Women's hormones
Heavy periods and your blood test: what a normal count still misses
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed August 20266 min read
Every Helvy guide is written by our health editors, then checked by a qualified clinician before it goes live and re-checked as the science moves. We name clinical roles, not individuals, until each reviewer has agreed to be credited publicly. This is wellness guidance to help you understand your own data, not a diagnosis.
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Heavy periods are a common reason women run low on iron. NICE tells your GP to run a full blood count, which flags anaemia only once haemoglobin has already dropped. It does not routinely include ferritin, the marker that shows iron stores draining first, so stores can empty while the count still reads normal.
Not sure which markers you need? Build your test →You plan your week around your period. You double up on products, you keep spares in your bag, and you are wiped out for days afterwards. Someone took your blood, and the result came back normal.
That word covers less ground than it sounds like. This guide sets out what the NHS test measures for heavy periods, what it leaves out on purpose, and where the gap sits.
1. What counts as a heavy period?
There is no single number. The NHS lists signs instead, and any one of them counts.
- You change a pad or tampon every one to two hours.
- You use two products together, such as a pad and a tampon.
- Your period lasts more than seven days.
- You pass clots larger than a 10p coin.
- You bleed through to your clothes or your bedding.
- You skip exercise, work or plans because of it.
The last sign on the NHS list is the one people discount: feeling tired or short of breath a lot. That is the iron cost of the bleeding, showing up between periods rather than during them.
2. What is your GP told to test?
One blood test, and NICE is unusually direct about it.
“Carry out a full blood count test for all women with HMB, in parallel with any HMB treatment offered.”
— NICE NG88, recommendation 1.2.6
HMB is heavy menstrual bleeding. A full blood count reads your haemoglobin, your red cell size and several other cell measures. It is a good test, and it is the right first move.
The two recommendations either side of it are the interesting part. NG88 also says do not routinely carry out a serum ferritin test for women with HMB. And it says do not test thyroid hormones unless other signs of thyroid disease are present.
Those are decisions about NHS resources across a whole population. They are not a claim that the markers hold nothing useful for you.
3. Why can a normal blood count still miss low iron?
Because the two tests answer different questions. Ferritin is your iron store. Haemoglobin is the protein that carries oxygen once that store has been spent.
Your body protects haemoglobin for as long as it can. Stores run down first, over months of heavier-than-average bleeding, and haemoglobin only falls when there is nothing left to draw on. So a normal count can sit alongside an emptying tank.
The scale of that gap has been measured in the UK. A 2025 study in Frontiers in Nutrition looked at 33,029 health-aware UK adults. It found signs of absolute iron deficiency in 31.6% of females, rising above 35% in women aged 18 to 49.
Roughly one in three. Our guide to low ferritin with a normal haemoglobin covers this pattern in detail, and it is the single most common version of “my bloods were fine” that we see.
4. What else is worth reading alongside iron?
Iron is the first question, not the only one. Heavy bleeding sits next to several other pictures, and a few of them are measurable.
| What you are noticing | Markers worth reading |
|---|---|
| Wiped out, breathless on stairs | Ferritin, iron, transferrin saturation |
| Cycle length all over the place | FSH and LH, read against your cycle day |
| Acne, extra hair growth, weight shifting | Testosterone, SHBG, free androgen index |
| Cold, dry skin, thinning hair too | TSH and free T4 |
| Foggy, tingling hands or feet | Vitamin B12 and folate |
The thyroid row is worth a note. NG88 says not to test thyroid hormones for heavy bleeding on its own. If you have other thyroid symptoms as well, that changes the picture, and our guide on underactive thyroid symptoms sets out what those look like.
For the cycle rows, our irregular periods guide and the women’s hormones hub cover which markers earn their place and when to sample.
5. What should you do next?
See your GP first if the bleeding is new, getting worse, or happening between periods or after sex. Bleeding after the menopause always needs a GP appointment. None of what follows replaces that.
- Ask for your printed numbers, not the word normal. Haemoglobin and MCV both travel better than a verdict.
- Ask whether ferritin was measured. Often it was not, and that is the guideline working as designed.
- Track a few cycles: days of bleeding, products used, how you feel in the week after.
- Leave iron tablets off for a few days before any iron test, since a recent dose lifts the reading.
A blood test measures biomarkers. It offers wellness insight rather than a diagnosis, and what any pattern means is a decision for a qualified clinician.
6. Which test reads iron in full?
The Vitamins & Minerals panel (£159) runs the full iron workup: iron, ferritin, transferrin, transferrin saturation and total iron binding capacity. It reads them with vitamin B12, folate, vitamin D and magnesium.
Two limits, stated plainly. It is not a full anaemia screen, because that also needs a full blood count and a clinical judgement. And ferritin rises with inflammation as well as with iron stores, which is why the report reads it against your other iron markers rather than alone.
If your bigger question is the cycle itself, the Hormone Balance panel (£99) measures FSH, LH, SHBG, testosterone and free androgen index. It does not include oestradiol or progesterone.
Not sure which fits? The build-my-test tool walks you through it in a couple of minutes.
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Helvy’s home blood tests report your iron markers with plain-English context, so a borderline ferritin never leaves you guessing. Samples are processed by UKAS-accredited UK labs.
Frequently asked questions
What blood tests should I have for heavy periods?
NICE NG88 tells GPs to carry out a full blood count for all women with heavy menstrual bleeding. That reads haemoglobin and red cell size. Ferritin, which shows your iron stores, is not part of the routine set, so many women ask for it separately.
Why did the NHS not test my ferritin?
Because NG88 says not to routinely carry out a serum ferritin test for women with heavy menstrual bleeding. It is a population-level decision about NHS resources rather than a view on your own iron stores.
Can you have low iron with a normal full blood count?
Yes. Iron stores run down first, and haemoglobin only falls once those stores are spent. So ferritin can be low while the count still reads normal. A qualified clinician should read the two together.
How common is low iron in UK women?
A 2025 study of 33,029 health-aware UK adults, published in Frontiers in Nutrition, found signs of absolute iron deficiency in 31.6% of females. Prevalence rose above 35% in women aged 18 to 49.
Should I have my thyroid tested for heavy periods?
NG88 says not to test thyroid hormones for heavy bleeding on its own. If you have other thyroid symptoms alongside it, such as feeling cold, dry skin or thinning hair, raise those with your GP as a separate question.
When should heavy periods be checked by a GP?
Book an appointment if the bleeding is new or getting worse, if you bleed between periods or after sex, or if you bleed at all after the menopause. Persistent tiredness or breathlessness is also worth raising.
Related guides
Low Ferritin, Normal Haemoglobin
The exact pattern behind most normal counts, and what it feels like.
Iron Deficiency Blood Test
Ferritin, iron studies and what each number in the workup shows.
Full Blood Count UK
Every line on the FBC your GP ordered, and what each one shows.
Irregular Periods Blood Test
Which hormone markers are worth reading, and when in your cycle.
Anaemia Blood Test UK
What anaemia looks like on a report, and the markers behind it.
Women's Hormones Hub
Every guide we have written on cycles, hormones and testing.