breakage.
It doesn't affect what happens at the follicle, which is where everything on this page operates. If you're shedding or thinning, this article's list of tests will do more for you than any topical product — our honest look at hair oil and growth explains the distinction.
It contains argan, sweet almond and apricot kernel oils, so it isn't suitable for anyone with a tree nut allergy. Patch test before first use.
The bottom line
Hair reflects what's happening in the rest of your body, usually on a two-to-four-month delay. Iron, protein, thyroid, hormonal changes and stress account for most unexplained shedding, and nearly all of them are identifiable with simple tests.
Get tested before you supplement, since the wrong supplement can make things worse. Treat the inside with your doctor, and the outside with gentle habits.
This article is for general information and is not a substitute for medical advice. Do not start iron or other supplements without testing and medical advice. Consult a doctor about hair loss, thinning, patchy shedding, or shedding with other symptoms. For the product mentioned: external use only, avoid contact with eyes, patch test before first use. Contains tree nut oils — not suitable for anyone with a tree nut allergy.
