Iron Deficiency: Symptoms, Testing and What Happens Next

Written by Dr. Simon Khela MBChB MRCGP, GMC Registered Doctor

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Tiredness that doesn't lift with sleep, hair that seems to be shedding more than usual, a craving for ice that makes no sense to the person experiencing it, iron deficiency has a habit of showing up in ways that don't immediately point to the obvious cause. It's also one of the most common things I test for in clinic, and one of the most frequently missed, partly because early symptoms are easy to dismiss as simply being run down.

This article works through the symptoms worth paying attention to, how iron deficiency is actually diagnosed on blood tests, and, importantly, what happens after a diagnosis, including when it warrants further investigation rather than simply starting supplements.

Quick Answer: Symptoms, Testing and Next Steps

• Common symptoms include persistent fatigue, pale skin, breathlessness, palpitations, brittle nails, hair shedding, and restless legs; less commonly, a craving to chew ice or other non-food items.
• Diagnosis starts with a full blood count and a ferritin level; ferritin below 30 micrograms per litre generally confirms iron deficiency, though this can be harder to interpret if there's inflammation present.
• Iron deficiency can exist before anaemia develops, so a normal haemoglobin doesn't rule it out.
• In men and postmenopausal women, unexplained iron deficiency anaemia should prompt investigation for a gastrointestinal cause, since this can occasionally be the first sign of bowel cancer.
• Treatment usually involves oral iron supplements, continued for around three months after levels normalise, alongside addressing whatever's actually causing the deficiency.

Iron Deficiency vs Iron Deficiency Anaemia: What's the Difference?

These terms are often used interchangeably, but they're not quite the same thing. Iron deficiency describes depleted iron stores in the body. Iron deficiency anaemia is what happens once those stores are low enough that your body can no longer produce sufficient haemoglobin, the protein in red blood cells that carries oxygen.

This matters clinically because you can have genuinely low iron stores, and experience real symptoms as a result, before your haemoglobin level has dropped enough to be classed as anaemic. If your private GP only checks a full blood count and it comes back “normal,” that doesn't necessarily rule out iron deficiency; a specific ferritin test is what actually assesses iron stores. Understanding how ferritin, haemoglobin and other blood markers fit together can make your results much easier to interpret. Our guide to understanding your blood test results explains what common blood test markers mean and how they are typically interpreted.

Common Symptoms

Some symptoms are well known; others are less obviously connected to iron in most people's minds:

• Persistent fatigue and low energy, often out of proportion to how much sleep you're getting
• Pale skin, particularly noticeable inside the lower eyelids or on the palms
• Shortness of breath, especially on exertion
• Noticeable heartbeats or palpitations
• Headaches and difficulty concentrating
• Restless legs, particularly at night
• Brittle nails, sometimes developing a spoon-shaped curve (koilonychia) in more marked deficiency
• Hair thinning or increased shedding
• A sore or unusually smooth tongue, or cracks at the corners of the mouth
• Pica: an urge to eat or chew non-food items, most classically ice, which is a genuinely recognised, if less common, presenting feature

Many people experience only a handful of these, and symptoms often develop gradually enough that they're easy to attribute to stress, poor sleep, or simply getting older, rather than a specific, correctable cause.

What Causes Iron Deficiency?

The underlying cause generally falls into one of a few categories, and working out which one applies matters for treatment. According to NHS guidance, the most common causes are:

• Blood loss: heavy periods are the most common cause in women of reproductive age; in men and postmenopausal women, gastrointestinal blood loss, from causes ranging from a stomach ulcer to, less commonly, bowel cancer, is the more typical concern.
• Dietary insufficiency: not eating enough iron-rich foods, more common in vegetarian and vegan diets if not carefully planned, though a well-balanced plant-based diet can meet iron needs.
• Malabsorption: conditions such as coeliac disease or inflammatory bowel disease can prevent iron from being properly absorbed, even with an adequate diet.
• Increased demand: pregnancy significantly increases iron requirements, as does rapid growth in childhood and adolescence.
• Medication effects: long-term use of proton pump inhibitors can reduce iron absorption, and regular NSAID use can contribute to low-grade gastrointestinal blood loss.

It's worth noting that more than one cause can be present at once. A woman with heavy periods and a diet that's naturally lower in iron, for instance, may be dealing with two contributing factors simultaneously, which is one reason a proper history matters more than simply prescribing iron and moving on.

How Is It Diagnosed? Understanding Your Blood Tests

Diagnosis typically starts with two tests: a full blood count, which shows your haemoglobin level and the size of your red blood cells (smaller than normal, or microcytic, red cells are a classic feature of iron deficiency), and a ferritin level, which reflects your body's iron stores. According to NICE guidance, a ferritin level below 30 micrograms per litre confirms iron deficiency in most adults.

