Low iron deserves a why
A woman in her thirties or forties presents feeling tired. Her blood tests show low iron stores or iron-deficiency anaemia.
She’s still having periods.
It can be very tempting to join those three dots and conclude:
“It must be menstrual blood loss.”
Sometimes that’s exactly the explanation.
But sometimes it’s not.
Periods are a common cause of iron deficiency in younger women. But they shouldn’t become an automatic answer that prevents us from asking the more important question:
Where is the iron going and does the proposed explanation genuinely account for it?
Bowel cancer is not exclusively an older person’s disease
Most bowel cancers still occur in older adults. However, colorectal cancer incidence is rising among younger Australians.
Australian Institute of Health and Welfare estimates show that between 2000 and 2025, the incidence rate more than doubled in people in their thirties -from 6.3 to 18.9 cases per 100,000. Rates also increased among people in their forties.
This doesn’t mean that every young woman with low iron has bowel cancer. Far from it.
It does mean that age alone can no longer be used to dismiss persistent bowel symptoms, unexplained rectal bleeding or iron-deficiency anaemia.
Are younger women being diagnosed later?
There is a strong and understandable concern that bowel symptoms in younger women can be interpreted through a gynaecological lens.
Abdominal discomfort becomes menstrual pain.
Bloating becomes hormonal.
Rectal bleeding becomes haemorrhoids.
Fatigue and low iron become heavy periods - even when the woman does not describe particularly heavy bleeding.
The research doesn’t allow us to say that every woman under 50 is diagnosed later than a man with the same disease. Sex differences in stage at diagnosis have not been completely consistent across populations.
There is, however, evidence of a potential problem.
A population study of younger adults with colorectal cancer found that women experienced a longer interval from first presentation to treatment than men -approximately 26 additional days. Most of that difference appeared to occur before the cancer was diagnosed. The researchers suggested that bleeding and anaemia being attributed to menstruation was one plausible explanation, while emphasising that their data could not prove the cause.
The fairest conclusion is therefore:
The possibility of diagnostic delay in younger women is real. The precise size and causes of that disparity remain incompletely understood.
Iron deficiency is a finding - not a diagnosis
Iron deficiency means that the body’s iron stores are depleted. Iron-deficiency anaemia occurs when the deficiency has progressed far enough to reduce haemoglobin production.
Replacing the iron is important, but replacing it doesn’t explain why it became low.
Potential causes include:
- Heavy or prolonged menstrual bleeding
- Pregnancy, childbirth or breastfeeding
- Insufficient dietary iron
- Regular blood donation
- Reduced absorption, including coeliac disease
- Gastrointestinal inflammation
- Bleeding associated with medications such as anti-inflammatories
- Ulcers
- Polyps
- Bowel cancer or another gastrointestinal malignancy
Sometimes more than one factor is present.
A woman may genuinely have heavy periods and also have coeliac disease. She may eat little iron-containing food and have an inflammatory bowel condition. Menstrual blood loss doesn’t provide immunity from every other cause.
How can a polyp or bowel cancer cause low iron?
Polyps and bowel cancers can bleed intermittently and in very small amounts.
The blood is often invisible. There may be no dramatic rectal bleeding and nothing obviously abnormal in the toilet.
Over weeks or months, that microscopic loss may gradually deplete iron stores. Fatigue, reduced exercise tolerance, shortness of breath, headaches, poor sleep quality and/or poor concentration may appear long before anyone suspects a gastrointestinal cause.
Iron-deficiency anaemia is recognised as one of the possible warning signs of early-onset colorectal cancer, together with rectal bleeding, abdominal pain and persistent changes in bowel habit.
What about a faecal occult blood test?
The modern test is generally called a faecal occult blood test, or FOBT.
It looks for tiny amounts of human blood in a stool sample.
Depending on the clinical circumstances, a GP might use FOBT when someone has anaemia, abdominal pain, altered bowel habits or unexplained weight loss. A positive result usually leads to further assessment, commonly colonoscopy. FOBT is generally not the appropriate test when there is already visible rectal bleeding. That bleeding needs direct clinical assessment.
FOBT can be very useful but it’s not perfect.
It doesn’t diagnose bowel cancer, it does not investigate the upper gastrointestinal tract, and a negative result should not automatically close the case when iron deficiency is persistent, recurrent or accompanied by concerning symptoms.
Recent specialist guidance describes FOBT as a way of helping to triage people with iron-deficiency anaemia while acknowledging a small but definite false-negative rate. The broader clinical picture still matters.
Colonoscopy remains the principal examination for directly inspecting the large bowel and removing or sampling abnormal tissue.
Not every woman with low iron needs a colonoscopy
This is important.
The answer is not to subject every menstruating woman with a low ferritin to immediate endoscopy.
The answer is to avoid reflex thinking.
A thoughtful assessment asks:
Are her periods actually heavy?
How many days does she bleed? Does she flood through clothes or bedding? Does she pass large clots? Has the bleeding recently changed?
Is the deficiency new, persistent or recurring?
Does it return whenever iron treatment stops? Is the fall disproportionate to her menstrual history?
Are there gastrointestinal symptoms?
Has there been a new and persistent change in bowel habit, rectal bleeding, abdominal discomfort, unexplained weight loss or loss of appetite?
Are there other plausible causes?
Dietary intake, pregnancy, blood donation, medications, coeliac disease and other gastrointestinal conditions all matter.
Is there a family history?
A family history of bowel cancer, advanced polyps or an inherited cancer syndrome may change the threshold for investigation.
No single answer automatically determines the next step. The pattern does.
Screening is different from investigating symptoms
Australia’s bowel screening program is designed for people who are well and don’t have any symptoms.
A person with iron-deficiency anaemia, rectal bleeding or a persistent bowel change is no longer asking a screening question. They need a diagnostic assessment, determined by their symptoms, history, examination and test results.
Completing a routine screening kit isn’t a substitute for discussing new symptoms with a GP.
When should you make an appointment?
Please speak with your GP if you have:
- Newly diagnosed or recurrent iron deficiency without a convincing explanation
- Iron deficiency that doesn’t improve as expected
- Rectal bleeding
- A persistent change in bowel habit
- Unexplained abdominal pain or bloating that’s new for you
- Unintentional weight loss
- Increasing fatigue or breathlessness
- A significant family history of bowel cancer or polyps
Most of these symptoms will have causes other than bowel cancer.
They are still worth explaining.
The bottom line
Heavy menstrual bleeding is common.
Iron deficiency is common.
Bowel cancer in younger women remains uncommon, but it is becoming more common than it used to be.
We should be able to hold all three facts at once.
Periods may be the reason for low iron. They should not be used as an explanation without first establishing that they genuinely account for it.
Not every woman with iron deficiency needs a colonoscopy. Every woman with iron deficiency deserves a credible why.
