How Periods Quietly Cause Iron Deficiency
A doctor explains how monthly blood loss drains iron stores over years, why ferritin falls long before hemoglobin does, and which symptoms to take seriously.
The short version
- Menstrual blood loss is the most common cause of iron deficiency in women of reproductive age worldwide, and it works slowly enough that most people never notice the decline.
- Iron stores fall first and hemoglobin falls last, so a normal full blood count does not exclude iron deficiency. Ferritin is the test that finds it early.
- Exhaustion, breathlessness on stairs, hair shedding, brittle nails, restless legs and craving ice are all recognized features of low iron rather than separate problems.
See a doctor promptly if
These are the signs that change this from something to read about into something to act on.
- Breathlessness at rest or on mild exertion, chest pain, or palpitations that are new
- Periods requiring hourly pad or tampon changes, flooding through clothing, or clots larger than a coin
- Iron deficiency with black or bloody stools, unintentional weight loss, or in anyone past menopause, where bleeding is not the assumed cause
Every period removes iron, and the body cannot always replace it from food quickly enough before the next one arrives. Repeat that cycle for years and iron stores drain gradually, which is why menstruation is the leading cause of iron deficiency in women of reproductive age and why the decline is so easy to miss.
Why it creeps up on people#
Iron loss through menstruation is small each month and relentless over time. Dietary iron absorption is limited. The gut takes up only a fraction of what you eat, and iron from plant sources is absorbed less readily than iron from meat. So a woman with moderately heavy periods, a diet low in red meat, and no particular illness can slide into deficiency over several years without a single dramatic moment.
The gradual pace also blunts the symptoms. There is no day when you feel suddenly anemic. Instead the stairs get slower, concentration frays, and people conclude they are simply getting older or working too hard. I see this constantly: the fatigue is attributed to everything except the one monthly event that has been quietly removing iron since adolescence.
Why ferritin matters more than hemoglobin#
The body defends circulating hemoglobin for as long as it can by pulling iron from storage. Deficiency therefore develops in stages: stores fall, then red cells start to shrink, then hemoglobin finally drops. A normal full blood count does not rule out iron deficiency, and if only the blood count is checked, the early and most treatable stage is invisible.
| What is measured | What it tells you |
|---|---|
| Hemoglobin | Whether you are anemic now. The last thing to fall |
| MCV (red cell size) | Falls as iron runs short; small cells suggest iron deficiency |
| Ferritin | Iron stores; the earliest and most useful marker |
| CRP | Inflammation, which can push ferritin falsely upwards |
What to do about it#
Two questions run in parallel: replace the iron, and reduce the loss. Replacement is a medical decision. The form, the dose and the duration all matter, and stores usually take months to refill even after blood counts normalize, so stopping early is a common reason for relapse. Taking iron with a source of vitamin C helps absorption; taking it with tea, coffee, milk or calcium supplements hinders it.
Reducing the loss is where many women are undertreated. Heavy menstrual bleeding has recognized treatments, including hormonal coils and non-hormonal medicines taken during bleeding days, and they are genuinely effective. Describing your periods concretely. How often you change protection, whether you flood, whether you plan your life around your cycle. Helps far more than saying they are heavy.
One caution: in anyone past menopause, or in men, periods cannot be the explanation, and iron deficiency then needs a proper search for another source of blood loss.
Where to take this next#
The fuller guide to iron deficiency anemia goes through the whole test panel, how treatment is monitored, and the other causes worth excluding when periods do not fully explain the numbers.
Common questions
How much blood is actually lost in a period?
Can I be iron deficient with a normal blood count?
Should I just take an iron supplement?
Sources
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