Ferritin has one important limitation worth understanding: it's what's known as an acute phase reactant, meaning it can rise in response to inflammation, infection, or chronic illness, regardless of your actual iron stores. This means a ferritin result that looks normal, or even high, doesn't always rule out iron deficiency if there's inflammation present elsewhere, which is why your GP may also check inflammatory markers like CRP alongside it, or interpret a borderline result in the context of your overall clinical picture.

Interpreting Your Results: A Quick Reference

Red Flags: When Iron Deficiency Needs Urgent Investigation

This is one of the most important parts of this article. NICE's suspected cancer guidance (NG12) specifically identifies unexplained iron deficiency anaemia as a reason for urgent, 2-week-wait referral for suspected colorectal cancer in adults aged 60 and over. In men of any age and in postmenopausal women, unexplained iron deficiency anaemia is generally investigated with bidirectional endoscopy (looking at both the upper and lower gastrointestinal tract), because gastrointestinal blood loss is the most likely explanation once heavy periods are no longer a possible cause.

• Men of any age with unexplained iron deficiency anaemia
• Postmenopausal women with unexplained iron deficiency anaemia
• Anyone aged 60 or over with new iron deficiency anaemia, particularly via urgent referral
• Iron deficiency anaemia alongside a change in bowel habit, rectal bleeding, or unexplained weight loss

I want to be direct about this without causing alarm: the large majority of people investigated this way do not have cancer. But because iron deficiency anaemia can occasionally be the first, and sometimes only, sign of a gastrointestinal cancer before other symptoms appear, this group of patients is investigated as a matter of routine, not because something is necessarily seriously wrong.

What Happens Next: Treatment Options

Once iron deficiency is confirmed, treatment usually involves two parallel strands: correcting the deficiency itself, and identifying and addressing the underlying cause.

• Dietary changes: increasing intake of iron-rich foods such as red meat, dark leafy greens, pulses, and fortified cereals, alongside vitamin C, which enhances absorption, and away from tea or coffee taken with meals, which inhibits it.
• Oral iron supplements: usually the first-line treatment, most commonly ferrous sulfate, ferrous fumarate, or ferrous gluconate.
• Intravenous iron: reserved for people who can't tolerate oral iron, whose gut can't absorb it properly, or who need levels corrected more quickly, such as before surgery or in late pregnancy.
• Treating the underlying cause: this might mean managing heavy periods, investigating and treating a source of gastrointestinal blood loss, or reviewing medications that affect absorption.

If you're prescribed oral iron, a few practical points make treatment easier to tolerate. Taking it with a source of vitamin C, such as a glass of orange juice, genuinely improves absorption. Taking it away from tea, coffee, calcium supplements, or antacids, which all reduce how much iron your gut actually takes up, matters more than most people realise. And darker stools are a normal, expected side effect of oral iron, not a cause for concern on their own.

A Note on How to Take Iron Supplements

This is worth knowing, because it goes against the instinctive assumption that more frequent dosing is always better. A randomised study by Stoffel and colleagues found that iron absorption was actually higher when supplements were taken on alternate days rather than every day, because a dose of iron temporarily raises a hormone called hepcidin, which blocks further absorption for roughly 24 hours. Taking iron every other day, rather than daily, can improve both absorption and, for many people, gastrointestinal tolerability.

This isn't universally the right approach for everyone, and your GP may still recommend daily dosing depending on the severity of your deficiency and how quickly correction is needed, but it's a reasonable thing to discuss if you're struggling with side effects on a daily regimen.

How Long Does Treatment Take?

Most people notice their haemoglobin starting to rise within two to four weeks of starting appropriate treatment, and a clear response confirms the original diagnosis. However, treatment usually continues for around three months after your haemoglobin returns to normal, because this additional time is needed to replenish your body's iron stores, not just correct the anaemia itself. Stopping too early, as soon as you feel better, is one of the more common reasons for iron deficiency to recur.

Common Myths and Misconceptions

• “If my full blood count is normal, I can't be iron deficient.” Iron stores can be low before anaemia develops; a specific ferritin test is needed to properly assess this.
• “A normal or high ferritin always rules out iron deficiency.” Ferritin can be falsely elevated by inflammation, so it needs interpreting in context, not in isolation.
• “Iron deficiency only happens if you don't eat enough meat.” Diet is one cause among several; blood loss and malabsorption are just as important, and often more so in adults.
• “Taking more iron tablets will fix it faster.” Beyond the recommended dose, extra iron isn't better absorbed and mainly increases the risk of gastrointestinal side effects.
• “Iron deficiency anaemia in older adults is usually just ‘getting older’.” It's never simply attributed to age without investigation, precisely because it can be the first clue to something that needs addressing.

Frequently Asked Questions about Iron Deficiency