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Hair loss: the causes worth testing for
Most sudden shedding is telogen effluvium and the trigger sits about three months before the hair started falling. Which nutritional causes are real and testable — iron, thyroid, rapid weight loss, excess vitamin A — which supplements only help if you are deficient, and which patterns of loss need a doctor rather than a shampoo.
This page explains the common causes of hair loss and which of them are worth a blood test. It does not diagnose, and it does not recommend a treatment. Hair loss with a red, scaly or painful scalp, hair loss in patches, or hair loss in a child needs a clinician.
The three-month rule
Each follicle runs its own cycle: a growing phase lasting years, a brief transition, and a resting phase of roughly three months at the end of which the hair is pushed out and replaced. Normally the follicles are out of step with one another, so shedding is a steady trickle you never notice. A systemic shock synchronises them: a large group leaves the growing phase together, waits out its three months together, and sheds together. That is telogen effluvium, and it is by far the most common reason someone suddenly finds hair in the shower drain.
The practical consequence is that the cause is almost never what you were doing the week the shedding started. Look at the twelve weeks before: a high fever, a hospital admission, an operation, childbirth, a crash diet or a rapid weight loss, a bereavement, a new or stopped medicine. Telogen effluvium is frightening and it is usually self-limiting — once the trigger has gone, the hair regrows over the following six to twelve months, and the shedding stops before the density visibly returns.
The patterns, and what each one means
| Pattern | What it looks like | What it usually is |
|---|---|---|
| Diffuse shedding | More hair everywhere, no bald patches, started suddenly | Telogen effluvium. Look three months back for the trigger. |
| Gradual thinning at the parting or temples | Slow, over years, follows a family pattern | Androgenetic (pattern) hair loss. Treatments exist and are medicines. |
| Smooth round bald patches | Sharply defined, skin looks normal | Possible alopecia areata, an autoimmune condition. Needs assessment. |
| Receding at the edges where hair is pulled | Along the hairline, matching the hairstyle | Traction alopecia. Reversible early, permanent if it continues. |
| Redness, scaling, soreness, follicle openings disappearing | The scalp itself is abnormal | Possible scarring alopecia. See a clinician promptly — this kind is irreversible. |
The nutritional causes that are real
Iron Real, common, testable
Iron deficiency is the nutritional cause most consistently linked to hair shedding, and it is common in menstruating women, in people with heavy periods, in vegetarians and vegans who have not planned for it, and after blood donation or pregnancy. The test is ferritin, usually alongside a full blood count. The threshold at which hair clinics act is debated and is often above the laboratory's lower limit of normal, which is exactly why this needs a clinician reading the result rather than a self-assessment. Do not take iron on suspicion: excess iron accumulates and causes harm. See the foods highest in iron and our blood test reference ranges.
Thyroid Real, testable
Both an underactive and an overactive thyroid cause hair loss, and both are easily missed because the other symptoms — tiredness, weight change, feeling cold or hot, period changes — get attributed to ordinary life. It is a simple blood test and it is on the standard list for anyone presenting with diffuse shedding.
Rapid weight loss and low energy intake Real, and self-inflicted
Crash dieting is one of the most reliable triggers of telogen effluvium there is, and it operates through the same three-month delay, so the shedding often begins just as someone reaches their target. Very low calorie intake, very low protein intake and sustained low energy availability in athletes all do it. If shedding began about three months after a diet started, that is very likely the answer, and eating adequately is the treatment. Our calorie needs reference gives the floor to work from.
Excess vitamin A Real, and counterintuitive
Too much preformed vitamin A causes hair loss. This matters because it is the one nutritional cause people create for themselves while trying to fix the problem: high-dose retinol supplements, cod liver oil taken generously, or frequent large portions of liver. Beta-carotene from vegetables does not do this. See the foods highest in vitamin A — and note the same substance topically, Retinol, is a different question entirely.
Zinc and selenium Deficiency yes, supplements no
Zinc deficiency causes hair loss, and so, at the other end, does selenium excess. Supplementing either without a demonstrated deficiency is not a treatment and carries its own costs: sustained high-dose zinc induces copper deficiency, and selenium has a narrow gap between enough and too much. Brazil nuts are the one food where a daily habit can genuinely overshoot on selenium. See the foods highest in zinc.
Biotin Not supported, and interferes with tests
Biotin is the headline ingredient of most hair supplements and there is no good evidence it helps anyone who is not deficient — and deficiency is rare on an ordinary diet. It also has a specific documented harm: high-dose biotin interferes with a number of laboratory immunoassays, including some thyroid and cardiac tests, and can push results in a misleading direction. If you take a hair supplement, say so before any blood test. The supplements reference covers what the fact sheets support.
Treatments are medicines, not shampoos
The treatments with real evidence for pattern hair loss are medicines: a topical solution or foam available without prescription in many countries, and oral prescription drugs whose suitability depends on your sex, your plans for pregnancy and your other conditions. One of those oral drugs must not be handled by anyone who could become pregnant. That is exactly why this page names none of them as a recommendation: the decision belongs to a doctor or pharmacist who knows your situation, and prescription-only medicines bought from an online seller are a category to avoid entirely.
What shampoos and supplements can do is limited and mostly cosmetic. What consistently helps is finding a treatable cause, and the three-month rule plus two blood tests find most of them.
When to see a clinician
- The scalp itself is abnormal — red, scaly, painful, or with follicle openings disappearing. Scarring hair loss destroys follicles permanently and is the one that should not wait.
- Smooth, round, sharply defined bald patches, which need a diagnosis rather than a supplement.
- Any hair loss in a child.
- Shedding with other symptoms: fatigue, weight change, feeling cold or hot, new period irregularity, or new coarse hair growth elsewhere.
- Shedding that has not settled after six months, or that is getting worse rather than better.
- Before starting iron or any high-dose supplement, so that the test is done first and the result means something.
Sources: EU Cosmetics Regulation (EC) No 1223/2009 and its annexes, as published in the European Commission's CosIng database, for every statement about what is permitted, restricted or banned · opinions of the Scientific Committee on Consumer Safety (SCCS) and safety assessments of the Cosmetic Ingredient Review (CIR) for the concentration positions · standard dermatology reference material for mechanism, tolerability and contact-allergy patterns · checked 2026-09-06. Deep links are deliberately omitted where a stable public URL could not be confirmed.
The hair-cycle timing, the pattern descriptions and the nutritional associations above follow standard dermatology reference material; the biotin interference with laboratory immunoassays is the subject of published regulatory safety communications. No dose is given on this page for any supplement or medicine.
Preguntas frecuentes
Why did my hair start falling out three months after I was ill?
Because that is how the hair cycle works. A systemic shock — a fever, an operation, childbirth, a crash diet, a severe stress — pushes a large number of follicles out of their growing phase and into a resting phase at the same time. Resting hair stays in place for roughly three months before it is shed, so the shedding you notice is a delayed echo of something that already happened. Look back twelve weeks, not at last week.
How much hair loss is normal?
Shedding is normal and is commonly quoted at somewhere around fifty to a hundred hairs a day, which is a rough reference rather than a measurement anyone should be attempting. What matters more than counting is change: hair coming out in much greater quantity than your own normal, visible thinning of the parting, or a scalp that is becoming visible where it was not.
Will taking biotin help my hair?
Only if you are deficient in biotin, which is rare on any ordinary diet. In people who are not deficient, biotin supplements have not been shown to improve hair. There is also a specific practical harm: high-dose biotin interferes with a range of laboratory blood tests, including some thyroid and cardiac tests, and can produce badly misleading results. Tell any clinician taking your blood that you take it.
Should I take iron for hair loss?
Not without testing. Iron deficiency is a genuine and common cause of hair shedding, particularly in menstruating women, and correcting it is worthwhile — but iron in someone who is not deficient does not help and iron overload causes real harm. Ferritin is the usual test, the threshold used in hair clinics is debated and is often higher than the laboratory's lower limit of normal, and the interpretation belongs to a clinician.
When is hair loss a reason to see a doctor quickly?
When the scalp itself is abnormal — red, scaly, sore, or with the follicle openings disappearing — because scarring types of hair loss destroy the follicle permanently and the window to intervene is early. Also for smooth round bald patches, for any hair loss in a child, and for hair loss alongside fatigue, weight change or period changes.
Blood test reference ranges
Ferritin, thyroid and the rest — what the numbers on the report mean.
Supplements and the evidence
What the fact sheets actually support, supplement by supplement.
The ingredient dictionary
What is in the products, and which claims survive contact with the evidence.
The trigger was three months ago. Was it in your log?
Illness, a diet, a stressful month, a stopped medicine — the cause of shedding is usually already behind you and rarely remembered accurately. TempoLife keeps the timeline so you can look back at the right twelve weeks.